School Science Lessons
Physics - Electrostatics, static electricity, capacitors
Updated: 2008-07-16
Please send comments to: J.Elfick@uq.edu.au
Table of contents
31.0.0 Static electricity,
electrostatics
31.0.0 Electrostatics, static electricity
31.1.0 Static charge, electrophorus,
electroscope
31.2.0 Coulomb's law, the coulomb, pith balls
31.3.0 Electrostatic meters
31.4.0 Conductors and insulators,
distribution of charge on a conductor, action of points
31.5.0 Induced charge, charging by induction
31.6.0 Electrostatic machines and devices,
electrostatic generators, Wimshurst machine, Van de Graaff generator
31.7.0 Electric fields and potential,
Faraday's ice pail experiment
31.7.1 Gauss' law
31.7.2 Electrostatic potential
31.8.0 Capacitance, capacitors, capacitance and inductance
31.8.1 Capacitors
31.8.2 Dielectric
31.8.3 Capacitors, energy stored, Leyden jar
31.8.4 RC circuits, resistance -
capacitance
circuits
31.0.0
Electrostatics, static electricity
31.01 Electrostatic charge, conventions from
effects of friction between substances
31.02 Movement of electric charge
31.03 Electrostatics and humidity, detecting
electrostatic charge, sand bath drying oven
4.44
Newspaper stays on the wall
4.45
Static electricity detector
31.1.0 Static charge,
electrophorus, electroscope
4.39
Static electricity from rubbing
4.40 Use a van de Graaff generator
4.41
Attract water to a comb
4.42
Balloon stays in place
4.43
Repulsing balloons
4.44
Newspaper stays on the wall
4.45
Static electricity detector
4.46
Pith ball indicator
4.47
Metal foil ball electroscope
4.48
Metal leaf electroscope
4.49
Two kinds of static charge
4.50
Many charges from one source
32.1.2
Piezoelectricity
31.1.01 Amber and rubbing experiments
31.1.02 Electrostatic series, triboelectric
series, ranking of insulators
31.1.1.1 Comb attracts and repels, bent water
stream, air cleaner
31.1.1.2 Comb on a turntable
31.1.2.1 Ebonite rods and glass rods
31.1.2.2 Hard rubber rubbing
31.1.3.1 Paper horse race
31.1.3.2 Paper strips diverge
31.1.3.3 Paper sticks to the wall
31.1.3.4 Paper snake strikes
31.1.3.5 Paper shapes dance
31.1.3.6 Paper loop attracted by plastic ruler
31.1.3.7 Puffed rice jumps
31.1.3.8 Pepper photocopy
31.1.4.1 Aluminium balls on gramophone record
31.1.4.2 Aluminium foil precipitator
31.1.5.1 Polythene bag for a nylon stocking
31.1.5.2 Clothes brush and flannel
31.1.6.1 Balloon charge
31.1.6.2 Balloons stick to a surface
31.1.6.3
Stretched rubber band
31.1.7.1 Body sparks, rug scuffing
31.1.8.1 Electrophorus
31.1.8.2 Electrophorus lights a Bunsen burner
31.1.8.3 Electrophorus lights an electric lamp
31.1.15 Mercury shaker (This experiment not
allowed in some school systems!)
31.1.16 Sulfur freeze
31.1.17 Cellulose acetate sheet
31.1.19 Metal on perspex, Volta's experiment,
voltaic pile
31.1.23 Neon lamp on Styrofoam
31.1.24 Cotton thread rises
31.1.25 Wool flannel lights a fluorescent light
tube
31.2.0 Coulomb's law,
the coulomb, pith balls
31.2.3 Coulomb's law balance
31.2.4 Aluminium foil covers pith balls and
ping-pong balls
31.2.5 Aluminium drink-cans as pith balls
31.2.6 Pith balls and Styrofoam balls show
electric charge
31.2.7 Balloons rubbed with wool or fur repel
each other
31.2.8 Jumping pepper, separate pepper from salt
31.2.9 Metal balls play music
31.2.11 Measure charge with a balanced metre rule
31.3.0 Electrostatic
meters
31.3.1 Electroscope
31.3.2 Braun electroscope
31.3.3 Aerated drink-can electroscope
31.3.4 Ping-pong ball electroscope
31.3.6 Balloon electroscope
31.3.7 Static electricity detectors, metal foil
ball electroscope, Kolbe electroscope
31.3.8 Make an electroscope
31.3.9 Charged ebonite rod on electroscope
31.3.10 A chocolate wrapper electroscope
31.3.11 Oppositely charged electroscopes
31.3.12 Electroscope with PVC and acrylic
2.143
Pith ball indicator
2.144
Metal foil ball electroscope
2.145
Metal leaf electroscope
31.4.0 Conductors and
insulators
31.4.1 Conductor and non-conductor
31.4.2 Test insulators
31.4.3 Wire versus string
31.4.4 Aluminium and acrylic
31.4.5 Distribution of charge, Kolbe cone
conductor
31.5.0 Induced charge,
charging by induction
31.5.1 Charging by induction
31.5.2 Induced charge
31.5.3 Electroscope charging by induction
31.5.4 Drink-can attracted to charged rod
31.5.5 Suspended electrophorus
31.5.6 Blow soap bubbles at Van de Graaff
generator
31.5.7 Paper sticks on the board
31.5.8 Cork attracted then repelled, familiarity
breeds contempt
31.5.9 Raleigh fountain
31.5.10 Kelvin water dropper
31.5.11 Charge by an electroscope induction
31.5.12 Charge a metal rod by induction
31.5.13 Pith balls attract and repel
31.6.0 Electrostatic
machines, electrostatic generators
31.6.1 Wimshurst induction machine, induction
generator
31.6.2 Van de Graaff generator
31.6.3 Simple electrostatic generator
31.6.4 Franklin's electrostatic motors
31.6.5 Atmospheric electric field motor
31.6.6 Toepler-Holtz machine
31.7.0 Electric fields
and potential
31.7.1 Gauss' Law
31.7.2 Electrostatic Potential
31.7.3 Electrostatic potential difference,
electrostatic potential difference (p.d.)
31.7.4 Lightning conductor, lightning stroke
31.7.5 Finger sparks, static electricity in the
body
31.7.6 Balloon sparks
31.7.7 Hair on end
31.7.8 Pith ball plate and flying balls
31.7.9 Streamers
31.7.10 Electric resin
31.7.11 Magnesium oxide smoke
31.7.12 Orbiting foil
31.7.14 Electric chimes
31.7.15 Electrostatic ping-pong
31.7.16 Aluminium bounces
31.7.17 Fuzzy in mineral oil shows electric
fields
31.7.18 Air bubbles in oil show electric field
31.7.19 Epsom salts on glass plate show electric
field
31.7.20 Ice filaments show electric field
31.7.21 Finger on the electrophorus
31.7.22 Extent of electric field
31.7.23 Mapping field potential voltage
31.7.24 Electric fields of currents
31.8.0.0
Capacitance, capacitors, capacitance and inductance, colour code,
variable capacitor, 500 pF, dielectric strength
31.8.1.0 Capacitors
31.8.1.1 Sample capacitors
31.8.1.2 Simple spherical capacitor
31.8.1.3 Parallel plate capacitor, variable air
capacitor
31.8.1.4 Capacitance and voltage
31.8.1.5 Battery and separable capacitor
31.8.1.6 Dependence of capacitance on area
31.8.1.7 Rotary capacitor
31.8.1.8 Inducing current with a capacitor
31.8.1.9 Water cup spark collector,
electrostatic storage, capacitor
30.06.8 Impedance, phase, resonance
30.06.9 Capacitor (formerly
"condenser"), capacitance in an A.C. circuit
30.06.10 Capacitors in series and
parallel
31.8.2.0 Dielectric
31.8.2.1 Permittivity
31.8.2.2 Capacitor with dielectric
31.8.2.3 Equation Q = CV
31.8.2.4 Force on a dielectric
31.8.2.5 Attraction of charged plates
31.8.2.6 Dissectible condenser
31.8.2.7 Bound charge
31.8.2.8 Impedance of a dielectric
31.8.2.9 Breath figures
31.8.2.10 Lichtenberg figures
31.8.2.11 Displacement current
31.8.3.0
Capacitors,
energy stored in a capacitor
31.8.3.1
Leyden jar
31.8.3.2
Experiments with capacitors
4.50
Many charges from one source
31.8.4.0 RC
circuits, resistance-capacitance circuits
31.8.4.1 Capacitor and light bulb, long RC time
constant
31.8.4.2 RC time constants
31.8.4.3 Series and parallel capacitors
31.8.4.4 Neon relaxation oscillator, blinking
neon bulb
See also:
See also: Interesting
websites
See also: History of this
document
31.0.0 Electrostatics, static electricity
Electrostatics is the study of charges mainly at rest on
non-conductors and the effects it creates. A neutral atom has equal
quantities of positive and negative charge. Charged bodies can be
positively or
negatively charged. Bodies with like charges repel each other and
bodies with unlike charges attract each other. The nature of a static
charge is due to an excess or shortage of electrons so you can say that
the natural
unit of electric charge should be the charge on an electron. A
negatively charged body contains more electrons than protons. A
positively charged body contains fewer electrons than protons. You can
use simple
electrostatic generation and testing equipment to show the nature of
attraction, repulsion and induction laws. Electrostatic charge refers
to electric charge at rest on the surface
of an insulator or insulated conductor. Electrostatic charge has an
associated electric field.
31.01 Electric charge, conventions from effects of
friction between substances
By convention the rubber rod rubbed with fur has negative
electrical charge, and the glass rod rubbed with silk has a positive
electrical charge. An uncharged body has equal amounts of positive and
negative charge.
1. Rub a rubber rod with fur. Suspend two pith balls so that they hang
parallel and vertically. Touch separately the two suspended pith balls
with the rubbed rubber rod. When you bring the two pith balls together,
they swing apart, so a repulsive force now exists between them.
2. Rub a glass rod with a silk cloth. Suspend two pith balls so that
they hang parallel and vertically. Touch separately the two suspended
pith balls with the glass rod. When you bring the two pith balls
together, they
swing apart, so a repulsive force now exists between them. When you
bring together one of each pair of pith balls, they swing
together, so an attractive force now exists between them.
31.02 Movement of electric charge
An electric current is a flow of electrons. The electron carries one
negative basic charge and the proton carries one positive
basic charge. You can define the original positive charge as the charge
on a glass
rod when rubbed with silk. Taking away electrons forms a positive
charge can be formed on an object. Adding electrons forms a negative
charge on a body. An electrical charge can be built up by means other
than
friction. A flow of electric charge is associated (a) heating effects
(b) lighting effects (c) resistance, conductors and insulators (d)
the motor effect (e) electrolysis (f) magnetic effects (g) currents
in nerve tissue.
Electric charge can be either positive or negative, and a neutral atom
has equal quantities of positive and negative charge. Like charges
repel, unlike charges attract. Rubbing a plastic bag with wool takes
electrons
from wool leaving the plastic bag negatively charged and wool
positively charged. Insulators do not allow charges to move over their
surface or through them, so any charge remains on the surface.
Conductors allow
charges to move over their surface or through them. When you bring a
charged object near a metal conductor, the like charge is repelled
along the conductor to the far end, and the near end is left with
opposite
charge. Charges separated this way are called induced charges. The
testing
for type of charge is repulsion by a like charge. If an isolated
conductor has charges induced on it, the charge that is repelled can be
made to
go to earth by touching the conductor briefly with a finger. The
opposite charge will be left on the conductor.
31.03 Electrostatics and humidity, detecting
electrostatic charge, sand bath drying oven
Electrostatics experiments may not work if the equipment is damp or
stored in a damp place or if the humidity is high. In some countries
electrostatics experiments may not work at all during the humid summer
months so teaching electrostatics in the winter months when humidity is
low is best. You can dry equipment with a sand bath drying oven. Fill a
shallow baking tray with sand and cover with a semicircular sheet of
tin-plate. Heat with a Bunsen burner. Put glass rods in the sand and
put paper and cloths over the tin-plate cover. Some equipment, e.g.
electrophorus, works better if first dried in front of a radiator.
31.1.0 Static charge, electrophorus,
electroscope
31.1.01 Amber and rubbing experiments
The writings of Thales of Miletus, about 600 BC mention that amber
rubbed with wood attracts light objects. The word electricity comes
from the Greek word for amber. In 1600 William Gilbert found that many
substances rubbed together attract other substances. Rub a fluorescent
tube with wool, rub an ebonite rod with wool, brass with an insulated
handle, amber, ebonite, glass, "Plexiglas", PVC, cat fur (rub an
ebonite
rod), silk cloth (rub a glass rod), foam rubber cloth, pith.
31.1.02 Electrostatic series, triboelectric
series, static electricity from rubbing, ranking of insulators
Produce static charge by rubbing or touching, contact charging. The
triboelectric series is a list of items sorted according to polarity of
charge produced by rubbing If you rub together any two of the
substances in
the triboelectric series, the substance higher in the list will lose
electrons to become positively charged, the substance lower on the list
will gain electrons and become negatively charged. For example, if rub
glass (6)
with silk (14), glass becomes positive and silk becomes negative. If
rub ebonite (25) with wool (9), ebonite becomes negative and wool
becomes positive. If rub perspex (23) with wool (9), perspex becomes
negative and wool becomes positive. The further apart the materials the
greater the charge produced. The relative polarity depends on the
specimen's molecular structure and the nature of the surface of that
specimen. The list below is a composite list from several authorities
with no two lists in complete agreement. If substances are touched
instead of being vigorously rubbed, the sequence may change.
Positive polarity +, collects positive charges
Most positive
1. Dry air
2. Human skin
3. Leather
4. Asbestos (Do NOT use asbestos in any experiment!)
5. Rabbit fur
6. Glass rod (borosilicate glass)
7. Mica
8. Human hair
9. Wool knitted, flannel
10. Nylon stocking
11. Cat fur
12. Polished glass, window glass
13. Lead rod
14. Silk cloth
15. Aluminium foil, chocolate wrapping ("silver paper"), zinc rod
16. Paper (newspaper, filter paper, puffed rice, popcorn, pepper)
17. Cellulose acetate, combs (acetyl cellulose, photographic film,
overhead projector transparency)
18. Cotton handkerchief, flannelette
19. Steel (iron compounds)
Negative polarity, collects negative charges
20. Dry wood, pith, cork (sunflower stem, artichoke stem, elder tree
pith)
21. Amber rod
22. Perspex / Lucite, plastic ruler (Plexiglas, PMMA, polymethyl
methacrylate thermoplastic, optical lenses, tubing)
23. Paraffin wax, resin (sealing wax, esters, beeswax, shellac,
turpentine)
24. Ebonite rod, hard rubber (polymerized isoprene resin + sulfur,
vulcanite, motor car tyres)
25. Polycarbonate polymer, car battery casing (PC, Lexan, CR-39,
spectacle lenses)
26. Brass rod, copper. nickel, cobalt, silver (metals)
27. Gold, platinum
28. Sulfur
29. Rayon (cellulose, artificial silk, tyre cord)
30. Celluloid, ping-pong ball (nitrocellulose polymer, spectacle frame)
31. Styrofoam, plastic Petri dish (Polystyrene, styrene resin).
32. Saran wrap (vinylidene chloride and vinyl chloride chlorofibre)
33. Orlon, polyesters (polyacrylonitrile polymer, acrylic resin,
Acrilan, imitation fur, carpets)
34. Polyurethane polymer, paints, rubber, foam plastics, tough
linings, car body parts
35. Polythene strip, plastic bag, cling film (Polyethylene, "Scotch
Tape", "Glad Wrap", bin liners, wash bottles)
36. Rubber, balloon (soft rubber, India rubber)
37. Polypropylene rod, plastic bucket (tubing connectors, heavy duty
bottles)
38. PVC, gramophone record (polyvinyl chloride polymer, Vinyl,
Vinylite, poly-chloroethane, tubing for burners, electrical cable)
39. Silicon
40. Teflon, non-stick surface of frying pans (polytetrafluoroethene
polymer, PTFE, non-lubricated bearings)
Most Negative
31.1.1.1 Comb attracts and repels, attracts a
water stream, bent water stream, deflection of a stream of water, air
cleaner
1. Rub a comb or plastic ruler vigorously with wool. Bring it near a
ping-pong ball on a smooth table. The charged comb attracts the
uncharged ping-pong ball. Let the comb touch the ping-pong ball.
2. Sprinkle bits of tissue paper on the
table. Bring the charged comb
close to the paper. The comb attracts the light paper but later they
shoot off the comb.
3. Draw a spiral snake on tissue paper. Draw
2 eyes and a mouth on
the head of the snake. Cut out the snake with sharp scissors. Bring the
charged comb or plastic rules near the head of the snake. The snake
rears up and tries to bite you!
4. Bring the charged comb to a bowl of puffed wheat or puffed rice.
They are at first attracted to the comb then shoot off.
5. Adjust the water tap to give a thin stream of water. Comb your hair
or rub the comb with wool. Bring the comb close to the thin stream of
water. The water stream bends towards the comb as the negatively
charged comb attracts the neutral water. Let the water touch the comb.
The comb loses its charge to the water and can no longer attract a
water stream.
6. Rub a plastic spoon with wool. Turn the
tap on gently and hold the
spoon near the fine stream. The falling water is pulled towards the
spoon. The plastic spoon becomes negatively charged because electrons
are
rubbed off the wool. The plastic spoon attracts the uncharged water
particles. When the water touches the spoon it can no longer attract a
water stream because the extra electrons flow onto the water surface
from
the spoon.
7. Attract water to a comb. Adjust a tap so
that a very thin stream
of water flows from it. Give a comb a charge by running it through the
hair several times. Hold the comb 2 cm from the stream of water. The
water is strongly attracted by the electrical charge on the comb.
8. Uses a comb as an air cleaner. Rub a comb with wool. Wave the
comb
through the air then look at the teeth of the comb with a magnifier.
See the dust from the air now attracted to the comb. This method is
used
to clean smoke in chimneys by "scrubbers" which attract dust by
electrostatic attraction.
31.1.1.2 Comb on a turntable
Make a turntable for electrostatic experiments to study the two
kinds of static charge. Put the top of an inverted test-tube into the
centre hole of a big cork or a rubber stopper. Hammer four pins
symmetrically
into the cork's upper plane without destroying the balance of the cork.
A swivel can be used in many static electricity experiments. Use it to
show that like charges repel each other. Put a plastic comb, after
being
rubbed against a piece of pelt, on the swivel table and make the comb's
centre of gravity coincide with the centre of the cork. Then observe
the rotation of the comb on the swivel table when another comb, after
being rubbed against a piece of pelt, close up the comb on the swivel
table.
31.1.2.1 Ebonite rods and glass rods
1. Ebonite rod rubbed with wool becomes negative because electrons
leave the wool and stick on the surface of the ebonite. Rub two ebonite
rods with wool. Suspend one rod. Bring the other rod near it. Note the
repulsion.
2. Glass rod rubbed with silk becomes positive because electrons
leave the glass and stick to the surface of the silk. Note that glass
rods dried in the flame of a Bunsen burner may become negative when
rubbed
with silk. However, glass rods dried in a sand oven always become
positive when rubbed with silk. Lead glass may charge better than soda
glass. Use glass rods dried in a sand bath oven. Rub the ends of glass
rods
with silk. Suspend one rod so that it hangs horizontally. Bring the
rubbed end other rod near it. Note the repulsion as the suspended rod
moves away. Note the force of attraction between the rubbed ends of
both
glass rids and the silk. With one charged rod on a pivot use another of
the same or opposite charge to show attraction or repulsion.
3. Suspend a charged ebonite rod. Bring a charged glass rod near it.
Note the repulsion.
Suspend a charged ebonite rod. Pass another charged rod gently
through the hand then bring it near the suspended rod. Note there is no
repulsion. Rub this uncharged rod on the back of the hand. Bring it
near
the suspended rod. Note the repulsion. Rubbing on the skin has
recharged the rod. Pass the charged rod quickly through a Bunsen burner
flame. Bring it near the suspended rod and note its loss of charge.
Fit a brass or other metal tube over the end of an uncharged
ebonite rod. Rub the brass tube only with fur. Bring it near a
suspended charged rod of ebonite. Note the repulsion. The brass is also
negatively charged.
31.1.2.2 Hard rubber rubbing
Investigate equality of charges. Rub a rubber rod against a similar rod
covered with wool in a Faraday ice pail. The electroscope shows no
charge unless either of the rods is removed. Or rub them together
outside
the pail and insert them separately and together.
31.1.3.1 Paper horse race
See diagram 31.1.3.1
Cut some small horses from folded dried paper and stand them on a
table. Rub a plastic article, such as a ruler, vigorously with wool and
use it to attract the "horses" in a race across the table.
31.1.3.2 Paper strips diverge
See diagram 31.1.3.2
Cut newspaper strips about 5 cm wide and 40 cm long. Dry them in an
oven or warm place. Hold two strips in one hand. Stroke them downward
several times with the finger and thumb of the other hand. The strips
fly apart. Use another two strips. Hold these two strips in one hand.
Stroke them downward with folded polythene plastic shopping bag or wool
cloth. The strips fly apart more strongly. Use paper strips as a "strip
test" to see which materials produce the greatest charge. In this way
you can produce your own triboelectric series.
31.1.3.3 Paper sticks to the wall
See diagram 2.141
1. Warm a sheet of newspaper paper in an oven. Put the newspaper on a
table and rub vigorously with a polythene shopping bag or dry cleaner's
cover. Put a round metal top from a glass jar on it. Hold each end
of the newspaper and lift it. Ask another student to bring a finger
near the round top. On a dry day a spark jumps. Repeat by rubbing with
a nylon stocking. Repeat by rubbing with a woollen jumper. Compare the
sizes of the sparks.
2. Spread out a sheet of newspaper and press it smoothly against a
wall on a dry day. Stroke the newspaper with a pencil or your hand all
over its surface several times. Pull up one corner of the paper and
then let
it go. Notice how it is attracted back to the wall. If the air is very
dry, you can probably hear the crackle of the static charges. If you
hold the charged paper near your cheek, you may receive a tickling
feeling.
Repeat the experiment by rubbing the paper with wool, fur, nylon,
plastic or celluloid.
31.1.3.4 Paper snake strikes
Cut a spiral shaped coil from a piece of very thin paper so that it
looks like a snake. Put it on a tin lid and bend its head up. Rub a
plastic ball point pen case with wool cloth and hold it over the snake.
The paper coil
rises like a snake! The ball point pen case takes electrons from the
wool and attracts the uncharged paper. On contact, the paper falls
because it takes part of the negative electric charge and gives it up
immediately
to the metal lid, which is a good conductor. Since the paper is now
uncharged again, it is again attracted upwards until the ball pen has
lost its charge.
31.1.3.5 Paper shapes dance
Put a sheet of glass over a metal plate. Cut out shapes from tissue
paper. Rub the glass with wool. The tissue paper shapes jump up and do
a funny dance. Rubbing charges the glass and attracts the tissue paper
shapes and charges them. The shapes repel each other, fall back on the
metal plate, give their charge to the metal plate and again are
attracted to the glass.
31.1.3.6 Paper loop attracted by plastic ruler
Cut a piece of paper 5 cm X 1 cm and join the ends with adhesive tape
to make a paper loop. Rub a plastic ruler vigorously and hold it on the
table in front of the loop. The loop rolls towards the ruler. If you
can
react quickly enough you can pull the ruler away from the loop and it
will follow the ruler and roll across the table.
31.1.3.7 Puffed rice jumps
Rub a plastic spoon with wool. Hold it over a dish of puffed rice
cereal. The grains jump up, hang on the spoon then jump off in all
directions. The uncharged puffed rice is attracted to the negatively
charged spoon
and hang on to it. Then electrons spread from the spoon into the puffed
rice both have the same charge. The charged puffed rice grains then
move in all directions because like charges repel each other.
31.1.3.8 Pepper photocopy
Evenly scatter finely ground pepper over the bottom of a plastic Petri
dish. Cut a piece of paper to exactly match the plastic lid of the
Petri dish. Use sharp scissors to cut a shape out of the paper, e.g. a
diamond
shape. Hold the paper over the Petri dish lid and rub vigorously with
wool over the diamond shape. The pepper jumps up and sticks to the
underside of the cover in a diamond shape. The diamond-shaped area of
the plastic cover rubbed with wool developed a negative charge that
induced positive charges on the upper surfaces of the pepper particles.
So the pepper particles were attracted to the plastic lid. Pepper
particles
do not have a regular shape and pepper is an electrical insulator so
the pepper particles remain charged and clinging to the plastic lid.
31.1.4.1 Aluminium balls on gramophone record
1. Use aluminium foil or "silver paper" to cut out the shape of a
soccer player and make a soccer ball. Attach it to the edge of an old
78 rpm gramophone record. Rub the record with wool cloth and put it on
an
inverted drinking glass. Put a tin sheet from a jam tin in front of the
player. Attach a thread to the aluminium foil soccer ball. Hold the
thread so that the ball is suspended near the foot of the player. The
extra
electrons on the record spread into the player and attract the ball
that has no charge. The ball touches the foot of the player, receives
extra electrons, then is forced away from the player because player and
ball are
now negatively charged. The ball hits the tin sheet, loses its
electrons and becomes attracted to the player again.
2. Rub an old gramophone record with wool and put it on glass. Drop
small aluminium foil balls onto the gramophone record. They jump from
the gramophone record in all directions. Static electricity produced by
rubbing is irregular. The aluminium ball take up the negative charges
and are repelled but they are attracted to positive charges still on
the gramophone record.
3. Use aluminium foil or "silver paper" to make little balls to
represent grasshoppers. Rub an old 78 rpm gramophone record with wool
and sprinkle the little aluminium balls onto the record. The
grasshoppers will
they will jump away from each other because they have the same charge.
31.1.4.2 Aluminium foil precipitator
See diagram 31.9.29
Build a simulation experiment to show the action of an electrostatic
precipitator. The outer electrode is a cylinder of aluminium foil
inside the walls of the jar. Turn on the aquarium pump. Air passes
hydrogen chloride
gas from the hydrochloric acid to react with the concentrated ammonia
solution from the next test-tube to form a smoke of ammonium chloride.
Notice the amount of smoke from the chimney. Gradually increase the
flow of air from the pump then turn on the induction coil to supply the
high voltage. Note any change in the smoke from the chimney. Adjust the
air flow by constricting the tube from the air pump.
31.1.5.1 Polythene bag for a nylon stocking
Rub the palms your hands with blackboard chalk to make them dry. Rub a
polythene shopping bag vigorously between the palms of your hands. Pick
up the bag. The side move out because of repulsion. Cut a strip
of polythene from the bag and hang it over a dry finger so that the
ends hang down. The ends repel each other. Hold a nylon stocking
against the wall and rub with a polythene bag. Pull it from the wall.
The stocking
fills out to its shape as the sides repel with the same charge. Bring
the polythene near the stocking and they will attract each other.
31.1.5.2 Clothes-brush and flannel
Brush a piece of dry flannel or brown paper with a clothes-brush and
note how the flannel will then cling to the wall.
31.1.6.1 Balloon charge
See diagram 31.2.3.1
1. Suspend two equally charged balloons by triangular suspension. The
forces on one balloon are as follows: F electric force, T tension in
string, mg weight of a balloon. Measure the distance d to the vertical
and
then weigh a balloon mg. F / mg = d / sqrt (s2 - d2)
2. Fill a toy balloon with air and tie the necks tightly. Rub the
balloon with wool. Attach the rubbed part to a wooden door. The balloon
will still stay there. Rub the balloon against a woollen sweater then
attach the
rubbed part to a student's face. The air balloon will also stay there.
You have rubbed some electrons from the wool onto the balloon to give
it
a negative charge. These electrons repel the electrons nearby on the
wooden door or student's face and these charges are then attracted to
the positive charges left on the door. The balloon will stick to any
uncharged surface but the charge will gradually leave the balloon
especially if
the humidity is high or if placed under running water. The balloons
filled with air will stick to the ceiling for hours because they are
attracted to the uncharged ceiling. After some time the extra electrons
on the
balloons pass to the ceiling so the balloons no longer kiss the ceiling.
31.1.6.2 Balloons stick to a surface
1. Blow up a toy balloon and rub it briskly with a piece of fur. Place
it against the wall and observe that it stays where you place it.
Repeat the experiment by rubbing the balloon on your hair.
2. Rub inflated balloons against wool, cotton
or hair. Let them stick
to the blackboard or the wall. The material rubbed against loses some
electrons and the balloons gain some electrons giving them a negative
charge. As the balloons approach the blackboard the negative charges on
the balloon repel the negative charges on the blackboard leaving a
positively charged surface on the blackboard near the approaching
balloon. So the balloon sticks to this positively charged area of the
blackboard. Later, as electrons flow from the balloon to the
blackboard, the balloon become electrically neutral and falls,
especially in humid weather.
31.1.6.3 Stretched rubber band
A stretched rubber band becomes charged positively Any amount of charge
can be removed by sliding along the band
31.1.7.1 Body sparks
1. Separating charge in common ways to separate charges, e.g. scuff a
rug and then discharge through a neon bulb. Identify charges with an
electroscope charged with known sign to test other charged objects.
2. Walk on a carpet or rug made of synthetic fibres while wearing
rubber shoes. Electrons are attracted from the carpet and spread across
your body. When you touch a metal rail or a door handle, some electrons
will jump away from your fingers as a small spark giving you a fright.
Touch the metal again. Nothing happens because you are no longer
charged.
3. Hold a fluorescent lamp tube while rubbing your shoes strongly on
the carpet. Keep hold of the tube and touch the other end on a metal
railing. The fluorescent tube produces a brief flash of light.
4. Comb your hair vigorously on a day when the humidity is low. Your
hair may stand on end!
5. Pull off a woolly jumper over a silk
shirt. On a very dry day you
may hear a crackling sound!
6. Motor car tyres can pick up extra electrons form the ground that
spread through the car and cover your body. When you get out of the car
you may experience a slight electric shock when the extra electrons
jump back to earth. Some vehicles, e.g. petrol tankers have a chain or
a metalled strip always touching the ground to get d of extra
electrons picked up by the tyres from the road.
31.1.8.1 Electrophorus
See diagram 31.1.8.1 | See
diagram 3.8.0: Conic sections, ellipse
Discharge of an electrophorus shows the actual movement of charge.
Alessandro Giuseppe Antonio Anastasio Volta, 1745 - 1827, was the
Italian experimenter who invented the "electroforo perpetuo",
electrophorus, described in 1775. Luigi Galvani, 1737 - 1798, another
Italian experimenter did the first work on the twitching of frogs' legs
by electric current Volta took up the study of frogs' legs and showed
Galvani that he was wrong about "animal electricity". An electret is
used in the same way as an electrophorus except it is permanently
charged.
1. Use a metal plate on a handle to transfer charge from a large
charged surface conducting sphere an ellipsoidal conductor a hollow
cylinder. Repeat charging a metal plate many times using Lucite or
polystyrene as
the sole making an electrophorus from sealing wax.
2. Use a neon discharge tube to show a flash by holding one end on
the electrophorus and then touching the other end.
3. You can use a plastic gramophone record to make an electrophorus,
but it slips about in use. Also, some records contain materials that do
not take the same charge consistently. You can use a block of sulfur as
an
electrophorus but handling it is difficult because it tends to crumble.
Brass, copper and aluminium work as well as a copper plate. Use short
perspex handles for convenience in storage with no metal screws
protruding through the perspex. It will nor work without a nearly
perfect insulator for a handle. Nylon clothes may crackle when pulled
over the head in dry weather because nylon on is an insulator. Friction
with dry
hair or even dry clothing electrifies the clothing. In dry weather the
charges accumulate and, when earthed, cause noisy little sparks. If the
weather is humid, the charges generated can be conducted off the
perspex,
through water vapour, on to the table and on to earth. In damp weather
dry the electrophorus with a radiator before use.
4. Rub a sheet of perspex, or slab of solidified sulfur or a plastic
gramophone record, with fur. The rubbing action wipes loose surface
electrons off the perspex onto the fur, so the perspex becomes
positively
charged. Hold a flat sheet of copper by an insulated handle on the
charged perspex. The copper sheet and perspex touch at only a few
points although both seem flat. The copper conductor has electrons that
can
move from atom to atom unlike an insulator. Positive charges from
excess protons in the perspex attract electrons from the flat sheet of
copper when it comes into the electric field of the charged perspex.
Unlike
charges attract, so the underside of the copper disc becomes negatively
charged because it has extra electrons attracted to it leaving the
upper side with fewer electrons. Touch the top of the sheet of copper
with
your finger. The human body is damp and an electrical conductor so when
you touch the copper plate, electrons flow through your body to the top
of the sheet of copper. These electrons replace the electrons lost
when the electrons in the sheet of copper flowed down to the perspex.
When you remove your finger from the copper, these replacement
electrons are left on the copper plate. The electrons lost from your
body are
replaced by electrons moving up your body from the earth, if you are
not wearing insulating rubber soled shoes. When you lift the copper
plate, the extra electrons are trapped on it. You can produce a spark
between the charged copper plate and your knuckle or the metal burner
because so many excess electrons are repelling each other on the
charged plate that they can be "pushed off" the plate. If you touch the
plate
with one end of a conducting copper wire, and touch the other end to
your finger or to the burner, these excess electrons quickly flow
through the wire and form an electric current. An electric current
through a wire
is simply a movement of and electrons. When all the excess electrons
have flowed from the plate through the wire, the current will stop.
When the charged plate is surrounded by dry air there is no conductor
through
which the excess electrons can flow to earth. When the plate is brought
close to a conducting object such as the metal of the burner, the
excess electrons on the plate first repel any free or conduction
electrons in the
burner as far as possible away from that part of the metal closest to
the charged plate. This part of the metal then becomes positively
charged. Electrons now on the plate try to push each other and are
attracted to
the positive part of the metal burner. Some electrons leap from the
charged plate through the air to the burner, knocking electrons out of
air molecules and these join in the electric current passing from plate
to
burner, producing a quick spark and a sharp cracking noise. In a
charged electrophorus, charge leaks away due to conduction of electrons
by moisture on the surface of the apparatus. The original energy comes
from you because work is done in lifting the plate off against the
attraction caused by the electric field. Pulling the charged copper
disc away from the oppositely charged perspex sheet takes energy from
you. When
you put your finger, which is "earthed", on the copper, some electrons
wiped off the perspex get back to the perspex where the copper touches
the electrophorus. However the points of contact are few on these
two surfaces so the electrons cannot spread all over the perspex
because it is a non-conductor.
31.1.8.2 Electrophorus lights a Bunsen burner
Use electrons trapped on the electrophorus to make a spark and to light
the gas in a Bunsen burner. Bring the copper plate of a charged
electrophorus near your knuckle and see if you can detect a spark as
the
electrons jump across the gap to your body and back to "earth" from
where they came. The electron flow is from the surplus to the
deficiency, that is, a current of electrons flows from negative to
positive. Charge
the plate again by laying it on the charged perspex, touching the
topside with a finger and lifting the plate off. This time, turn the
gas on a little and nearly touch the Bunsen burner with the copper
plate. If you can
make a long spark, it will light the gas. Electrons running from a
charged copper plate can momentarily light up a fluorescent tube. The
electron flow is from the surplus to the deficiency, i.e. a current of
electrons
flows from negative too positive.
31.1.8.3 Electrophorus lights an electric lamp
See also 4.118: Fluorescent lamp
Charge the copper disc of the
electrophorus. Hold the end of the lamp
in one hand and touch the terminal of the other end with the copper
disc. You may need to darken the room slightly to be able to notice the
flash. To measure the amount of charge stored on an electrophorus,
repeat the experiment. How many times you can cause the lamp to flash
without recharging the plastic plate? If the original charge on the
plastic
does not leak away, the lamp can be flashed many times.
31.1.15 Mercury shaker (This experiment not
allowed in some school systems!)
Shake mercury in a bottle Put some mercury in a plastic bottle with a
conducting rod sticking through a stopper. Shake the mercury and invert
to charge the rod for a positive charge. Invert a second time for
negative. A glass tube containing some mercury is covered with tin foil
on one end. Either positive or negative charge may be produced. A
mercury tube that emits light when shaken.
31.1.16 Sulfur freeze
Allow molten sulfur to solidify on a glass rod check with an
electroscope
31.1.17 Cellulose acetate sheet
See diagram 31.9.30: Measure electrostatic
force
Stick 3 lengths of elastic yarn to the edges of a triangular piece of
polystyrene foam. Suspend the polystyrene triangle horizontally from
the tripod stand so that one angle is outside the legs of the tripod.
Record the
height of point 1. Roll cellulose acetate sheet to form a cylinder and
rub with a dry cloth. Raise the cellulose acetate cylinder slowly under
the exposed angle of the polystyrene triangle until the exposed angle
dips
down and sticks to it. Record the height of point 1. Lower the
cellulose acetate cylinder. Record the height of point A when the
polystyrene triangle pulls away from the cellulose acetate cylinder.
Remove the
cellulose acetate cylinder. Put squares of graph paper on point A until
the height is the same as before. The weight of the squares of graph
paper = the electrostatic force between the polystyrene triangle and
the
cellulose acetate cylinder.
31.1.19 Metal on perspex, Volta's experiment,
voltaic pile
See diagram 31.1.19: Volta's experiment
Fit a tin plate metal disc with a plastic handle and attached a metal
screw to the metal disc. Rub the top face of a sheet of perspex plastic
with wool to make a positive charge on the perspex. Take hold of the
plastic
handle and lay the metal disc on the charged sheet of perspex. Touch
the metal disc briefly to earth it. Hold the plastic handle and pick up
the metal disc. The metal disc now has a positive charge. In the dark,
bring
your finger close to the screw in the metal disc and observe an
electrical discharge. Note how many times can you repeat the discharge
without rubbing the perspex sheet again.
31.1.20 Paper dancers
Use very light paper to cut out the shapes of dancers. Put the dancers
on a metal plate. Put a glass roof over the dancers supported by two
books. Rub the glass with wool. Be careful! Do not rub too hard and
break the glass! The dancers start dancing when the uncharged dancers
are attracted to the charged glass, pick up extra electrons from the
charged glass, become negatively charged then repel other dancers with
the same charge. The dancers fall down onto the metal plate, give their
extra electrons to the plate and are again attracted to the glass.
31.1.23 Neon lamp on Styrofoam
Use a voltage tester, shaped like a screwdriver. In its handle is a
small neon tube that you can easily remove. Hold one metal end firmly
and rub the other on a piece of hard Styrofoam plastic, which is often
used for
packing or insulation. The lamp begins to glow as you rub it to and
fro.
You can see this particularly clearly in the dark. Since the plastic is
soft, its layers are rubbed against one another by the movement of the
lamp
and become strongly charged with electricity. The electrons collect on
the surface, flow through the core of the tiny lamp, which begins to
glow, and into your body.
31.1.24 Cotton thread rises
Hold a long cotton thread from each in front of a television. The
thread rises as electrons from inside the television set hit the screen
and give it an electric charge that attracts the uncharged thread.
31.1.25 Wool flannel lights a fluorescent light
tube
Rub a fluorescent light tube in the dark with a piece of flannel.
31.2.0 Coulomb's law, the coulomb, pith balls
Coulomb's law, pith balls, balloon
repulsion, Coulomb's torsion balance, electrostatic fields and
Coulomb's Law, quantitative treatment of the potential differences in
the fields of a point charge and parallel
plates F = kQ1Q2 / r2, E = F / Q, V = Ed E = kQ / r2, V = kQ / r
Electric charge, Q, the coulomb, C, Coulomb's law
Electric charge, symbol Q, is measured in coulombs, SI unit 3. A
conductor carrying one amp for one second is carrying one coulomb of
charge. Coulomb's law relates electrical charge to mechanical force.
The
French physicist Charles Augustin Coulomb invented a torsion balance
that he used in 1785 to show the forces of attraction or repulsion
between two charged spheres. Coulomb's Law states that the
electrostatic
force between two charged spheres depends directly on the charges on
the spheres and inversely as the square of the distance between their
centres. If two charges, q1 coulombs, C, and q2 coulombs, C, have the
same sign (+ or -) and are distance, r, apart, the force on one charge
from the other charge, F = constant, k X (q1q2) / r2. The
Coulomb's law constant, k =. 9 X 109 Nm2,
about = 1 / (4piE), where E,
permittivity of the medium = 8.85 X 10-12C2 / N.m2.
So F = q1q2 / 4piEr2.
The very small forces involved in Coulomb's law must be measured with a
torsion dynamometer, e.g. 0.01 N, and very light conducting spheres.
31.2.3 Coulomb's law balance
Adapt a soda straw balance to make a simple Coulomb's law balance.
31.2.4 Aluminium foil covers pith balls and
ping-pong balls
Spend two small pith balls and show either attraction or repulsion.
Suspend two small pith balls from a common support. Suspend two pith
balls coated with Aquadag in a clear framework on the overhead
projector. Hollow aluminium foil balls are charged with a Van de Graaff
generator. Wrap aluminium foil around a marble or ping-pong ball and
then remove the ball to make a replacement for a light. Use metal
painted ping pong balls. Two silver coated ping pong balls are
suspended from separate supports. Gas filled balloons repel. A small
charged pith ball is repelled from a large charged sphere
31.2.5 Aluminium drink-cans as pith balls
Aluminium drink-cans are used instead of pith balls to show repulsion
of like charges.
31.2.6 Pith balls and Styrofoam balls show
electric charge
See diagram 31.2.6
1. Use two perspex rulers or rods, a dry woollen cloth, a pith ball
stand, two ebonite rods and two clock glasses. Make a pith ball by
cutting open a dead sunflower or artichoke stem to take out the light
pithy
centre part. Make two little balls of pith and hang them by fine
threads of nylon or silk from an insulated stand. Rub the perspex rod
with the flannel and bring the rod near the pith balls. Both pith balls
rush over and
touch the charged rod. Both pith balls fly apart after they have
contacted the perspex rod or when you pull the charged rod out of
contact with them. The pith balls now repel each other. Rub a black
ebonite rod
with dry flannel and bring it near the pith balls that are repelling
one another. The pith balls are strongly attracted to the ebonite rod.
Rub another ebonite rod with flannel and rest it on a clock glass. When
the
previously rubbed perspex and ebonite rods are in turn brought near
this rod, you find that it is attracted by the perspex and repelled by
the ebonite.
2. To find the electric potential, bring a charged pith ball close
to a like charged conductor. Suspend a pith ball or Styrofoam ball from
a thread attached to a stand. Rub a plastic comb with the wool. Bring
the
comb near the hanging pith ball without touching it. The pith ball is
attracted to the comb. Let the comb touch the pith ball. The charged
object plastic comb attracted the uncharged pith bal. When the comb
touched
the pith ball, some charges on the comb are transfer to the pith ball.
The pith ball now repels the comb because like charges repel. The pith
ball is attracted to the uncharged finger, When the pith ball touches
the
finger, the charges are neutralized so there is no attraction or
repulsion between the pith ball and the finger.
31.2.7 Balloons rubbed with wool or fur repel each
other
1. Fill three air balloons with air equally and use three pieces of
one metre long thread to fasten their openings separately. Grasp the
ends of the threads by one hand and hang the three air balloons in the
same height.
Remove one balloon without changing the position of the others and rub
it against wool or fur. Let it drop down to resume its original
position. Note the states of the other two balloons.
2. Blow up two balloons equally. Attach them with a one metre long
string. Rub both balloons with wool or fur. Hold the centre of the
string and let the balloons drop down. The balloons will not come
together
because both became negatively charged when they took extra electrons
from the wool. The balloons fight to move away from each other because
like charges repel each other. Suspend the balloons in a doorway
where there is a slight wind. The balloons keep spinning around each
other. Bring one hand near one balloon but not touching. The balloon is
attracted to the hand. Put your hand between both balloons. The
balloons come together to touch the hand. The negative charges on the
balloons have repelled the negative charges on the hand leaving the
hand with net positive charge, so the balloons are attracted to the
hand.
3. Blow up two balloons equally. Attach them
with a one metre long
string. Rub one balloon with wool or fur. Rub the other balloon with
Saran Wrap or a polythene plastic shopping bag. Hold the centre of the
string and let the balloons drop down. The balloons are strongly
attracted to each other but when they touch they hang straight down.
The balloon rubbed with wool or fur has extra electrons. The balloon
rubbed
with Saran wrap, or polythene plastic shopping bag, has a shortage of
electrons. So the two balloons attract each other and come together.
When the balloons touch the balloon with extra electrons, it gives them
to
the balloon with shortage of electrons so they become neutral, no
longer attract or repel each other and hang straight down.
31.2.8 Jumping pepper, separate pepper from salt
See diagram 31.5.3
1. Put ground pepper in a small plastic box with a flat lid. Rub the
lid with wool, e.g. a woollen jumper. The pepper jumps up and sticks to
the inside of the lid. Touch the lid with an opened wire paper
clip. The pepper moves sideways away from the end of the paper clip or
drops down.
2. Put some coarse salt and ground pepper in a plastic box and cover
the box. Shake the box many times to mix the contents thoroughly. Rub
the cover of the plastic box with a piece of wool cloth, or the dry
palm
of your hand. Turn the plastic box upside down then turn it back again.
The pepper sticks to the cover.
3. Mix salt and pepper then put the mixture on a piece of paper.
Hold a plastic ruler rubbed by a piece of wool cloth above the mixture.
The pepper flies out of the mixture and sticks to the ruler.
4. Sprinkle pepper and salt on the table. Rub a plastic spoon with
wool and hold it above the mixture. The plastic spoon becomes
negatively charged because electrons are rubbed off the wool. The
pepper is
attracted to the plastic spoon. The pepper jumps up and sticks to the
plastic spoon. The plastic spoon attracts both the uncharged pepper and
salt but the pepper rises first because it is lighter than the salt.
Hold the
plastic spoon lower to attract the salt. The uncharged pepper particles
are attracted to the charged surface of the plastic spoon because
opposite charges were induced in the pepper particles. No charges are
induced in the salt particles. Separate the two substances by
sprinkling the mixture over
the water surface in a container. The salt sinks and the pepper
particles float.
31.2.9 Metal balls play music
Charge by touching can make a hanging small conductor swing. Use two
equal sized metal balls and insulated stands with same height. Put them
4 cm apart. Hang a screw nut by a nylon thread. The upper end of the
thread hangs on the iron bar of the stand. Adjust the height of the
hanging point to let the screw nut being just in the line connected two
conductors and equal distance from them. Charge the two conductors with
opposite charges. Incline a conduct ball to touch the screw nut gently
(it cannot discharge with another ball), then back to its original
position. The screw nut will swing between two conductors. After the
screw nut
touches a ball, it has the same charge as that ball. The nut is acted
on either by repulsion of that ball or by attraction of another ball,
leading to it moving to another ball and touching it. This results in
the nut having the
same charge with another ball and leading to it moving to the original
ball. Thus motion is repeated until the two balls complete the
discharge and the energy of swing is used up. If this discharge occurs
when you first
incline the ball, you must charge the balls again. If it still
discharges, you can increase the distance between the balls.
31.2.11 Measure charge with a balanced metre rule
Balance a metre rule on an inverted round bottom flask. Rub a comb or
an ebonite rod with wool cloth. Bring the comb ebonite rod near the end
of the metre ruler and note the movement of this sensitive balance.
31.3.0 Electrostatic meters
31.3.1 Electroscope, gold leaf electroscope,
charging and discharging, electrostatic meters,
tests of conduction with gold leaf electroscope
31.3.2 Braun electroscope
31.3.3 Aerated drink-can electroscope
The tab of the soft drink-can supports the electroscope leaves.
31.3.4 Ping-pong ball electroscope
Repulsion of two charged ping-pong balls hung from nylon cord.
31.3.6 Balloon electroscope
Helium filled balloon can be painted with aluminium and charged with a
Van de Graaff.
31.3.7 Static electricity detector, metal foil
ball electroscope, Kolbe electroscope
See diagram 31.2.7 | See
diagram 2.144
Use a light weight sphere hanging by a nylon thread to show that like
charges repel each other, when electric charges distribute equably over
the sphere's surface soon after the sphere is charged. Fasten the top
of
the nylon thread on a pen, and then put the pen upon a wide mouth
bottle. You can use the wax to join the nylon thread and the sphere.
The sphere can be made of a piece of 6 cm2 metal
foil, shaped and pressed
into a sphere. Use a table tennis ball, or popcorn to make the sphere
with a layer of metal membrane packed around the sphere. Smearing a
liquid containing egg white and aluminium powder or scrap iron evenly
on
the surface of the sphere. When a rubbed plastic ruler approaches the
light conductor sphere, the sphere will jump.
31.3.8 Make an electroscope
See diagram 31.3.8
Use a plastic or glass jar with a metal cap. Punch two holes in the lid
to hold a wire inside the jar. Cut two leaves of metalled paper or
light aluminium foil so that they can swing freely over a hook at the
end of the
wire. Use the electroscope for testing electrostatic charges.
31.3.9 Charged ebonite rod on electroscope
Charge an ebonite rod negatively by rubbing it with wool. Roll the
charged rod on the metal lid of an electroscope. Some electrons
transfer to the lid. When you remove the ebonite rod, the electroscope
is left with
excess electrons. Electrons can move through the metal lid, wire and
leaves. Both leaves of the electroscope become negatively charged.
31.3.10 Chocolate wrapper electroscope
Push a piece of copper wire through a hole in the lid of a jam jar.
Bend the end of the wire into a hook and hang a folded strip of
aluminium foil, or "silver paper" from a chocolate wrapper, over the
hook. Rub a
plastic spoon, or ball pen casing with wool or comb your hair with a
plastic comb. Touch the top of the wire with these charged objects. The
ends of the foil strip move apart. Electrons flowed from the charged
objects through the wire to the ends of the strip that now have the
same negative charge and so move away from each other.
31.3.11 Oppositely charged electroscopes
Investigate equal and opposite charge. Two electroscope are charged
equal and opposite then the charge is transferred from one to the other
If tape is pulled off an electroscope plate charge will result and the
tape
will also charge a second electroscope with the opposite charge.
31.3.12 Electroscope with PVC and acrylic
Rubbing rods, fur and silk. Use a PVC rod and felt, acrylic rod and
cellophane with the Braun electroscope as a charge indicator. Rub
acrylic and rubber rods with wool and place on a pivot. Strike a
student sitting
on an insulated stool on the back with a fur. Rub a rod with a cloth
place on a pivot show attraction between rod and cloth.
31.4.0 Conductors and insulators
31.4.1 Conductor and non-conductor
Suspend two light pieces of conducting material from a thread (nylon,
rayon or silk). The conducting material could be aluminium bottle tops,
cooking foil or tinsel (metalled yarn), rolled into a ball. Charge a
rod, e.g.
a plastic ruler, by rubbing with wool and bring it near the suspended
conducting objects. They will be attracted towards the charged ruler.
Immediately on touching the ruler, however, they will be repelled both
from
the ruler and from each other. Repeat the experiment with a
non-conductor, e.g. two balls of ordinary writing paper. What happens?
How does the behaviour differ from that of the conductor?
31.4.2 Test insulators
To test whether an object is a conductor or insulator, touch a charged
electroscope with the object held in the hand. The better the
insulator, the longer it will take for the leaves to collapse.
31.4.3 Wire versus string
Connect two electroscope together with wire or string and charge one
electroscope. Connect a wire or silk thread to an electroscope and show
the difference in conductivity.
31.4.4 Aluminium and acrylic
Aluminium and acrylic rods are mounted on a Braun electroscope. Bring a
charged rod close to each rod conductors and insulators.
31.4.5 Distribution of charge, Kolbe cone conductor
Use the Kolbe cone conductor to show the relation between charge
density distribution and the surface shape of the conductor. It is a
metallic cylinder with a metal cone attached so that you can attach
charge to the
inside or outside of the cylinder and cone. The cone conductor has an
insulated handle.
31.5.0 Induced charge, charging by induction
Electrophorus, electrostatic induction, electroscope, gold leaf
electroscope, charge by induction, deflection of a water stream
31.5.1 Charging by induction
Charging by induction using two balls on stands with an electroscope
for a charge indicator. Use two metal spheres a charged rod and an
electroscope.
31.5.2 Induced charge
Use electroscope and proof planes to show charging by induction
31.5.3 Electroscope charging by induction
Use conductors on the top of two electroscope that can be brought into
contact to show charging by induction. Large metal bars on two
electroscope are apart when charging by induction charging.
Touch the plate of an electroscope while holding a charged rod nearby.
Pith balls touching both ends of a conductor are charged when a charged
rod is brought towards one end. Use another test charge to show the
polarity at each end.
31.5.4 drink-can attracted to charged rod
A hoop of light aluminium is attracted to a charged rod
31.5.5 Suspended electrophorus
Raise an electrophorus off the plate with a helical spring touch the
disc to remove induced charge and show the spring lengthens.
31.5.6 Blow soap bubbles at Van de Graaff generator
A low neutral soap bubbles at a Van de Graaff generator for intriguing
induction effects. Try double bubbles.
31.5.7 Paper sticks on the board
Hold a piece of paper on a slate blackboard and rub it with fur. Rub
paper with cat fur while holding it on the board.
31.5.8 Cork attracted then repelled, Familiarity
breeds contempt
Cork filings are first attracted to a charged rod by induced charge
then repelled as they become charged by conduction.
31.5.9 Raleigh fountain
A charged rod held near a stream of water directed upward breaks it
into drops.
31.5.10 Kelvin water dropper
Sparks are produced by water falling through two rings connected by an
arrangement to opposite receivers. Water dropper made with shower
heads. Water drops through a paraffin coated funnel into a brass cup.
The funnel and cup are connected to a electroscope.
31.5.11 Charge an electroscope by induction
See diagram 31.5.11
Charge an electroscope by induction as follows: Charge an ebonite rod
negatively by rubbing it with wool and bring it near, but not touching,
the lid of the electroscope. 2. Still holding the ebonite rod in
position,
touch the lid of the electroscope with your finger. (3) Remove your
finger from the lid of the electroscope. (4) Remove the ebonite rod.
You have now positively charged the electroscope and the leaves are
apart.
The ebonite rod near the metal top of the electroscope repels some
electrons that move down to the two leaves and they fly apart. The top
becomes positively charged. When you touch the electroscope with your
finger, electrons repelled by the ebonite rod, move through your body
to earth. Electrons are attracted back from the leaves and the leaves
collapse. When you remove the ebonite rod, the excess positive charge
is
distributed through the conducting part of the electroscope.
31.5.12 Charge a metal rod by induction
Put a metal rod or metal spoon AB on an insulating stand, e.g. a
beaker. Hang an aluminium foil metal ball from a string so that it
hangs near end B of the metal rod. Charge an ebonite rod negative by
rubbing it with
wool then bring it near end A of metal rod. A positive charge is
induced at end A and a negative charge is induced at end 2. The
aluminium foil ball is attracted to end B where it receives a negative
charge, is
repelled by end B and moves away again. Move the aluminium foil ball
far from the metal rod. Bring the charged ebonite rod near the
aluminium foil ball. They repel each other showing that the metal rod
had lost
some of its electrons to the aluminium foil ball.
31.5.13 Pith balls repel and attract
See diagram 31.2.6
Use a thin thread to suspend a pith ball on the arm of the iron stand.
Rub a plastic comb vigorously against woollen material. When the comb
approaches the suspended pith ball, the comb attracts it. Let the comb
approach the pith ball several times and observe its movement. Do not
let them make contact. Move the comb towards the pith ball slowly until
the comb contacts it. Note the movement of the pith ball. Move a
finger towards the pith ball to contact it. Note the movement of the
pith ball. The pith ball is attracted by the comb and moves with the
comb when the charged comb approaches the pith ball. A charged body
attracts an uncharged body. The pith ball bounces off the comb when the
comb and the pith ball contact each other because some charges from the
comb flow to the pith sphere and the two bodies then have like
charges. Like charges repel each other, so the two bodies repel each
other. The pith sphere moves along with the finger's direction when the
finger approaches the charged pith ball is the same as the principle
that
the charged comb attracts the uncharged pith sphere. Because the finger
is uncharged and the pith ball is very light, the pith ball closes with
the finger. However, when the finger and the pith ball contact each
other, all
of the charges of the pith ball run off through the person's body and
the pith ball becomes neutral. So the attraction between the pith ball
and the finger will disappear.
31.6.0 Electrostatic machines and devices
Wimshurst induction machine, Van de Graaff generator, distribution of
charge on a conductor, proof plane, action of points, lightning
conductor
31.6.1 Wimshurst machine, induction generator
Two parallel plates rotate in opposite directions. Charge due to
friction in collected by combs then stored in two integrated high
voltage capacitors, Leyden jars, connected in parallel to an adjustable
spark gap up to
30 mm.
31.6.2 Van de Graaff generator
This electrically driven generator with a 200 mm conducting sphere,
capacity 15 pF, can be use to generate high direct voltages of 15 to
200 KV using a high speed fabric belt to accumulate charge in a large
Faraday cage, i.e. the conducting sphere. A charge is applied to the
belt from a point below, then carried up into the hollow sphere where a
collector removes the charge from the belt and stores it on the sphere.
Examine sparks from a Van der Graaff generator to a nearby grounded
ball.
31.6.3 Simple electrostatic generator
Built using a hand drill LP record and fur.
31.6.4 Franklin's electrostatic motors
polyethylene bottle spins as a Wimshurst is connected to brushes
alongside the bottle
31.6.5 Atmospheric electric field motor
Electret type and corona type motor for operation from the earth's
electric field.
31.6.6 Toepler-Holtz machine
You can use a large antique Toepler-Holtz machine to generate high
voltages.
31.7.0 Electric fields and potential
Field
mapping, hair on end, exploring electric fields, lines of force, grass
seed dipole patterns, electric balance, plot electrostatic fields using
simple measurement of equipotentials in
the field, electrostatic potential, potential difference, surface
charge density, lightning rod
An electric field exists when a charge experiences an electric force.
The strength of an electric field at any point is the force per unit
positive charge, and the direction is that in which a positive charge
would tend to
move. Electric field lines are drawn to show the direction of an
electric field. They must leave a charged positive surface
perpendicular to it, and arrive at a negative surface perpendicular to
it. Where the lines are
concentrated, the field is strongest. Opposite charged parallel plates
will produce a uniform electric field between them, except at the ends.
Electric field due to a charged sphere is proportional to charge and
inversely proportional to square of distance, F = 1 / (4piE0) X
(q1q1 / r2). Investigation of the nature of the electrostatic field
surrounding point charges and between parallel plates.
31.7.1 Gauss' law
Faraday ice pail, electroscope
in a cage, Faradays butterfly net, hollow conductors, surface of
conductors, electric field E = volt / metre
A Faraday cage is an earthed metal cage to shield a device from
electric fields. Induced
currents cannot move around the cage.
31.7.2 Electrostatic potential
Potential difference, electrostatic
potential difference (p.d.), potential difference V = joule / coulomb,
electroscope, the earth is zero potential, surface charge density,
lightning rod, electric potential, point
discharge, spark discharge
The potential difference between two points A and B is work done
against electrical forces to move a unit charge from A to B, i.e. work
done per unit charge, joule / coulomb = volts. V = J / 3. In Australia
and
other
countries you connect the negative side of a potential difference to
earth, and to call this zero potential. The positive side of the
potential difference is called the high potential.
31.7.4 Lightning conductor, lightning stroke
See diagram 31.1.5
Lightning is a discharge of electrons from a point where there is a
surplus of electrons to a point where there are fewer electrons.
Lightning can be discharged from one part of the cloud to another
(in-cloud), from cloud to cloud (from negative part of one cloud to
positive part of another cloud), from cloud to the atmosphere, from
cloud to earth (lightning bolt). In lighting bolt starts when a
forking stream of charge with sharp points goes down from the cloud
towards the earth. Before it reaches the ground an upward stream of
positive charge (upward streamer) reaches one of the points and the
lightning bolt lights up from the bottom (return stroke). So a
lightning bolt "moves up and down". Lightning flickers when other
sources of charge in the same cloud use the same channel as the first
lightning bolt. A distinctive lightning bolt is sometimes called
fork lightning compared to "sheet lightning" but they are the same
except that sheet lightning is a lightning bolt (forked lightning)
hidden by cloud so you only see the diffused reflection.
This electrophorus experiment explains lightning, which is the same
effect occurring on a much larger scale. In the atmospheric
disturbances of a heavy storm, clouds often become strongly charged
electrically. Just
as the copper plate in the electrophorus experiment they might have an
excess of electrons produced by the atmospheric disturbances. If the
charged cloud approaches another cloud that is not so heavily charged
giant sparks can jump from one cloud to the other. This is a flash of
lightning with the accompanying noise of thunder. If the cloud is low,
electrons may jump between the cloud and a building on the ground. The
building is struck by lightning. Tall buildings usually have a long
metal rod running from the ground vertically up the side of the
building and extending higher than the building. The long metal rod is
called a lightning
conductor. It protects the building from being struck by lightning. If
electrons jump from a charged cloud overhead to the building, they can
pass through the conducting rod to earth leaving the building free from
damage. Lightning conductors end in trident points because points
discharge electrons from the earth into the clouds easier than any
other shape. Lightning conductors may also have a metal ball to help
electrons to
land on in case of discharge is from cloud to earth. It is not true
that lightning does not strike twice in the same place and you should
not try to run away from it if you are caught in a thunderstorm. If
lightning hits a
tree nearby, electricity may flow into a runner because the body
conducts the electricity better than the ground. Avoid lightning by
sitting with feet together in a depression in the ground. An aircraft
becomes charges
electrically in flight due to friction with the air. Most of the charge
is removed while the plane is in flight with graphite coated nylon rods
fitted with tungsten points placed at the extremities of the aircraft
and linked by
metal strips. On landing, the plane is completely discharged by a wire
spring coil fitted between the wheels that strikes the ground to allow
electrons either to flow on to, or away from, the aircraft. Otherwise
passengers would risk an electric shock when alighting and a fire when
refuelling caused by a spark due to discharge.
31.7.5 Finger sparks, static electricity in the
body
See diagram 31.2.7
1. Use a box cover made of metal. Rub off the paint on its surface
with sand paper. Make a handle by fixing a candle in the centre of the
cover. Use a balloon that has no oil or dirt on its surface. Pump air
into the
balloon, then rub the surface of the balloon with strength by a piece
of dry wool cloth. Hold the candle by your right hand to put the cover
on the surface of a rubbed balloon. Then touch the upper surface of the
metal cover with the left hand index finger. Then will probably be a
small spark on your finger. First remove the left hand finger, then
take off the metal cover from the balloon, still touch the metal cover
by left hand
finger but this time the finger is near the edges of the cover,
immediately is a beam of electric spark jumps up between the finger and
edge of the cover.
2. Inflate a balloon and rub it with wool. Attach a candle to a
candle holder. Put the candle holder on the rubbed surface of the
balloon, using the candle as a handle. Touch the upper side of the
candle holder with a
fingertip. Observe a small spark jumping to your finger. Lift the
candle holder from the balloon by holding on to the candle. Touch the
bottom edge of the candle holder with a fingertip. Observe a small
spark
jumping to your finger. Rubbing the balloon with the wool leaves the
balloon negatively charged. Bringing the candle holder near the charged
balloon pushes all negative charges to the upper side of the candle
holder.
Touching the upper side of the candle holder removes the negative
charges leaving a positive induced charge on the electrophorus.
Negative charges will jump from the finger when brought near the bottom
edge of
the electrophorus.
31.7.6 Balloon sparks
Rub a tightly blown up balloon strongly against your woollen sweater
then quickly press the balloon against your ear. Hear the crackling
noise of the sparks between the balloon and your ear. Make a balloon
spark.
Put a metal tray on glass. Rub a balloon with wool sweater and put it
on the tray. Hold your finger near the edge of the tray. A spark jumps
between the metal tray and your finger.
31.7.7 Hair on end
While standing on an insulated stool charge yourself up with a Van de
Graaff generator.
31.7.8 Pith ball plate and flying balls
Place a plate with pith ball hanging on strings on an electrostatic
generator. Place a cup filled with Styrofoam balls on an electrostatic
generator.
31.7.9 Streamers
Attach ribbon streamers to the top of a Van de Graaff generator. Fray
the end of a nylon clothes-line and charge with an electrostatic
machine to show repulsion. A bunch of hanging nylon strings are charged
by
stroking with cellophane causing repulsion. Charge a mop of insulating
strings.
31.7.10 Electric rosin
Melt rosin in a metal ladle and attach to a static machine. When the
machine is cranked and the rosin slowly poured out, jets of rosin
follow the electric field.
31.7.11 Magnesium oxide smoke
Fill an unevacuated bell jar with MgO smoke to form three dimensional
chain-like agglomerates between electrodes.
31.7.12 Orbiting foil
Throw a triangle of aluminium foil into the field of a Van der Graaff
and it comes to equilibrium mid air. Give it a half twist and it will
orbit in a horizontal circle below the sphere.
31.7.14 Electric chimes
Insert a metallized ping-pong ball between two highly charged metal
plates. Toss a small foil near the charged sphere and then bring a
grounded ball close to show the chime effect.
31.7.15 Electrostatic ping-pong
Conductive ping pong balls bounce between horizontal plates charged
with a Wimshurst machine. A fluffy cotton ball travels back and forth
between an electrostatic generator and a lighted cigar.
Bounce a conducting ball hanging between two plates charged with a
Wimshurst.
31.7.16 Aluminium bounces
Aluminium powder bounces between two horizontal plates attached to a
static machine. Metallized pith balls bounce between an electrode at
the top of a bell jar and the plate.
31.7.17 Fuzzy in mineral oil shows electric fields
Fur in mineral oil aligns along field lines from charged electrodes.
Fine black fibre clippings in castor oil are used to show electric
field between electrodes. Charged electrodes are placed in a tank of
mineral oil
containing velveteen and the pattern is projected on the overhead
electric fields between electrodes. Bits of material suspended in oil
align with an applied electric field. A pan on the overhead projector
contains
particles in a liquid that align with the electric field
31.7.18 Air bubbles in oil show electric field
A stream of air bubbles in an oil bath are repelled in the region of an
inhomogeneous field.
31.7.19 Epsom salts on glass plate show electric
field
Sprinkle Epson salts on a glass plate with two aluminium electrodes.
Tap to align the crystals.
31.7.20 Ice filaments show electric field
An ice filament pattern shows the electrical field configuration Place
a PZT transducer on a block of dry ice.
31.7.21 Finger on electrophorus shows electric
field
Charge and electrophorus then trace a circle on it with your finger and
probe the resulting field with a pith ball on a long thread finger on
the electrophorus
31.7.22 Electroscope near electrostatic machine
shows electric field
Hold an electroscope several feet away from a static machine and
observe the electroscope leaves rise and fall as sparking occurs.
31.7.23 Metal wire in candle flame shows electric
field
To mapping field potential voltage
A wire held in the flame of a candle and attached to a grounded
electroscope is held near a Van de Graaff generator. Mount two candles
on a insulator and attach the second to the case of the electroscope to
measure voltage. A small spirit lamp attached to an electrostatic
voltmeter can be used to map potential fields.
31.7.24 Electric fields of currents
Current carrying conductors are made of transparent conducting ink on
glass plates. Sprinkle on grass seeds to show the electric lines
of force inside and outside the conducting elements.
31.8.0.0 Capacitance, capacitors, capacitance
and inductance, colour code, variable capacitor, 500 pF, dielectric
strength
Capacitors store electrical energy,
to be released at a later stage.
Capacitors are usually made from two conductors separated from an
insulator, dielectric. Capacitance, C, in farads, F, is the capacity
for parallel
plates to hold charge = size of charge on either conductor / size of
potential difference between conductors. C = q / V farads. Capacitors
in
parallel C = C1 + C2 + C3. Capacitors in series C = 1 / C1 + 1 / C2 + 1
/ C3.
Energy stored in a capacitor of capacitance with charge q and potential
difference V = ½ (q2 / C)
C (farads) = Q (coulombs) / V (volts), capacitors, capacitance and
area, capacitor with dielectric, capacity of condenser and distance /
medium, Leyden jar, Fa parallel plate capacitor, force on dielectric
Charge stored, Q = capacitance X potential difference. Coulomb, the
unit of charge transferred by one ampere in one second. Energy changed
= current2 X resistance X time, I2Rt. Energy
changed = p.d. X current
X time, W = VIt. Potential difference = work done / charge moved.
Voltage (potential difference, p.d., potential), the volt, V
31.8.1.0 Capacitors (formerly condensers)
Capacitors are made of layers of flat conductor separated by layers of
insulator (dielectric), e.g. vacuum, waxed paper, mica, glass, plastic,
air (for tuning high frequency oscillators). The capacity to hold
charge,
capacitance, is the ratio of total charge on the parallel plates /
potential difference between the plates (volts). Capacitors store
electrical energy. Capacitance is measured in farads. When you switch
off the power to
your computer the indicator lights keep glowing for a while because
electrical energy is stored in capacitors in the computer.
31.8.1.1 Sample capacitors
Examine many capacitor examples.
31.8.1.2 Simple spherical capacitor
Charge a sphere several times with an electrophorus then repeat with a
insulated conductor near then repeat with a grounded conductor near.
The number of sparks required to reach a potential varies.
31.8.1.3 Parallel plate capacitor, variable air
capacitor
See diagram 31.08
1. Before use, electrolytic capacitors should be correctly connected
across the battery for minute to ensure that the plates are formed.
Charge a large capacitor, e.g. 500 muF (50 V working) with no resistor
in the
circuit. Connect 4 volts from a 12 volt battery or power pack to the
two plates of the capacitor through two galvanometers, or microammeter,
each side of the capacitor. When the switch is closed, see the
momentary pulses of current, positive charge to one plate and negative
charge to the other plate, then no more current. Remove the battery
from the circuit with the flying lead to discharge the capacitor and
observe
momentary current in the opposite directions.
2. Charge a 500 muF capacitor through a high resistor. Repeat (a)
with a high resistance, e.g. 4.7 kohm in series with the capacitor.
Observe the slow charging process as the current dies exponentially as
the charge
rises to full value. Remove the battery from the circuit with the
flying lead to discharge the capacitor and observe momentary current in
the opposite directions. You can see the same current patterns using 5
kV from
a power pack to charge 0.001 muF (20 kV working) capacitor.
3. Charge a 0-001 muF capacitor using a Van de Graaff generator and
then short circuit it. Charge the 0.001 muF capacitor by holding the
capacitor horizontally in a clamp and connecting the stud mounting end
to
the earthed negative terminal of the power supply. Connect the positive
terminal to the capacitor through a 100 ohm resistor. Include a very
high resistance in series to avoid damaging the capacitor, e.g. wet
string.
Connect the end of the resistor to the top of the capacitor with an
insulated flexible lead held by hand. After a few seconds, remove this
flexible lead. Use another insulated lead to short circuit the
capacitor. Hold the
insulated flying lead by hand against the sphere so that it can readily
be removed from contact and used to short circuit the capacitor. Be
careful! 2 cm sparks can be obtained from a capacitor charged in this
way!
This experiment shows that the charged capacitor can produce sparks
when it has been charged from an electrostatic source. The capacitors
may not be designed for use at these voltages and may break down
4. Change the spacing of a charged parallel
plate capacitor while it
is attached to an electroscope.
1. Vary the spacing of a charged parallel plate capacitor while the
voltage is measured with an electroscope field and voltage.
2. Charge a simple capacitor of two parallel movable plates and the
divergence of electroscope leaves varies as the plates are moved.
3. Charge parallel plates with a rod watch the electroscope as the
distance between the plates is changed
31.8.1.4 Capacitance and voltage
Separate charged plates while an electroscope is attached.
31.8.1.5 Battery and separable capacitor
Charge a parallel plate capacitor to then move the plates apart until
an electroscope deflects.
31.8.1.6 Dependence of capacitance on area
As a chain is lifted out of a hollow charged conductor on an
electroscope the deflection decreases When let back down it increases
again. A long rectangular sheet of charged tin foil is rolled up
while attached to an electroscope. Hook up a charged radio tuning
condenser to an electroscope.
31.8.1.7 Rotary capacitor
Charge a large rotary capacitor with a rod and watch an
electroscope as the overlap is changed.
2. C= i / dv / dt demonstrator. Vary a potentiometer so that a constant
current is maintained while charging a capacitor from a volt battery.
Measure the time.
31.8.1.8 Inducing current with a capacitor
A charged ball moving between the plates of a parallel plate capacitor
will induce a current in the external circuit inducing current with a
capacitor
31.8.1.9 Water cup capacitor, water cup spark
collector, electrostatic storage
See diagram 31.1.4: Water cup spark
collector
Put a plastic cup 3 / 4 full of water into a plastic container
containing
water the same height as the water in the cup. Bend two pieces of bare
copper wire at on end so that they can stand upright. Put one piece of
copper wire in the cup and the other in the plastic container. Rub a
glass rod with a piece of silk then touch the copper wire standing in
the plastic cup. Repeat this action 10 times - you are charging the
capacitor.
Hold the end of the copper wire in the outer container with a
clothes-peg and move it towards to the centre copper wire. Observe a
spark jumping between the two copper wires. This apparatus is similar
to the
Leyden jar except that the collecting surfaces are water separated by
plastic instead of aluminium separated by glass to store charges. When
a glass rod is rubbed with a piece of silk, the centre wire water
accumulates positive charges. When a plastic rod is rubbed with fur,
the centre wire accumulates negative charges.
31.8.2.0 Dielectric
Dielectric, waxed paper capacitor,
electrolytic capacitor, capacitor with dielectric, force on a
dielectric, dissectible condenser, waxed paper capacitor, variable air
capacitor
See diagram 31.8.2.0
A dielectric can be a solid, liquid or gas that can keep an electric
field constant, i.e. an insulator. Use a dielectric for electric
cables, electric terminals and capacitors
31.8.2.1 Permittivity
Permittivity, E0, represents the permittivity of free space, i.e. in a
vacuum. If the charges are surrounded by a material, e.g. air, induced
charges in the material decrease the force between the charges.
Permittivity for
vacuum = 1. Permittivity for air = 1.0006. Permittivity for porcelain
is X 7 permittivity for air.
31.8.2.2 Capacitor with dielectrics
Insert and remove a dielectric from a charged parallel plate capacitor
while it is attached to an electroscope. The voltage is measured with
an electroscope as dielectrics are inserted between parallel plates of
a
charged capacitor. Various dielectrics are inserted between two charged
metal plates to show the difference in deflection on an electroscope.
Bring a charged rod close to an electroscope and interpose various
materials between the two.
31.8.2.3 Equation Q = CV
The bottom of a parallel plate capacitor is mounted on an electroscope.
Charge the top plate touch the bottom. Lift off the top.
31.8.2.4 Force on a dielectric
A counterbalanced acrylic dielectric is pulled down between parallel
plates when they are charged with a small Wimshurst generator. A
microscope slide is pulled into the gap between parallel
plates of a capacitor. Elongated paraffin ellipsoid in a parallel plate
capacitor turns when the field is turned on. Kerosene climbs between
parallel plates.
31.8.2.5 Attraction of charged plates
A brass plate fitted with an insulating handle can lift a lithographic
stone plate when dc is applied. Fix the top plate of a parallel plate
capacitor on a triple beam balance to measure the force with and
without
dielectrics as the voltage is varied. Measure the permittivity of free
space with a Mettler balance to find the force between the plates of a
parallel plate capacitor.
31.8.2.6 Dissectible condenser
A capacitor is charged disassembled passed around assembled and
discharged with a spark. The inner and outer conductors of a charged
Leyden jar are removed and brought into contact then reassembled and
discharged. Charge a capacitor and show the discharge then charge again
and take it apart.
31.8.2.7 Bound charge
You can ground the two coatings of a Leyden jar successively without
much loss of charge. A discharge occurs when you connect the two
coatings.
31.8.2.8 Impedance of a dielectric
Place a small parallel plate capacitor in series with a phonograph
pickup. Insert different dielectrics. High dielectrics have low
impedance.
31.8.2.9 Breath figures
Blow on a glass plate that has been polarized with the image of a coin.
31.8.2.10 Lichtenberg figures
Trace a pattern on a dielectric from the two polarities of a charged
Leyden jar. Litharge and flowers of sulfur sprinkled on adhere to the
areas traced out with the different polarities.
31.8.2.11 Displacement current
A toroidal coil is either placed around a wire leading to a large pair
of capacitor plates to show Ampere's law or inserted between the
capacitor plates to show displacement current. Measure the
displacement current in a barium titanate capacitor. (Comment: The
experiment in has nothing to do with displacement current in Maxwell's
sense!)
31.8.3.0 Energy stored in a capacitor, short a
capacitor, light a bulb with a capacitor, lift a weight with a
capacitor, charging a capacitor, charging with a battery
31.8.3.1 Leyden jar
See diagram 31.1.3
1. A Leyden jar is a glass jar with metal foil on inside and outside
surfaces invented in the Netherlands' town of Leyden about 1745. It was
the early form of a capacitor and can act as a high voltage capacitor,
e.g.
2000 pF. Put aluminium foil into a 250 mL wide mouth glass jar to one
third of the height of the jar. Cover the jar externally with aluminium
foil to the same height as the internal foil. Push a large nail through
a cork.
Use pliers to twist the end of the nail to form a hook. Make a hole in
the plastic lid of the glass jar so that the cork fits tightly through
it. Fix the cork into the hole in the lid. Connect a chain of paper
clips to the hook.
Touch the head of the nail with a plastic rod rubbed with fur. Repeat
this action ten times to accumulate electric energy to the Leyden jar.
The paper clip chain carries charge from the nail head to the aluminium
foil in
the jar. The plastic lid of the glass jar is an insulator. When the jar
is fully charged, use an insulated wire to connect the nail head with
the external aluminium foil. A spark bounces out from the point of the
contact.
2. Make a grounded Leyden jar. Sparks from a Wimshurst machine are no
longer but are much more intense when a Leyden jar is connected. Charge
a capacitor with a Wimshurst machine and ground each side
separately. Make a spark to show that the charge is still there.
3. Make series and parallel condensers. Charge four Leyden jars in
parallel and discharge singly and with three together Next charge three
in series with one in parallel and discharge singly and three in
series.
Compare the length and intensity of sparks. Charge a single capacitor
two series capacitors and two parallel capacitors to the same potential
and discharge through a ballistic galvanometer.
4. Show the addition of potentials. Charge Leyden jars in parallel
and discharge charge in parallel again and connect in series before
discharging. Compare length and intensity of the sparks.
5. Show residual charge. Charge and discharge a Leyden jar. Wait a few
seconds and discharge it again. After charging a Leyden jar, light a
neon tube up to 100 times. Also show the polarity of charge on the
dielectric with a triode residual charge.
6. Use Leyden jars with a Toepler-Holtz machine. The Toepler-Holtz
machine produces weak sparks without the Leyden jars and strong less
frequent sparks with the jars connected.
31.8.3.2 Experiments with capacitors
1. Short a capacitor by charging a large electrolytic capacitor, e.g.
5600 microF, to 120 V and short with a screwdriver.
2. Explode capacitors by charging four 1000 microF capacitors to 400
V storing about 320 Joules, then short them with a metal bar.
3. Use a capacitor with a calorimeter. Discharge a capacitor into a
resistor in an aluminium block with an embedded thermistor to measure
the temperature increase.
4. Light a bulb with a capacitor. Charge a large electrolytic
capacitor, e.g. 5600 microF, to 120 V then discharge it through a light
bulb.
5. Lift weight with a capacitor. Discharge a capacitor through a small
d.c. motor to lift a weight.
6. Compare charge on a capacitor. Charge different capacitors to
different voltages and discharge through a ballistic galvanometer.
31.8.4.0 RC circuits, resistance-capacitance
circuits
31.8.4.1 Capacitor and light bulb, long RC time
constant
A large electrolytic capacitor a light bulb and a 120 V d.c. supply in
series show a long time constant. Charge and discharge a 5600 microF
capacitor through 7.5 and 40 W light bulbs. A 5600 microF capacitor, a
light bulb and a 120 V d.c. supply in series show a long time constant
where the bulb dims as the capacitor charges. Charge a capacitor with
d.c. and discharge through a light bulb then try the same thing with
A3.
Charge a capacitor through a resistor and read the voltage with a meter.
31.8.4.2 RC time constants
1. Use RC time constants with a galvanometer. Use a series RC circuit
with a galvanometer. Use a voltage follower to isolate the circuit from
the display.
2. Use RC time constant with an oscilloscope. Charge and discharge a
circuit with a slow time constant (.1 - 10 sec.) and display the
current and voltage on a dual trace storage oscilloscope.
3. Observe an RC charging curve, time constant. Use a battery on an
oscilloscope to show charging and discharging a RC circuit. Show the
time constant from an RC circuit on an oscilloscope. Show the time
constant of a RC circuit driven by the calibration signal on an
oscilloscope.
31.8.4.3 Series and parallel capacitors
Use two 2 microF capacitors in series or parallel with a 40 W lamp.
31.8.4.4 Neon relaxation oscillator, blinking
neon bulb
Use a neon bulb in parallel with a capacitor to light periodically as
the capacitor charges and discharges. Use an RC relaxation oscillator
has a neon lamp across the capacitor to provide a visible discharge.