School Science Lessons
31. Electrostatics, static electricity, induced charge, van de Graaff
generator, capacitors, electroscopes, Coulomb's law
2012-05-04c SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
31.0.0 Electrostatics, static electricity
31.8.0 Capacitance, capacitors, capacitance
and inductance
31.4.0 Conductors and insulators
31.2.0 Coulomb's law, the coulomb, pith balls
31.7.0 Electric fields and potential, Faraday's ice
pail experiment
31.1.8.1 Electrophorus
31.6.0 Electrostatic machines, van de Graaff generator,
Wimshurst induction machine
31.0.0 Electrostatics, static electricity
31.5.0 Induced charge, charging by induction
31.7.4.0 Lightning, sparks
4.39.0 Static electricity experiments,
UNESCO
31.8.0 Capacitance,
capacitors, capacitance and inductance
31.8.0.0 Capacitance, capacitors, capacitance and inductance
31.8.1.0 Capacitance, capacitors
38.4.1 Capacitors, capacitor charge
and discharge, (Electronics)
31.8.2.0
Dielectrics, conductors and insulators, electrical conductivity
31.8.4.0 RC circuits, resistance-capacitance
circuits
31.8.1.0 Capacitance, capacitors
31.8.1.5 Battery and separable capacitor
31.8.1.4 Capacitance and voltage
31.8.1.1 Capacitors (formerly condensers)
31.8.1.6 Dependence of capacitance on area
31.8.1.8 Inducing current with a capacitor
31.8.3.1 Leyden jar
31.8.1.3 Parallel plate capacitor, variable air
capacitor
31.8.1.7 Rotary capacitor
31.8.1.2 Simple spherical capacitor
31.8.1.9 Water cup spark collector, electrostatic
storage, capacitor
31.4.0 Conductors and insulators
31.4.4 Aluminium and acrylic
31.4.1 Conductor and non-conductor
31.4.5 Distribution of charge, Kolbe cone conductor
31.4.2 Test whether an object is a conductor or
insulator
31.4.3 Wire versus string
31.4.6 Superconductors, Meissner effect
31.2.0 Coulomb's law, the coulomb, pith balls
31.2.0 Coulomb's law, the coulomb, pith balls
31.2.5 Aluminium drink-cans as pith balls
31.2.4 Aluminium foil covers pith balls and ping-pong
balls
31.2.7 Balloons rubbed with wool or fur repel each
other
31.2.3 Coulomb's law balance
31.2.11 Measure charge with a balanced metre rule
31.2.9 Metal balls play music
31.2.6 Pith balls and Styrofoam balls show electric
charge
31.2.8 Separate pepper from salt and pepper, jumping
pepper
31.7.0 Electric fields and potential
31.7.0 Electric fields and potential
31.7.18 Air bubbles in oil show electric field
31.7.16 Aluminium bounces
31.7.14 Electric chimes
31.7.24 Electric fields of currents
31.7.10 Electric resin
31.7.15 Electrostatic ping-pong
31.7.2 Electrostatic potential difference, (p.d.)
31.7.19 Epsom salts on glass plate show electric
field
31.7.22 Electroscope near electrostatic machine
shows electric field
31.7.17 Fur in mineral oil shows electric fields
31.7.1 Gauss' Law
31.7.7 Hair on end
31.7.20 Ice filaments show electric field
31.7.11 Magnesium oxide smoke
31.7.23 Metal wire in candle flame shows electric
field
31.7.12 Orbiting foil
31.7.8 Pith ball plate and flying balls
31.7.4.1 Saint Elmo's fire
31.7.9 Streamers
31.6.0 Electrostatic machines, van de Graaff generator,
Wimshurst induction machine
31.6.5 Atmospheric electric field motor
31.3.0 Electrostatic meters, electroscopes
31.6.4 Simple electrostatic motor
31.6.3 Simple electrostatic generator
31.6.6 Toepler-Holtz machine
4.40 van de Graaff generator
31.6.1 Wimshurst induction machine, induction generator
31.3.0 Electrostatic meters, electroscopes
31.3.6 Balloon electroscope
31.3.9 Charged ebonite rod on electroscope
31.3.10 Chocolate wrapper electroscope
31.3.3 Drink-can electroscope
4.144.0 Electroscope, metal foil ball
electroscope, metal foil ball
electroscope
4.144.1 Gold leaf electroscope
31.3.12 Rods balanced on a pivot
31.3.8 Make an electroscope
4.144.0
Metal foil ball
electroscope
31.3.11 Oppositely charged electroscopes
31.3.4 Ping-pong ball electroscope
4.147 Pith ball indicator
4.142 Static
electricity detector
31.5.0 Induced charge, charging by induction
31.5.6 Blow soap bubbles at Van de Graaff generator
31.5.1 Charge electroscope by induction
31.5.12 Charge metal rod by induction
31.5.8 Cork attracted then repelled
31.5.4 Deflection of s compass needle
31.5.5 Drink-can attracted to charged rod
4.144.1 Gold leaf electroscope
31.5.14 Induction coil
31.5.10 Kelvin water dropper
31.5.7 Paper sticks on the board
31.5.13 Pith balls repel and attract
31.5.2 Proof plane to test charge on an object
31.5.15 Rub a magnetic pocket compass, hand-held
compass
31.5.9 Raleigh fountain
31.0.0 Electrostatics, static electricity
31.0.0 Electrostatics, static electricity
4.39.0 Static electricity experiments,
UNESCO
31.1.4.1 Aluminium balls on gramophone record
31.1.4.2 Aluminium foil precipitator
31.1.01 Amber and rubbing experiments
4.138 Attract water to a comb
31.1.6.1 Balloon charge
4.139 Balloon stays in place
31.1.6.2 Balloon sticks to a wall
31.1.7.1 Body sparks, rug scuffing
31.1.1.1 Comb attracts and repels, bent water
stream, air cleaner
31.1.1.2 Comb on a turntable
31.1.5.2 Clothes brush and flannel
31.1.17 Cellulose acetate sheet
31.1.24 Cotton thread rises
31.1.2.1 Ebonite rods and glass rods
31.01 Electric charge, conventions from effects of
friction between substances
31.02 Electric charge movement
4.50 Electrophorus, Many charges from
one source
31.1.8.1 Electrophorus, many charges
from one source
31.7.2 Electrostatic potential difference, (p.d)
31.1.02 Electrostatic series, triboelectric series,
ranking of insulators
31.03 Electrostatics and humidity, detecting electrostatic
charge, sand bath drying oven
See
pdf: Fun Fly Stick, electrostatic generator
31.1.2.2 Hard rubber rubbing
31.1.15 Mercury shaker (This experiment not allowed
in some school systems!)
31.1.19 Metal on Perspex, Volta's experiment,
voltaic pile
4.144.0
Metal foil ball
electroscope
4.145 Metal leaf electroscope
31.1.23 Neon lamp on Styrofoam
4.141 Newspaper sticks to the wall
31.1.3.1
Paper horse race
31.1.3.6 Paper loop attracted by plastic ruler
31.1.3.5 Paper shapes dance
31.1.3.2 Paper strips diverge
31.1.3.4 Paper snake strikes
31.1.3.8 Pepper photocopy
32.1.2 Piezoelectricity, Voltage produced
by mechanical stress to crystals
4.147 Pith ball indicator
31.1.5.1
Polythene bag for a nylon stocking
31.1.3.7 Puffed rice jumps
4.140 Repulsing balloons
4.142 Static electricity detector
4.39.0 Static electricity (experiments)
4.137 Static electricity from rubbing
31.137 Static electricity from rubbing
31.1.6.3
Stretched rubber band
31.1.16 Sulfur freeze
4.146 Two kinds of static charge
4.40
van de Graaff generator
31.1.25 Wool flannel lights a fluorescent light
tube
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.
3. Rub a glass rod with a silk cloth then bring it near an insulated conductor
.The silk is now negatively charged and the glass is positively charged
so the side of the insulated conductor nearest the glass rod is negatively
charged and the side of the insulated conductor away from the glass rod
is positively charged. The total charge on the insulated conductor is unaltered.
If the original charge on the insulated conductor was zero then it is still
zero. You can verify these charges with a proof plane, an aluminum-covered
conductive disc attached to an insulated handle. It is used to sample the
charge density on charged conductive surfaces. By touching the insulate
conductor at different points it will be the same potential as that point
and this can be verified with an electroscope.
31.02 Electric charge movement
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 with the following: 1. Heating
effects, 2. Lighting effects, 3. Resistance, conductors and insulators,
4. The motor effect, 5. The electrolysis process, 6. Magnetic effects, 7.
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.
31.1.01 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, wool felt cloth,
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 rod, (polymerized isoprene resin + sulfur, vulcanite, motor
car tyres), 25. Polycarbonate polymer, car battery casing, (PC, Lexan, 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 rod, 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, bent water
stream, 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. To show deflection of a water stream, 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
(Michael Faraday 1791-1867)
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.
31.141 Paper sticks to the wall
See diagram 31.141: stays on the wall
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 × 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-shaped.
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 31.9.2: Electrostatic precipitator
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 / √ (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 Balloon sticks to a wall
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. Blow up a toy balloon and rub it with a piece of fur or your clothing.
Place it against the wall and note that it stays where you place it. The
electrons collected on the rubber balloon from the fur or clothing repel
electrons in the surface of the wall leaving a positive charge on the wall
that attracts the surface of the balloon opposite the wall which pulls the
rest of the balloon with it. Repeat the experiment by rubbing the balloon
on your hair.
3. 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, rug scuffing
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.
31.1.15 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
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 de 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
× (q1q2) / r2. The Coulomb's law constant, k = 8.99 ×
109 Nm2 (newton m2/C2), about
= 1 / (4πE), where E, permittivity
of the medium = 8.85 × 10-12C2 / N.m2.
So F = q1q2 / 4πEr2.
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.
The coulomb is the practical unit of charge (C)
1 C = 6.24 × 1018 elementary charges
1 elementary charge = 1.60 × 10-19 C
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: Perspex rods | See diagram 31.2.6.2: Plastic combs
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.
3. When two pith balls, A and B, are suspended by insulated threads, charged
and suspended near each other, they approach, touch, then repel each other.
If only A was charged positively, the side of B nearer A will become negatively
charged and the other side of B will become positively charged. There will
then be a force of attraction between A and the negative charge on B, and
a repulsion between A and the positive charge on B. However, the negative
charge on B is nearer to A than the positive charge on B, so the resultant
force is an attraction between A and B and the pith balls approach each
another. When the pith balls touch, the positive charge on A is shared between
A and B, so now they have like positive charges and they repel each another.
If A was now earthed, it would become charged negatively by induction from
B, the pith balls would therefore approach each another again, and when
they touched the positive and negative charges would completely or partially
neutralize each another, depending upon whether or not they were equal.
If they were equal, the pith balls would now be both uncharged and so would
neither attract nor repel each another. If the positive charge were greater
than the negative charge the excess positive charge would be shared between
the pith balls, and they would repel each another, similarly if the
negative charge was greater than the positive charge the excess negative
charge would be shared between the pith balls, and they would repel each
another.
31.2.7 Balloons rubbed with wool or fur repel each
other
See diagram 31.2.7
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 Separate pepper from
salt and pepper, jumping pepper
See diagram 31.2.8: Separate pepper from salt
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 on a dry surface then put the mixture on a piece
of paper. Hold a plastic spoon or plastic ruler rubbed by a piece of wool
cloth or woollen sock above the mixture. The pepper jumps up out of the
mixture and sticks to the plastic spoon or ruler. Lower the plastic spoon
and the salt also jumps up. The pepper jumps up first because it is lighter.
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.
31.3.3 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 31.144
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. Rub the PVC
rod, (- ve), with the wool felt and touch the electroscope. Rub the acrylic
rod, (+ ve), with cellophane and touch the electroscope.
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 Rods balanced on a pivot
Rub both ends of a rod and balance it on a pivot. Rub a PVC rod, (- ve),
with the wool felt. Rub an acrylic rod, (+ ve), with cellophane. Rub one
end of a glass rod and hold its charged end near the end of the rod on the
pivot.
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 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.
31.4.6 Superconductors, Meissner
effect
Order online: Levitron Omega, Meissner
effect, magnetic top floats in space
An electric current can pass through a superconductor with almost no
resistance. Aluminium, mercury, lead, zinc cooled close to close to 0 Kelvin
with liquid nitrogen have superconductor properties. Some ceramics, may
be superconductors at higher temperatures. The Meissner effect is when magnets
levitate over superconductors because the magnet induces a magnetic field
in the superconductor. This effect is used in Maglev trains, e.g. in Shanghai,
and in magnetic resonance imaging, MRI.
31.5.1 Charge electroscope by induction
See diagram 31.5.11: Charging by induction
1. Charge an ebonite rod negatively by rubbing it with wool and bring
it near, but not touching, the lid of the electroscope. 1.1 Still holding
the ebonite rod in position, touch the lid of the electroscope with your
finger. 1.2 Remove your finger from the lid of the electroscope. 1.3 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.
2. 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.
3. Give a charging rod a negative charge by rubbing it with cats fur
using a flicking motion. Bring the charging rod near the cap of the electroscope
so that the leaves diverge to indicate potential. Allow electrons to flow
to earth by touching the cap of the electroscope with your finger it while
holding the charged rod is still near it. First remove the earth connection
and then the charged rod to leave a net positive charge on the electroscope.
31.5.2 Proof plane to test
charge on an object
A glass rod, charged by being rubbed with silk, is brought close to,
but not touching, an insulated conductor. The silk will be left negatively
charged and the glass positively charged, so the side of the conductor nearest
to the rod will be negatively charged, and the far side will being positively
charged. However, the total charge on the conductor will remain the same and
be zero if not previously charged. You can verify this with a small proof
plane. It is a little conducting plane mounted on an insulating handle that
can be touched on to the large conductor at different points so that it will
be at the same potential as the touched point, and have the same nature of
charge. Later you can measure the charge on the proof plane with an electroscope.
If you wanted to determine whether a metal object made of copper can
be charged by rubbing with silk the copper must held in some insulating material,
otherwise the charge produced by rubbing would pass to earth through the body
of the person holding the copper.
31.5.4 Deflection of s compass
needle
If the cover glass of a prismatic compass is rubbed in one small area
on a dry day with a handkerchief to remove some dirt, the compass needle
may be deflected towards that area of the glass, causing an error in reading.
This occurs because the region of the cover glass rubbed will become electrically
charged, and then the needle will become oppositely charged by induction
and attracted to that region. The compass needle is insulated at its pivot.
31.5.5 Drink-can attracted to charged rod
A hoop of light aluminium or even a drink-can may be attracted to a charged
rod
31.5.6 Blow soap bubbles at Van de Graaff generator
Blow 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
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.
31.5.12 Charge metal rod by induction
See diagram 31.5.12: Charge 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
1. Use a charged rod, rubbed with fur, to contact hanging pith balls.
The pith balls are repelled from the rod and from each other.
2. 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.
31.5.14 Induction coil
See diagram 31 4 14: Induction coil
An induction coil (spark coil, Ruhmkorff induction coil is a kind of transformer
where low voltage direct current in a primary coil is interrupted to induce
higher voltage in the secondary coil. The primary circuit, P, is a few
turns of thick wire, a switch, K, a rheostat (variable resistance), R, a
battery, B, a make and break (interrupter), MB, and a condenser, C, in parallel
with the make and break. The secondary circuit is many turns of thin wire
wound on the primary coil with a spark gap. The greatest distance between
the spheres of the spark gap is a measure of the induced e.m.f. An iron core,
IC, a bunch of insulated iron wires, can be placed within the primary coil.
When current is started or stopped in a conductor, an induced e.m.f. occurs
in that conductor, self-induction, with direction such as to oppose the
change (Lenz's law). So when the current starts to flow, the self-induced
e.m.f. opposes the rise in current, back e.m.f. However, when the current
is broken, the back e.m.f. tends to cause the current to keep flowing to
cause the current to falls gradually to zero instead of falling instantly.
The unit of self induction, the henry, is the self induction of a circuit
in which the back e.m.f. is 1 volt when the current alters at the rate of
1 ampere per second. Mutual self induction for two adjacent circuits is the
e.m.f induced in one circuit caused by the unit rate of change in the current
flowing though the other circuit. In the induction coil, the induced e.m.f.
is proportional to the number of turns per unit length of the primary circuit
to the number of turns in the secondary circuit, and is proportional to the
time rate of change of current in the primary circuit. When an iron core
is placed inside the primary coil the currents magnetizes the iron to increase
the induced e.m.f. in the secondary coil. The back e.m.f. can flow
across the gap in the make and break causing a spark. However, instead this
current can charge a condenser in parallel with the make and break. The charged
condenser immediately discharges to send current through the primary
circuit in a direction opposite to the original current.
Commercial
IEC Ruhmkorff induction coil, tungsten vibrating points, high quality
capacitor reduces arcing, fuse fitted to protect primary winding, secondary
winding contains thousands of turns and can be slide from the primary winding
to reduce coupling, runs on 6 to 9 volts, spark length 25 - 30 cm, removable
and adjustable needle points with insulated handles, 300 mm × 125
mm × 110 mm
31.5.15 Rub a magnetic pocket
compass, hand-held compass
Use a handkerchief to rub a small area of a magnetic pocket compass, hand-held
compass, on a dry day. The compass needle deflects towards the rubbed area
of the cover glass. The rubbed area of the cover glass has become electrically
charged by induction, the compass needle has become oppositely charged by
induction and has become attracted to the rubbed region. The compass needle
is insulated at its pivot. This experiment works only if the hand-held compass
is a "dry" compass, i.e. not filled with fluid. Instructions for using a
dry compass may include the warning not to position the thumb over the cover
glass because movement of the thumb over the glass may cause induction and
lead to errors in navigation.
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.
31.6.3 Simple electrostatic generator
Built using a hand drill LP record and fur.
31.6.4 Simple electrostatic
motor
See pdf:
Make an electrostatic motor
31.6.5 Atmospheric electric
field motor
Electret type and corona type motor for operation from the earth's electric
field.
Electret is a permanently polarized piece of dielectric material having
the same function as a permanent magnet.
31.6.6 Toepler-Holtz machine
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 equipotential 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) × (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 difference, (p.d.)
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.
31.7.7 Hair on end
1. While standing on an insulated stool charge yourself up with a Van
de Graaff generator.
2. In very cold countries wear a wool cap outside the house, enter the
house then pull off the wool cap. Your hair stands out and up. The wool in
the wool cap scrapes electrons from the outer layers of your hair so each
hair has a net positive charge. The hair strands all positively charges d
so they try to move apart. However, small hairs close to the negatively charged
skin lay down on the skin.
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 Fur 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.
31.7.22 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
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 Capacitors, capacitance, farad
Capacitors store electrical energy in the form of an electric field,
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, i.e. store electric charge = size of charge on either conductor
/ size of potential difference between conductors, or between a single
conductor and Earth. So capacitance is the ratio between the charge on
either plate of a capacitor and the potential difference between the plates.
The unit of capacitance is the farad, F, the capacitance of a capacitor
charged with one coulomb with a potential difference of one volt between
its plates, 1 coulomb/volt, 1 F = 1 CV-1 However, the farad
is too large so for most applications the microfarad, 10-6F,
is used. (Michael Faraday 1791-1867)
Capacitance, 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)
31.8.1.1 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.
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 31.8.1.3: Parallel plate capacitor
The capacitance of a parallel plate capacitor is inversely proportional
to the distance of separation.
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 1. 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 breakdown
4. Change the spacing of a charged parallel plate capacitor while it
is attached to an electroscope.
4.1 Vary the spacing of a charged parallel plate capacitor while the
voltage is measured with an electroscope field and voltage.
4.2 Charge a simple capacitor of two parallel movable plates and the
divergence of electroscope leaves varies as the plates are moved.
4.3 Charge parallel plates with a rod watch and 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. When
you 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.