Physics experiments
Please send comments to: J.Elfick@uq.edu.au
Updated: 2008-07-19
Table of contents
4.1.0 Heat as energy
4.5.0 Expansion
4.10.0 Latent heat
4.12.0 Density
4.14.0 Thermometers
4.16.0 Conduction of heat
4.24.0 Convection
4.32.0 Radiation
4.37.0 Quantities of
heat
4.39.0 Static electricity
4.51.0 Current electricity
4.67.0 Magnetism
4.78.0 Electromagnetism
4.85.0 Make waves
4.93.0 Sound
4.103.0 Producing light
4.106.0 Reflection
4.114.0 Refraction
4.132.0 Colour
4.145.0 Balances
4.147.0 Gravity
4.155.0 Inertia
4.158.0 Centipetal force
4.162.0 Force and motion
4.170.0 Machines
4.188.0 Liquid pressure
4.200.0 Buoyancy
4.212.0 Surface tension
4.223.0 Atmospheric pressure
4.1.0 Heat as energy
4.1
Temperature rise and
quantity of heat intake
4.2
Transfer kinetic energy to
heat energy
4.3 Plug and ring experiment
4.4
Expansion of a solid when heated
4.5.0 Expansion
4.5
Bimetallic
strip, compound bar
4.6
Expansion and contraction
of liquids
4.7 Expansion and
contraction of a liquid
4.8 Expansion of air
4.9 Burning candles over water
6.35 Burn candle over water (Primary)
4.10.0 Latent heat
4.10 Heat energy to change solid
to liquid,
melting point, latent heat of fusion
4.11 Heat
energy to change liquid to
vapour, boiling point, latent heat of vaporization
4.12.0 Density
4.12 Density of a solid
4.13 Density of a liquid, relative
density
4.14.0 Thermometers
4.14 Test a liquid in glass thermometer
4.15
Thermoscope to compare
absorption of radiation
4.16.0 Conduction of heat
4.16
Reduce heat loss with
insulation
4.17
Conduction of heat by metals
4.18 Solids that conduct
electricity
4.19 Liquids that conduct
electricity
4.20
Copper coil snuffer
4.21
Conduction of heat by a coin on
paper
4.22
Conduction in a metal bar
4.23
Water is a poor conductor of
heat, boil water in a paper cup
4.24.0 Convection
4.24 Convection currents in a test-tube
4.25 Convection currents in a container
4.26
Convection current from an ink bottle
4.27 Convection currents in air, convection box
4.28
Trace convection currents from a lighted
candle
4.29 Convection disc, heat snake, convection wheel
4.30
Convection currents and
ventilation
4.31
Temperature of water at
maximum density, 4oC
4.32.0 Radiation
4.32
Transfer heat by radiation
4.33
Focus radiant heat waves
4.34
Reflect radiant heat waves
4.35
Feel heat radiation
4.36
Different surfaces affect
heat radiation and absorption
4.37.0 Quantities of
heat
4.37
Heat and temperature
4.38
Calorific value of fuel
4.39.0 Static electricity
4.39
Static electricity from rubbing
4.40 van de Graaff generator
4.41
Attract water to a comb
4.42 Balloon sticks to the wall
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, electrophorus
4.51.0 Current
electricity
4.51
Electricity from two coins
4.52
Electricity from a lemon
4.53 Dry cell,
electric torch (flashlight) battery, Leclanche cell
4.54
Dry cells in an electric
circuit
4.55
Simple switch
4.56
Switches in a circuit.
4.57
Electric torch (flashlight)
4.58
Conductors and
non-conductors of electricity
4.59 Circuit board
4.60
Cells in series
4.61
Cells in parallel
4.62
Electric light bulbs in
series and parallel
4.63
Make a fuse
4.64
Use a fuse
4.65
Model electric light bulb (incandescent filament lamp)
4.66
Electric current
detector
6.37 Electric circuit (Primary)
6.38 Electricity conductors (Primary)
6.39 Electric torch - flashlight
(Primary)
4.67.0 Magnetism
4.67
Simple compass needle
4.68
Magnetic dip
4.69
Make a magnetizing coil
4.70
Freely-suspended magnet
4.71
Natural magnets
4.72
Artificial magnets
4.73
Identify magnetic substances
4.74
Magnetic poles and pin
chains
4.75
Cut an iron wire magnet
4.76
Magnetic fields in two
dimensions
4.77
Magnetic fields in three
dimensions
6.40 Hanging magnets (Primary)
2.10 Magnetic pin chain (Primary)
4.78.0 Electromagnetism
4.78 Cylindrical electromagnet
4.79
Horseshoe electromagnet
4.80
Test the strength of
electromagnets
4.81
Magnetic field from electric
current in a wire
4.82
Magnetic field inside an open
coil, open solenoid
4.83
Electricity from a magnet
and a coil
4.84
Make a simple electric motor
6.41 Make electromagnets (Primary)
4.1 Temperature rise
and quantity of heat intake
See diagram 4.1: Temperature rise
and quantity of heat intake
1. Put a large iron bolt and a nut for the bolt in a container of
boiling
water to bring them to the same temperature. Put equal volumes of water
in two containers with each volume enough to immerse the bolt. Put the
hot bolt in one container and the hot nut in the other container.
Record the temperature of the water in each container after the same
period. The difference in temperature change of the water in the two
containers is because of the different amounts of heat stored in the
iron
bolt and the iron nut.
2. Check if your temperature sense is reliable. Use containers of hot
water, warm water and cold water. Put both hands in the warm water. The
hands feel the same temperature. Put one hand in the hot water and the
other hand in the cold water. Quickly dry your hands and put them both
into the warm water again. The two hands do not feel the same
temperature.
Is your temperature sense reliable? This may be a silly experiment but
it shows that our temperature sense is not always reliable.
4.2 Transfer kinetic energy to heat energy
See diagram 4.2: Transfer kinetic energy to heat
energy
Use a small piece of lead sheet wrapped around one end of a piece of
thin iron wire. Hold the other end of the wire. Hit the lead several
times with a hammer. Feel the temperature rise as heat moves along the
wire towards your hand.
4.3 Plug and ring experiment
See diagram 4.3: Plug and ring experiment
Use a large metal screw and a screw-eye through which the head of
the screw just passes. Alternatively, use a metal ball that just
passes through a metal ring, or a bar that will just pass through a
gauge. Attach the screw and screw eye into the ends of a stick. Hold
the stick to heat the head of the screw in a burner flame. Try to pass
the screw through the screw eye. The screw cannot pass because of
expansion because of heating. Keep the screw hot and heat the screw eye
in
the flame simultaneously. Now the screw head can pass through the screw
eye. Keep the screw head in the flame and cool the screw eye in cold
water. The screw head cannot pass through the screw eye. When you
cannot
open a jar with a metal screw top, hold the jar upside down so that the
metal screw top is touching hot water. The metal screw top expands and
you can open the jar.
4.4 Expansion of a solid when heated
See diagram 4.4: Expansion of a solid when heated
Use a 2-m piece of stout copper tubing. Put it on a table and fix one
end by a clamp. Underneath the other end put a bicycle spoke to act as
a roller. A drinking straw fixed to the roller by wax will show any
movement of
the rod resting on it. Blow steadily down the tube between the fixed
end and the middle. This arrangement detects the expansion of the tube
caused by the hot breath. Pass steam through the tube, and note the
motion of the pointer. Repeat the experiment with different types of
tubing.
4.5 Bimetallic strip,
compound bar
See diagram 4.5: Bimetallic strip
Strips of dissimilar metals bonded together bend when heated. Heat
a bimetallic strip of brass and steel in a Bunsen burner flame. Mount a
pointer on the end of a bimetallic strip. Use two 25 cm strips of brass
and invar steel welded together as a bimetallic strip.
A pair of iron and brass strips rivetted together bends when
heated
because of the difference of expansion of the two metals. Make the
holes
with a nail and fix small tacks as rivets. Another way of fastening the
strips together is to cut them with projections at equal intervals and
bend the projections over to interlock. Bimetallic strips switch
thermostats
on or off.
4.6 Expansion and contraction of liquids
See diagram 4.6: Expansion and contraction of
liquids 1
1. Fit a flask with a
one-hole stopper and a 30 cm length of glass tubing that extends into
the flask. Add coloured water to the flask so that it extends 5 cm up
the glass tubing. Slowly heat the flask while carefully watching the
level of coloured water in the glass tubing. When you heat the flask,
the water level initially falls as
the glass in the flask expands then rises as the water expands. Cool
the
flask under the tap. The level of coloured water in the glass tubing
first rises as the glass in the flask contracts then drops as the
coloured water cools and contracts. So the expansion of
liquid you see
in a thermometer is really the expansion of liquid less the expansion
of
the glass tube.
2. Use two identical small bottles fitted with one-hole stoppers and
glass tubing passing though into the bottles. Fill the bottles with
different
liquids. Put the bottles in a container of hot water. The different
rise of liquids inside the glass tubing shows the difference in
expansion of
the different liquids.
4.7 Expansion and
contraction of a liquid
See diagram 4.7: Expansion and contraction of
liquids 2
Place some coloured water in a flask. Insert a one-hole stopper and
glass tube so that it extends downward into the fluid and upward. Pour
warm water over the flask and the coloured water rises in the tube.
Pour cold water and the
coloured water drops inside the tube.
4.8 Expansion of air
See diagram 4.8: Expansion of air
1. Use a flask fitted with a one-hole stopper and glass tube that
extends into the flask. Put a small amount of oil in the glass tube to
trap air in the flask. Hold the flask in your hands. The oil moves up
the tube because the heat from your hands causes the trapped air to
expand. If you look carefully note that the oil first moves down
because the heat from your hands first causes the glass of the flask to
expand. When you cool the flask under the tap, the oil moves down.
2. Fit a hard-glass test-tube with a one-hole stopper that has a length
of glass tubing through it. Invert the test-tube so that the end of the
tubing is in a container of water. Clamp the test-tube in an inverted
position so that you can heat it with a burner. Heat the test-tube and
note the bubbles from the end of the tube in the container of water.
Heat has caused the air to expand. Cool the test-tube by pouring cold
water over it. Water moves up the glass tubing as the cooling air
contracts.
3. Fit a toy balloon over the neck of a small flask. Put the flask
in a container of water. Heat the water. The balloon expands as the
heated air in the flask expands. Partially inflate a balloon and tie
the neck tightly. Leave it in
a warm place or in the sunlight. The balloon becomes fully inflated as
the air inside expands when heated.
4.9 Burning
candles over water
See diagram 4.9: Burning candles
Attach a tall candle and
a short candle to the bottom of a trough.
Add water to the trough and note the water level. Light both candles.
Put a large jar upside down over the candles. The tall candle
extinguishes first then the short candle. Hot gas products of
combustion including carbon dioxide gas have filled the jar from the
top down to extinguish the candle flames. Some hot gases push out
under the rim of the jar to form bubbles around the jar in the trough.
When the
candles are extinguished, the hot gases cool and contract to form a
partial vacuum and the water level rises inside the jar.
Some decreae in volume will becaused by the candle wax burning to
form carbon dioxide and water. Som eof the carbon dioxde will dissolves
in the water from the trough and the water vapor formed will condense
to form liquid water.
4.10 Heat energy to change solid to liquid,
melting point, latent heat of fusion
See diagram 4.10: Liquid naphthalene solidifies
Put crushed naphthalene or ethanamide (acetamide) in a test-tube in a
container of water. Heat gently until all the substance has melted.
Remove the test-tube from the container and fix a thermometer with its
bulb in the melted substance. Stir the substance with a thermometer
while the substance cools and record the temperature every 30 seconds
for 6 minutes. Plot a graph of temperature against time. At first the
temperature drops while the substance remains liquid. Then the
temperature remains the same while the substance changes from liquid to
solid. When all the substance is solid, the temperature starts to drop
again. The melting point, m.p., is the temperature when a solid changes
to a liquid. The specific latent heat of fusion of a substance, L, is
the quantity of heat required to change one kilogram of the substance
from solid to liquid without change in temperature. The unit is joule /
kg, J kg-1. The specific latent heat of fusion of ice = 3.34
X 105 joule / kg, 334 kJ kg-1.
4.11 Heat energy to change liquid to
vapour, boiling point, latent heat of vaporization
See diagram 4.11 Heat required to vaporize a
liquid
1. Weigh a container, add 50 mL water and weigh again. Heat the
container and water. Put a thermometer in the water and record the rise
in temperature every 30 seconds. Plot a temperature against time graph.
Draw the line of best fit and calculate the average temperature
increase per minute. Assume that all the heat goes into the liquid and
the heat absorbed by the flask is small. Calculate the heat absorbed by
the liquid per minute by multiplying the mass of the liquid by its
specific heat and by the temperature increase per minute.
2. Weigh a container, add 50 mL water and weigh again. Heat the
container and water and allow to boil for 10 minutes. Leave to cool
then weigh the container and water. Calculate the mass of water lost by
evaporation. This will be the heat of vaporization of the liquid.
3. Put a known mass of water in a boiling flask and a known mass of
water in a container. Record the temperature of the water. Heat the
boiling flask and pass all the steam into the water in the container so
that all the steam condenses to water. When most of the water in the
flask has evaporated, stop heating and record the temperature of the
water in the container. Leave the apparatus to cool to room
temperature, weigh the water remaining in the flask and the water in
the
container. The condensing steam loses latent heat of fusion when it
condenses and loses heat when its temperature (100oC) falls
to the temperature of the water in the container. The specific latent
heat of vaporization of water is 4.26 MJ kg-1.
4.12 Density of a solid
The density of a solid is the ratio of mass to
volume (mass per unit
volume). Use a balance to measure the mass. If the solid is insoluble
in water, measure the volume by displacement of water. Half fill a
graduated cylinder with water. Note the reading. Immerse the solid in
the water and note the reading again. The volume of the solid is the
difference in the two readings. Examples of the densities of elements,
in g cm-3, are as follows: aluminium: 4.70, carbon
(graphite): 4.25, carbon (diamond): 3.51, copper: 8.92, gold: 19.30,
helium: 0.147, hydrogen: 0.070, iron: 7.86. lead: 11.30, magnesium:
1.74, mercury: 13.60, nickel: 8.90, platinum: 21.40, silver: 10.50,
uranium: 19.10, zinc 7.14. In SI units, measure density in kg m-3,
e.g. density of dry air at sea level = 1.29 kg / m3.
Measure the density of examples of different metals then decide whether
they are pure substances.
4.13 Density of a liquid, relative density
Weigh a small container with the liquid inside. Pour the liquid into a
graduated cylinder to find the volume of the liquid. Use a balance to
find the mass of the container and the mass of liquid transferred to
the measuring cylinder. Obtain the density by dividing the mass of the
liquid by the volume. The density of water is close to 1 g per cc, cm3,
so you can compare the density of substance with the density of water
as relative density. Relative density (formerly specific gravity), is
the ratio of mass of a volume of a substance to the mass of an equal
volume
of water, at 4oC. Relative density
has no units because it is a ratio, e.g. petrol r.d. 0.70, ethanol r.d.
0.79, ice r.d. 0.90, olive oil r.d. 0.92, water r.d. 1.00, sea
water r.d. 1.03, glass r.d. 4.50, mercury r.d. 13.60, gold r.d. 19.30.
A special
bottle, a density bottle, gives an accurate measure of relative
density. Let mass of empty density bottle = A, mass of bottle + liquid
= B,
mass
of liquid = B - A, mass of bottle + water = C, and mass of water = C -
A.
Relative density = B - A / C - A. Use a small bottle to measure the
density of different liquids. A more convenient way to measure
the density of a liquid is to use a hydrometer.
Find the density of a cola drink in an aluminium drink can. Weigh the
full aluminium can. Open the aluminium can and drink the cola. The
weight of the aluminium can is approximately 13 g. The volume of the
cola is written on the side of the aluminium can, e.g. 375 mL or 355 mL
(12 oz). Calculate the density of the cola. Weight of aluminium
can - 13 g / volume of cola. Repeat the experiment with "diet"
cola where sugar is substituted by a chemical sweetener, e.g.
phenylalanine, aspartame.
4.14 Test a liquid in glass thermometer
Use a thermometer with a scale, e.g. a mercury or alcohol thermometer,
-10oC
to
110oC.
Also, use a tall flask containing
coloured
water fitted with a one-hole stopper and glass tube extending into
the
bottle. Attach a blank scale to the glass tube. A thermometer scale has
two fixed points, the lower fixed point and the upper fixed point.
Put the bulb of a thermometer
in crushed ice that is melting. Check
that the temperature is 0oC on the calibrated thermometer.
Mark the lower fixed
point on the blank scale.
Put a
thermometer in steam immediately above the surface of boiling water.
Check that the temperature is 100oC
on the calibrated thermometer. Mark the lower fixed point on the blank
scale. Divide the distance
between the upper and lower fixed point to obtain 100 marks
representing a
temperature difference of 1oC. If you do the experiment on a
mountain at a high altitude, the temperature of boiling water will be
below
100oC because of the reduced atmospheric pressure. If you do
the experiment in a submerged submarine, the temperature of boiling
water
may be above 100oC because of the increased pressure with
depth. The thermometer in the boiling water
reads
exactly 100oC only at sea level or where the barometer
reading
is 760 mm of mercury.
4.15 Thermoscope to compare absorption of
radiation
See diagram 4.15: A simple thermoscope
Experiment with different materials before doing this experiment
because for most cloths the absorption of infrared is almost
independent of colour. The amount of surface area pointing towards the
source is also a variable. Use two identical clear plastic bottles. Put
a dark coloured piece of cloth or plastic in one bottle. Put an
identical amount of white cloth or shiny metal foil in the other
bottle. Fit the bottles with one-hole stoppers with 20 cm of glass
tubing. Into each glass tube introduce a bead of water or oil. Place
each bottle in the sun, or about 50 cm from a bright light bulb or 1
metre from a fire or 20 cm from a burning lamp or candle. Note the rate
at which the beads of water or oil rise in the tubes.
4.16 Reduce heat loss with insulation
Use four large tin cans of equal size and four smaller tin cans of
equal size. Inside the first large can put a small can on two corks in
a large can. This is the control. Select types of insulating material,
e.g. sawdust, cork pieces, newspaper, plastic. Put a small can inside
each large can. Pack one type of insulating material under and around
each of the smaller cans. Put a cardboard cover on each large can. Make
a hole in each cover for a thermometer. Fill each small can to the same
depth with water that is nearly boiling. Record the initial temperature
of the water in each can. Record the temperature of the water in each
can at five minute intervals. Draw cooling curve graphs by plotting
temperature against time for each can. Note which material is the best
insulator.
4.17 Conduction of heat by metals
See diagram 4.17: A model Davy lamp
1. Hold a wire coat hanger horizontally over a flame with your
fingers, a small distance from directly above the flame. Soon the wire
becomes too hot to hold. Move your fingers back but keep the coat
hanger in the same position. Feel heat moving along the wire.
2. Use identical lengths of different metal bars or rods with the
same diameter, e.g. copper, brass, aluminium, iron. Put blobs of melted
candle wax at the same intervals along the bars. Push small nails or
metal pieces into the wax while the wax blobs are still soft. Heat one
end of each metal bar. The blobs of wax melt and the nails fall down as
heat moves along the bar. The metals do not conduct heat equally.
3. Hold a sieve or a piece of metal gauze, e.g. 1 mm iron gauze or
metal fly-wire screen, over the flame of a small candle. (Some fly-wire
screens consist of fibreglass or plastic so do not use this type of
screen.) As you lower the wire gauze, the flame gets smaller. The flame
does not go through the wire netting. The flame becomes smaller because
the wire conducts the heat away from the flame so the temperature is
lowered. Sir Humphry Davy used this observation to invent the miners'
safety lamp. Metal gauze around the flame in the lamp conducts away the
heat so that the flame cannot ignite explosive gas in the coal mine.
4. Put an unlit burner under a tripod stand and cover it with 1 mm
iron gauze. Turn on the gas and ignite it above the metal gauze. The
gas burns only above the wire gauze screen because it conducts away the
heat and prevents the gas below it from reaching ignition temperature.
5. Hold a piece of paper above a candle flame. The paper chars. Put a
metal coin or a key on the paper and hold it over the candle flame. The
metal conducts the heat away from the paper and leaves a pattern where
the metal touches the paper.
4.18 Solids that conduct electricity
1. Use a 6 V dry cell or lead cell
accumulator and a 1.5 V light
bulb. Fix electrodes from old 6 V dry cells in a cork to keep
them at a constant distance apart. Test the conductivity of solids by
making a good contact between the surfaces of the test solid and the
two electrodes. Test metals and non-metals, e.g. scissors, nails,
plastic, paper, naphthalene, wax, sugar, sodium chloride, and water.
Record
which substances are conductors and non-conductors, insulators.
2. Test conductivity of glass
Test the conductivity of a glass rod at room
temperature. Heat the
glass rod until it becomes very hot and begins to soften. Test the hot
soft part with the conductivity apparatus. Molten glass can be a good
conductor of electricity.
4.19 Liquids that conduct
electricity
1. Test melted substances. If you heat the following substances, heat
very gently and cautiously because they may ignite and burn: sulfur,
wax, naphthalene, polyethylene, tin, lead, a low melting point salt,
e.g. lead bromide, m.p. 488oC, or potassium iodide, m.p. 682oC.
To test the conductivity of the melt, dip the electrodes in the
melt and wait for the electrodes to reach the same temperature as the
melt. Make sure that the electrodes are in contact with the liquid melt
and not the solidified melt. Scrape and clean the electrodes between
each test.
2. Test methylated spirit, acetone, vinegar, sugar
solution, copper (II) sulfate solution, sodium chloride solution, and
other substances dissolved in water. Clean and dry the electrodes
between each test.
3. Test demineralized water. Put the electrodes
into a container of deionized water. The light bulb does not light.
Slowly add small crystals of sodium chloride to the demineralized
water. Observe the light bulb as the salt dissolves.
4. Test tap water. Note whether you get the same result as for
deionized water.
4.20 Copper coil snuffer
See diagram 4.20: Snuffing out a candle flame
with a copper coil
Place a coil of heavy copper or aluminium wire over the flame of a
small size candle. Why does the flame go out? You can snuff out a
candle flame by depriving it of oxygen but here the oxygen can easily
get to the flame. The fire goes out because the wire conducts the heat
away from the flame so fast that the temperature is lowered below the
kindling point. This shows that copper and aluminium are good
conductors of heat. If the flame is too large, it will produce heat
energy too rapidly to be carried away by the coil. If the coil is
already hot before the experiment, the temperature of the flame may not
be lowered enough to put it out.
4.21 Conduction of heat by a coin on paper
Hold a piece of paper above a candle flame: it will char if brought
near. Place a metal coin on the paper and repeat the experiment: the
metal will conduct the heat away and leave a pattern on the
paper.
4.22 Conduction in a metal bar
Use a bar of copper, brass or aluminium at least 30 cm long. Place
blobs of melted paraffin wax at 3 cm intervals. While the paraffin
blobs are still soft, push the pointed ends of nails or tacks into
them. Heat one end of the box with a flame. Note the evidence that heat
moves along the bar by conduction.
4.23 Water is a poor conductor of heat, boil
water in a paper cup
1. Use your bare fingers to hold the bottom of a test-tube containing
water. Tilt the test-tube over a flame so that you can heat the water
in the upper part of the test-tube. You can hold the bottom of the
test-tube until the water in the upper part boils because water is a
poor conductor of heat.
2. Boil water in a paper cup. Pour some water in a paper cup and hold
it over a flame.
3. Put small pieces of ice in the bottom of a test-tube containing
water. Heat the water near the top of the test-tube with a spirit
burner. The water will start to boil, yet the ice will not melt. The
warmed water is already at the top, so no convection takes places, and
the conduction by water is very small. Little heat transfers to the ice.
4. Put a small fish in a test-tube full of water. Tilt the test-tube
and heat the top 1 cm of water. The water will boil and not harm the
fish. However, this experiment upsets some students.
4.24 Convection currents in a test-tube
Fill a test-tube with cold water. When the water is still, add a very
small crystal of potassium manganate(VII) and let it fall to the bottom
leaving little colour trace. Hold the test-tube in the bare fingers
near the top but not above water level. Heat with a very small burner
or candle flame at the bottom of the tube. You can hold the warm
test-tube with bare fingers. Note the movement of the coloured dye from
the crystal in the convection current. Repeat but heat very gently near
the top of the water surface, while holding the test-tube near the
bottom.
4.25 Convection currents in a container
Weigh an empty container. Fill a container exactly with cold water and
weigh it again. Empty the container and fill it exactly again with the
same volume of hot water and weigh it again. The same volume of hot
water weighs less than cold water. When you heat water the lighter warm
water displaces the heavier cold water and convection currents occur.
Hot water is less dense than cold water. This is the cause of
convection currents.
4.26 Convection current from an ink bottle
See diagram 4.26: Convection currents in water
Use a small ink bottle, fitted with a two-holes stopper. Cut two pieces
of
glass tubing. One piece should extend from the stopper almost to the
bottom of the bottle. The other piece should extend 5 cm up from the
stopper. Fill a large container with cold water. Fill the small bottle
with hot coloured water. Put the small bottle in the bottom of the
large container while holding the fingers over the ends of the tubing.
The hot coloured water rises in the large container as the cold water
enters the bottle.
4.27 Convection currents in air, convection box
To make a convection box, cut away one side of a box
and replace it with glass. Cut two holes 2 cm diameter and 10 cm apart
in the top of the box. Attach two tubes above the holes to be chimneys.
Put a candle in the box under one chimney. Light the candle. Hold the
smoking paper above each chimney. See the convection currents through
the glass side of the box.
4.28 Trace convection currents from a lighted
candle
See diagram 4.28: Air currents from a lighted
candle
Hang a T-shape piece of
cardboard from
the rim of a large container. The stem of the T-shape should reach half
way down the container. Use a wire loop to lower a lighted candle into
one side of the container. Use smoking paper to find the convection
currents in the container. Use smoking paper to trace the air currents
around a candle, in a room heated with a stove, at different
levels above the floor, with windows open at the top and bottom, in a
doorway between a warm and cold room.
4.29 Convection disc, heat snake, convection wheel
See diagram 4.29:
Convection disc, heat snake
1. Use a disc of thin tin from the end of a cylindrical metal can. Cut
four blades all round the disc and pivot it on a bent knitting needle.
Hold the disc above a candle flame, and it will revolve rapidly. A
paper spiral supported on a knitting needle will revolve in a similar
way.
1. Use a disc of tin cut from the top of a
metal can. Make four radial cuts and bend the tin to form four
propeller
blades. Balance the disc on the end of a bent wire. Hold the disc above
a candle flame. The disc revolves as rising air hits the blades.
2. Make a
more sensitive convection wheel from the metal foil top of a milk
bottle.
3. Cut paper into a spiral. The centre of the spiral is like the
head of a snake. Support the head of the snake on a wire over a candle.
The heat snake turns around the candle.
4.Look at an object on the other side of a
hot engine or a hot road. The object will appear distorted because the
refractive indexes of warm and cold air are different. This is one
cause of mirages in the desert.
4.30 Convection currents and ventilation
See diagram 4.30: Convection current ventilation
Use a box with grooves for a lid and cut a glass window that slides in
the grooves to make an airtight fit. Bore four holes in each end. Each
end represents a window. The top holes of each side are the top halves
of each window. Put four candles in the box, light them and close the
sliding glass. To study the best conditions for ventilation, put solid
corks in the openings, close completely both windows, and note the
candles.
Try the followingdifferent combinations of opening:
1. one window open at
the top and bottom, i.e. all four holes in one side open,
2. one
window open at the top and the other at the bottom,
3. both windows
open at the top, one window open at the bottom,
4. both windows open
at the bottom, one window open at the top. Find which window openings
provide the best ventilation.
4.31 Temperature of
water at maximum density, 4oC
1. Fill a bottle with water and put the top on securely. Wrap the
bottle in a cloth, to prevent the shattered glass from falling. Put the
bottle into the freezing compartment of a refrigerator. After 24 hours,
remove the bottle and examine it. The bottle may be cracked because of
pressure from the expanding ice.
2. Put a large piece of ice into a glass of
water. Arrange two
thermometers so that they measure the temperatures near the top and the
bottom of the water. The water cooled by the ice falls to the bottom.
This fall continues until the water at the bottom of the glass reaches
a temperature of 4oC. The water stays at this temperature
for a long time, the colder water remaining higher up near the ice. So
water at 4oC is denser than the water at 0oC, so
a pond freezes from the surface downwards while the bottom seldom falls
below 4oC.
3. To study the expansion of freezing water,
use two identical
drinking cups. Fill the first cup with tap water at room temperature so
that the water heaps up to form a meniscus. Put the second cup in the
freezing compartment of the refrigerator then add extra water to the
cup to get the highest possible meniscus. When the water in the cup is
frozen, compare the meniscus of the frozen water with the meniscus at
room temperature. The frozen water heaped up because it had expanded.
Water has a maximum density at 4oC. When water cools from
room temperature to 4oC, it contracts in volume. When water
cools from 4oC to 0oC, it expands in volume. At 4oC
the density of water is 1000 kg m-3 (1 g per cc). At 0oC
the density of water is 999.87 kg m-3 and the density of
ice is 918 kg m-3, so ice floats on water.
4.32 Transfer heat by radiation
Hold the palm of your hand very close to, but not touching, your cheek.
Feel the radiation from your hand. Heat travels by radiation almost
instantaneously. Hold your hand under an unlighted electric light bulb,
the palm upward. Turn on the electricity and feel the heat from the
light bulb. The heat could not reach your hand so quickly by conduction
because air is a very poor conductor of heat. The heat could not reach
your hand by convection because convection carries the heat upward and
away from your hand. The heat came to your hand carried by short
electromagnetic waves of wavelengths longer than light. Radiation
carries heat in every direction from the source. Put a piece of glass
between a light bulb and your hand to block any movement of air. Feel
the radiated heat.
4.33 Focus radiant heat waves
Use a magnifying glass to focus the rays of the sun on a piece of paper
tissue. The paper chars and catches fire. Repeat the experiment with
paper tissue soaked in black ink. The black paper catches fire sooner
than the white paper. Repeat by focussing the sun's rays on your arm. A
bright spot forms and you can feel the hot spot. Note the distance of
the lens from your arm when the spot is smallest and brightest. This
distance is the focal length of the lens. Notice the distance when the
spot feels hottest. The two distances are different.
4.34 Reflect radiant heat waves
Heat tissue paper with a magnifying glass. Note the distance from the
reading glass to the tissue paper. Put a tilted mirror half way between
the lens and the paper. Feel with your hand above the mirror until you
find the point where the heat waves are focussed. Hold a piece of paper
tissue at this point. The paper ignites.
4.35 Feel heat radiation
1. Stand near an open window to feel the radiation from the sun on
your cheek. Close the window. You can still feel the radiation from the
sun on your cheek.
2. Hold your cheek 25 cm from a hole in a wooden sheet placed in
front of a heating element. Feel the radiation on your cheek. Put a
piece of glass between your cheek and the hole. Feel the radiation on
your cheek. Repeat the experiment using more sheets of glass.
4.36 Different surfaces affect heat radiation
and absorption
See diagram 4.36: Heat radiation
and absorption
1. Use three same size metal drink-cans. Paint the first can white.
Paint the second can black. Let the third metal drink-can remain shiny.
Fill the cans to the same level with warm water at the same
temperature. Record the initial temperature. Put cardboard covers with
holes for thermometers over each can. Put cans in a cool place. Record
the temperature of the water in each can at five minute intervals.
Describe the difference in the rate of cooling. The second can cools
fastest because a black surface is the best radiator of heat.
2. Fill the same metal drink-cans with very cold water and record the
initial temperature. Put cardboard covers with holes for thermometers
over each can. Put the cans in a warm place in the sun. Record the
temperature of the water at five minute intervals. The black metal
drink-can is the best absorber of heat.
3. Use a pair of old shoes. Paint the left
shoe black and the right
shoe white. Your left foot becomes your hot foot.
4.37 Heat and temperature
The joule, J, is the SI unit of work and
energy. A joule is equal to the amount of work done when the point of
application
of a force of one newton moves one metre in the direction of the force.
The c.g.s. unit, the
calorie,
is the amount of heat required to raise the temperature of 1 gram of
water
by 1oC at 15oC
(room temperature). Nowadays the SI unit the joule, J,
is
used. 1 calorie (cal) = 4.184 J, commonly, 4.2 joules. You may see
"kilocalories", 1000 calories, in
nutritional information about weight loss. In some "calorie counter"
books, 1000 calories
is a "Calorie", so in their tables 1 "Calorie" = 4.2 kilojoules.
Suspend a metal can containing 50 mL water and a thermometer over a
small Bunsen burner flame or a candle. Record the initial temperature.
Heat it for two minutes, constantly stirring, and record the final
temperature in degrees Celsius, oC. Empty the water and
repeat the experiment with 100, 150, 200 mL water, using the same
flame. Assume 1 mL (1 cm3) water = 1g. Find the product of
mass of water X by rise in temperature. As the same heat is given out
by the flame to each mass of water (100, 150, 200 mL), a convenient
unit of amount of heat would be the amount of heat absorbed by 1 g
water rising in temperature by 1oC. This unit is the
calorie.
4.38 Calorific value of fuel
‘Calorific value’ could refer to the number of joules of energy
released when 1 g of a fuel burns completely. A 1oC change
in temperature of 1 mL of water requires 4.2 J. Hang a small metal can
from a stand. Pour 100 mL of cold water into the can. Record the
initial
temperature, t1. Put a small piece of candle on a tin lid
and weigh
them, w1. Put the candle and tin lid under the can of water.
Light
the candle. Stir the water with a thermometer as the temperature rises.
When
the temperature reaches 60oC, t2, blow out the
flame.
Weigh the tin lid and candle again, w2. The calorific value
of the
fuel = 100 X 4.2 X (t2- t1) / (w2- w1).
However, the calorific value of fuels is
usually expressed in megajoules per kilogram, MJ kg-1, e.g.
petrol
45, natural gas 40, coal 35, ethanol 30, dry wood 15.
Nutritional information usually expresses
calorific value in kilojoules per gram, kJ g-1, e.g. fat 40,
cheese
30, sugar 16, potatoes 5.
4.39 Static electricity from rubbing
See diagram 4.39: Obtaining
electricity by rubbing things together
Make circular pieces of paper with a hole puncher or make a pile of
cork particles by filing or cutting a cork. Rub a plastic comb or
plastic rule or plastic ball pen case with a woollen jumper or your dry
hair, or rabbit fur or flannel or silk. Note which rubbing attracts the
most circular pieces of paper or pieces of cork.
4.40 van de Graaff
generator
It has an endless belt made of insulating material, e.g. rubber or
plastic, that is pulled over a Perspex roller by an electric motor. The
upper end of the endless belt is inside a large metal dome. The
moving belt forced charges onto the dome so that it gets to a very high
voltage.
1. Attach a wire to a needle. Touch the other end of the wire to
the metal dome and point the needle at a candle flame. The
flame appears to be blown away by a wind.
2. Bring a small metal sphere near the metal dome and note the
"lightning" spark.
3. Touch the metal dome and note your hair standing on end.
4.41 Attract water to a comb
1. Turn on a tap so
that a thin continuous stream of water flows. Charge a
comb by combing your hair several times. The friction between you hair
and the comb cause excess electrons on the comb so it becomes
negatively charged. Hold the comb near the stream
of water where it comes out of the tap. The comb attracts the water
because of the negative electrical charges
on the comb attract positive charges in the water molecules. The
charges on the water molecules inside the water stream neutralize each
other but the charges on the surface of the stream opposite the comb
cannot be neutralized so the side of the stream opposite the comb is
attracted to the comb pulling the rest of the stream along with it.
2. Repeat the experiment using "Golden Syrup" or treacle or thin honey
instead of water.
4.42 Balloon sticks to the wall
1. 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.
2. Repeat the
experiment by rubbing the balloon on your hair.
4.43 Repulsing balloons
Blow up two balloons and tie with strings one metre long. Rub each
balloon with fur. Hold the strings together and note how they repel.
Put your hand between them and note what happens. Bring one balloon
near your face. Repeat, using three balloons.
4.44 Newspaper stays on the wall
See diagram 4.44: The newspaper stays on the
wall
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, 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.
4.45 Static electricity detector
See diagram 4.45: A static electricity detector
Cut a strip 2 cm by 10 cm from thin cardboard. Fold it in half
lengthways and balance it on a pencil point. The pencil point should
indent but not perforate the paper, so that the paper can turn easily.
Charge a comb by rubbing on hair or wool and hold it near one end of
this detector.
4.46 Pith ball indicator
Use the white pith from inside a plant stem. Dry the pith thoroughly
and then press it tightly into small balls 5 mm in diameter. Coat the
pith balls with aluminium powder in egg white, colloidal graphite or
metal paint. Attach each pith ball to a silk thread or fishing line 15
cm in length. Bring objects rubbed with silk, fur or flannel near the
pith ball and note how it behaves. This equipment is an electroscope.
In place of pith balls, use grains of puffed wheat, puffed rice,
expanded polystyrene, Styrofoam balls, ping-pong balls, or any light
object.
4.47 Metal foil ball electroscope
See diagram 4.47: Metal foil ball electroscope
Roll metal aluminium foil from a chocolate packet into a ball. Use
adhesive tape to attach a piece of thread to the ball. Tie the free end
of the thread to a plastic ball pen sleeve. Place the ball pen sleeve
across the mouth of a container so that the ball of foil hangs in the
centre of the container, clear of the sides. Bring a charged body near
the metal ball. At first the charged body attracts the ball then the
ball jumps away. Rub another ball pen sleeve on a plastic protractor.
Hold the pen near the ball and let it take a charge. Bring the
protractor near the charged ball.
4.48 Metal leaf electroscope
Use the equipment as in 4.144 but instead of a ball of metal foil
attached to a thread, hang a folded piece of tissue paper or strip of
aluminium foil over the ball pen sleeve so that they do not touch the
sides of the container. Bring a charged body near the ball pen sleeve.
The leaves of the paper fly apart because they have the same kind of
charge.
4.49 Two kinds of static charge
See diagram 4.49.1: Positive and negative
charges attract each another. Negative charges repel each another. | See
diagram 4.49.2: Use an uncharged pith ball electroscope. | See diagram 4.49.3: Use a charged pith ball
electroscope.
The basic observations of electrostatics are as follows:
Observation 1. Rub a
plastic comb with fur. The plastic comb becomes -ve and the
fur becomes +ve.
Observation 2. Rub a glass rod with silk. The glass rod becomes
+ve and silk
becomes -ve.
1. Like static charges repel each other and unlike charges attract
each other. Make a turntable by driving a long nail through a wood
base. Push a test-tube into a hole made in a large flat cork. File the
end of the nail to a sharp point and invert the test-tube over it. Set
pins in the top surface of the cork, they brace the objects you put on
the turntable. Use two test-tubes or glass rods, a piece of silk, two
plastic combs, an ebonite rod, some wool, and a piece of fur or
flannel. Rub a comb with fur and set it on the turntable. Rub the other
comb with fur and bring it near the comb on the turntable.
2. Rub a glass rod with silk and put it on the turntable. Again rub a
comb with fur and bring it near the glass rod. Repeat until you are
sure of your observations. When you rub the comb with fur, the plastic
takes a negative charge of electricity and the fur takes a positive
charge. When you rub glass with silk, the glass takes a positive charge
and the silk a negative charge.
3. Rub an ebonite rod with a piece of wool and bring the rod near an
uncharged pith ball electroscope. Note that the pith ball is first
attracted and then repelled.
4. Rub a glass rod with a piece of silk and bring the rod near an
uncharged pith ball electroscope. The pith ball is at first attracted
to the glass rod and then repelled.
5. Charge a pith ball negatively by touching it with an ebonite rod
rubbed with wool. When you bring a negatively charged plastic comb near
the negatively charged pith ball, they repel each other. When you bring
a positively charged glass near the negatively charged pith ball, they
attract each other.
4.50 Many charges from one source, electrophorus
See diagram 4.50: Many charges from one source,
electrophorus
1. Use a flat bottom aluminium foil pie dish. Push a drawing pin up
through the centre of the pie dish. Press a rubber eraser down onto the
upturned point of the drawing pin. Rub the bottom of a flat polystyrene
dish with wool. Hold onto the end of the eraser and use it as a handle
to lift up the pie dish then place it in the poylystryrene bowl. The
head of the drawing pin now connects the alumininm pie dish with the
polystyrene bowl. Touch the aluminium pie dish with your finger then
lift it up using the rubber eraser handle.Touch the aluminium pie dish
again. A spark may jump between your finger and the pie dish each time
you touch it.
2. Use a piece of aluminium or a cake tin. Heat the metal evenly over a
flame. Touch a wax candle to the centre of the aluminium until it melts
and sticks solidly to it as a handle. Use a plastic dish pan or bowl
larger than the cake tin. Put the bowl or pan on a table and stroke the
inside bottom of the pan briskly with a piece of fur or flannel for
half a minute. Put the aluminium on the plastic and press it down hard
with your fingers. Remove the aluminium pan, put your finger near the
metal and you should get a spark. You can take many charges from the
plastic without more rubbing. Press the metal against the plastic,
press with your fingers and lift by the handle.
4.51 Electricity from
two coins
Take two coins made of different metals. Clean them well with steel
wool or fine sand paper. Fold some paper into a pad so that it is
larger than the coins. Soak the paper in saltwater. Place one coin on
top of the pad and the other underneath. Hold them between your thumb
and finger. Connect both leads of a sensitive galvanometer or
multimeter to the coins and note the deflection.
4.52 Electricity from a lemon
See diagram 4.52: Electricity from a lemon
Connect a wire to a piece of zinc. Use zinc cut from the can of a used
dry cell, torch battery. Connect another wire to a piece of copper.
Roll a lemon on the table with your hand to break up some tissue
inside. Push the zinc and copper strips through the skin of the lemon
so that they do not touch. Connect both leads of a sensitive
galvanometer or multimeter to wires and note the deflection. Repeat the
experiment using a potato. Note whether the distance between the metal
strips affects the galvanometer reading.
4.53 Dry cell,
electric torch (flashlight) battery, Leclanche cell
See diagram 4.53: Investigating a dry cell
The term "battery" refers to several joined electrical cells, but one
dry
cell is commonly called a "battery", e.g. a torch battery, flashlight
battery
A Leclanche cell (Georges Leclanche 1839-1882) is a primary voltaic
cell with a carbon rod anode, zinc cathode, dilute ammonium chloride
solution electrolyte and e.m.f. approximately 1.5 volts.
Zn + H2SO4 --> (discharge) ZnSO4 + H2O
+ H2 (g)
A torch
"battery" is the dry cell version of the Leclanche cell. It has
manganese dioxide [manganese (IV) oxide] around the carbon rod to
oxidize hydrogen and so depolarize the anode.
2MnO2 + H2 --> Mn2O3 + H2O
The electrolyte is
in the form of a water paste so the dry cell is not really "dry".
Remove the outer covering from an old dry cell. Use a saw to cut the
cell in half and note its structure. Note the carbon (+ve) pole in the
centre. The zinc container is the negative (-ve pole). The material
between the two poles is the electrolyte. Note how the zinc has been
eaten away by the chemical.
4.54 Dry cells in an electric circuit
Connect an electric light bulb, e.g. 4.4 volts, V,
0.5 amps, A, and a lampholder, to
the +ve and -ve terminals of a dry cell or a lead cell accumulator or a
low
voltage power supply. Notice the filament made of tungsten carbide.
Passage of the electric current through the tungsten carbide wire
causes it to become very hot and give off light. Reverse the
connections to the source of electricity and the lamp still operates
although the electricity is flowing in the opposite direction. Draw a
diagram to show the path of the current through the light bulb and
around to the other end of the cell. This is a simple electric circuit.
Use circuit diagrams to represent the electrical components in a
circuit.
4.55 Simple switch
See diagram 4.55: A simple switch
Fasten the end of a piece of wire to a pencil with two rubber bands. A
second wire makes a connection.
4.56 Switches in a circuit.
Put a knife switch in a circuit with a dry cell and a light bulb. Turn
the light on and off by operating the switch. Replace the light bulb
with a bell or buzzer and operate the switch. Replace the knife switch
with a push button switch. Examine the construction of different
switches, e.g. household tumbler switch, rocker switch. Use them in a
circuit.
4.57 Electric torch (flashlight)
See diagram 4.57: Workings of an electric torch
(flashlight)
A Glass screen in front to protect the light bulb, B Small incandescent
light bulb (lamp), C Reflector, D Electric
switch, E Batteries, F Cover that can be gripped in the hand and
containing part of
the electric circuit, G Spring to keep batteries tightly together, H
Screw opening at the end for battery replacement
Take
apart an electric torch to see
the following different parts. Note the directions of
insertion of batteries. The batteries must be in series.
Note the rating on the side of the light bulb, e.g. 4.4 V, 0.5 A.
Larger light bulbs are rated in volts, V and watts, W, e.g. in
Australia, 240 V 40 W. Note the lamp type, fitting, e.g. screw or
bayonet.
4.58 Conductors and non-conductors of
electricity
See diagram 4.58: Conductor or non-conductor?
Use a simple electric circuit to test whether different substances
conduct electricity, e.g. paper, rubber eraser, plastic, key, coin,
cloth, string, chalk, glass, pin, nail file, insulated wire, bare wire,
finger, water. Test these in a circuit across an open knife switch.
Materials that carry electricity are electrical conductors, conductors.
Materials that do not carry electricity are
non-conductors, insulators. The copper core of bell wire is a
conductor. Its covering is an insulator.
4.59 Circuit board
Use a piece of heavy cardboard 30 X 30 cm as a base. Fixed clips on it
for holding the cells, and sprung metal strips for providing
connections between cells. Screw brass curtain rod holders into the
base. Make spring connectors of varying lengths from curtain wire with
hooks at each end. Put light bulb holders into circuits with
curtain wire connectors or heavy uninsulated copper wire. Make other
connections with lengths of uninsulated copper wire attached to
crocodile clips.
4.60 Cells in series
See diagram 4.60 Cells in series
Cells connected in the same direction in series each add their own
voltage, e.m.f., to the total voltage. However, each cell has an
internal resistance, r. So if connecting three cells of voltage V1, V2
and V3, if current through each of the cells is I amps, then total
voltage = V1 +V2 + V3 - (Ir1 + Ir2 + Ir3).
Connect two dry cells or lead cell accumulators so that the negative
terminal of one is in contact with the positive terminal of the other.
Connect them in series. Put a light bulb in the circuit. Close the
circuit with one cell, two cells, three cells in series. Record the
changes in the brightness of the light bulb. The brightness of the
light depends on the number of cells connected in series. When you
connect cells in series, the total voltage is the sum of the individual
voltages of the cells. If you use 1.5 V cells, then two cells give 3
V, and three cells give 4.5 V, four cells give 6 V.
4.61 Cells in parallel
See diagram 4.61: Cells in parallel
When three identical cells are connected in parallel the total voltage
is as if for one cell. However, the total resistance for one cell is
1/3 r. So total voltage = V -3I(1/3 r). So motor car batteries
may be connected in parallel to provide the extra current needed to
start the engine.
Connect two or three fresh dry cells or lead cell accumulators so that
you join their positive terminals and they join their negative
terminals. They are connected in parallel. Set up a circuit on a
circuit cardboard with three cells in parallel. Disconnect one or two
of the cells. The circuit is not broken and the brightness of the light
does not change. The voltage drop in the circuit is the same if you use
one, two or three cells. The total current is unchanged. If four cells
in the circuit, the total current is 0.125 X 4 = 0.5 A.
4.62 Electric light bulbs in series and
parallel
See diagram 4.62.3: Resistors in series | See diagram 4.62.4: Resistors in parallel
If resistors with resistance R1, R2 and R3 are connected in
series, they have the same current, I, passing through them. and total
resistance of the circuit = R1 +R2 + R3 ohms.
If resistors with resistance R1, R2 and R3 are connected in
parallel, they have a common potential difference across them, V, and
the total current through them is the sum of the separate currents I1 +
I2 + I3. If total resistance is RT, then 1/RT = I/R1 + I/R2 + I/R3. So
the total resistance will be less than the smallest resistance in
parallel.
1. Connect one, two and three identical light
bulbs in series. Record
the brightness of the light bulbs.
2. Connect one, two and three light bulbs in parallel. Record the
brightness of the light bulbs.
If you connect six light bulbs in series in a circuit containing a 6
V battery, each light bulb receives 1 V. If you connect six light
bulbs in parallel in a circuit containing a 6 V battery, each light
bulb receives 6 V.
4.63 Make a fuse
See diagram 4.63: How a fuse works
Examine normal and burnt out fuses. You use fuses to protect electric
circuits against overloading. The fuse wire melts and breaks the
circuit when an unsafe amount of current is flowing. Use a thin strip,
no more than 0.5 mm wide, of metal foil cut from a chocolate wrapper or
a thread of steel wool. Fasten it between the ends of two wires
projecting through a cork. Pass electric current through the fuse until
the fuse wire melts and breaks.
A short circuit is the deviation of a current from the planned path
along a path of less resistance. However, this excess current can be
stopped if a suitable fuse exists in the circuit.
4.64 Use a fuse
See diagram 4.64: Uses of fuses
1. Place the model fuse from experiment 4.160 in a circuit in series
with three cells and a light bulb. Use a crocodile clip to
short-circuit the light bulb. If the fuse does not melt, cut a thinner
strip
of foil. Try different kinds and widths of foil until the foil carries
the current when connected properly but melts when a "short" occurs in
the circuit. Then replace the fuse and add more light bulbs in parallel
until the fuse burns out.
2. Open the fuse box at your school or home.
Note the different kinds
of fuses, how to "trip" a fuse, and to replace the fuse wire. A fuse
box should contain spare fuse wire. When you use several appliances
simultaneously, the wires carrying the current may become overheated
and cause a fire. Putting a coin behind a fuse to allow more current to
flow is a very dangerous practice. Use the correct fuse wire. A 30
ampere, 30 A, fuse in a circuit designed for a 15 ampere, 15
A, fuse is unsafe.
4.65 Model electric light bulb
(incandescent filament lamp)
See diagram 4.65: Getting heat and light from
electricity
A substance is incandescent if it emits light as a result of its
temperature being raised.
1. Push the ends of two pieces of copper wire through a cork in a small
bottle. Connect the ends of the copper wire inside the bottle with a
stand of steel wool. Connect this model electric lamp model in a
circuit with one or more dry cells, or lead cell accumulators, and a
switch. Close the switch until the fine wire filament begins to glow.
At first the heated iron wire produces light but soon the iron combines
with the oxygen of the air inside the bottle and burns.
2. Examine a
manufactured electric light bulb. It contains argon but no oxygen. It
has a tungsten
carbide wire filament that glows without burning when heated to a high
temperature. The argon restrains the blackening of the inside of the
bulb by deposition of tungsten vapour. Fluorescent lamps containing
mercury vapour or neon gas are much more energy-efficient.
4.66 Electric current detector
See diagram 4.66.1: Compass in a coil | See diagram 4.66.2: Compass in a match box
Wrap 50 to 60 turns of bell wire to form a coil around a container 8 cm
in diameter. Remove the coil from the container and bind it with short
pieces of wire or insulating tape. Mount the coil on a piece of
cardboard. Attach a 16 mm plotting compass to a cork and fix it inside
the vertical coil. Rotate the coil until it is in line with the compass
needle. Connect a battery to the coil and note the deflexion of the
compass needle. Reverse the connections, and note the deflexion of the
compass needle again. Make a more sensitive instrument by putting a
compass in the tray of a match box then winding the coil wire over the
box.
4.67 Simple compass needle
See diagram 4.67.1: Simple compass needles
1. | See diagram 4.67.2:
Simple compass needles 2
1. Magnetize a sewing needle by stroking it with a bar magnet. Make a
simple compass by the following methods:
1.1 push the magnetized needle through cardboard
and
suspend it on a thread,
1.2 push the needle through the
projections of a cloth-covered button,
1.3 attach the needle to a
strip of cardboard and balancing it over an inverted test-tube
supported on a long pin.
Label the end of the magnet that tends to
point north.
2. Make another simple compass needle by the following methods:
2.1. push two magnetized
sewing
needles through the holes of a large press stud and balancing it on the
end
of a needle pushed into a cork,
2.2. push a magnetized needle through
thin cardboard and suspend it on
a
thread inside a glass jar.
3. Compare the north direction shown by a plotting compass with the
directions shown by the simple compass
needles. A compass needle is marked "N" at on end. This end points
towards the north magnetic pole so it is the "north-seeking pole"
of the magnet. The other end is the "south-seeking pole".
4.68 Magnetic dip
See diagram 4.68: Magnetic dip
Push a steel knitting needle through cylindrical cork at right angles
to its long axis. Push a pin into the centre of each end of the cork to
act as an axle. Balance the cork through its axle of pins on knife
edges. Magnetize the steel knitting needle using a magnetizing coil,
see 4.166. Balance the cork again. The earth's magnetic field pulls one
end of the needle downwards. Fix a spirit level, or a glass tube
containing a bubble in water, above the knitting needle. Use a
protractor to measure the angle of dip between the horizontal spirit
level and the knitting needle. At the north magnetic pole or at the
south magnetic pole the
needle should point straight down. At the equator the knitting needle
will be about parallel to the spirit level.
4.69 Make a magnetizing coil
See diagram 4.69: Magnetizing coil
Use glass tubing wound with close turns of insulated copper wire to
magnetize steel knitting needles.
4.70 Freely-suspended magnet
See diagram 4.70: Suspended magnet
Use loops of cotton to suspend two magnets freely. Bring each pole of
the two magnets close to, but not touching, each other. Show that like
poles repel and unlike poles attract.
4.71 Natural magnets
A form of magnetite, iron (II, III) oxide, called lodestone, acts as a
magnet when freely suspended. It was probably first discovered in China
where they used it for the first magnetic compasses.
4.72 Artificial magnets
Look for low-cost artificial magnets in discarded loudspeakers,
telephone receivers and other equipment. Artificial magnets have
different shapes, e.g. "Alnico", horseshoe magnet, pairs of bar magnets
with a soft iron keeper, cylindrical magnets. Store artificial magnets
in pairs in a box, north to south, and south to north. Be careful! Keep
magnets away
from computer diskettes (floppy discs) and colour television screens.
4.73 Identify magnetic substances
Collect objects made of different substances, e.g. paper, wax, brass,
zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold,
silver, wood, tin. Test each object with a magnet to see which objects
a magnet attracts or does not attract. Bring a soft iron wire and hard
steel or piano wire near a compass needle to see if a magnetic field
affects it.
4.74 Magnetic poles and pin chains
1. Use a 6 cm length of iron wire. Draw one end of a magnet along it
once only and in one direction from end to end. Lay the wire on a piece
of paper then test for magnetism by sprinkling iron filings over it.
The iron filings are not attracted equally along its whole length. The
areas of strongest attraction are the magnetic poles of the piece
of wire. Use adhesive tape to removes iron filings from a strong magnet.
2. Pick up a pile of pins with the magnet. Leave one pin attached to
the magnet. Take off another pin and bring it close the end of the
first pin. They will stick together by magnetic force. Connect all the
pins to make a magnetic pin chain.
4.75 Cut an iron wire magnet
Cut in half the magnetized steel wire from 4.171. Test both ends of
each broken portion. The magnetism found on each side of the break has
opposite polarity. Cut off a very small piece of the wire magnet and
test it with iron filings. The smallest piece of the wire is a magnet
with opposite poles.
4.76 Magnetic fields in two dimensions
See diagram 4.76.1: Magnetic fields 1| See
diagram 4.76.2: Magnetic fields 2
1. Sprinkle iron filings evenly on a thin card. Hold the card high
over a bar magnet then carefully lower it until it almost touches the
magnet. Tap the card gently with the end of a pencil. The iron filings
move into a pattern showing the magnetic field.
2. Repeat the experiment
with two bar magnets in different
positions. The iron filings tend to line up in "lines of force", "field
lines". Hold a plotting compass above the lines of force and compare
their direction with the direction of the compass needle. Put an
unmagnetized piece of soft iron near two bar magnets on the desk and
observe the interesting magnetic fields formed.
3. Make permanent records of the magnetic field by the following methods
3.1. Spray over the
iron filings with a paint sprayer.
3.2 Replace the card with
photographic paper in a dark room. Shine a bright light on it and
develop the print.
3.3 Dip a white sheet of paper in melted wax. Let it
cool then sprinkle iron filings on the solid wax. Hold the paper over a
strong magnet to allow the iron filings to move into lines of force
patterns. Hold a hot iron over the iron filings to let them sink into
the wax.
3.4 Photocopy the iron filings on transparent paper, but do
not use a strong magnet near a photocopy machine.
4.77 Magnetic fields in three dimensions
Add oil to iron filings in a container. Shake to see if the filings
will go into suspension in the oil. Use a concentration of oil that
allows the iron filings to remain suspended then bring a magnet to the
container to develop a pattern of iron filings in three dimensions.
Make a permanent record using water glass or liquid plastic.
4.78 Cylindrical electromagnet
See diagram 4.78: Cylindrical electromagnet
1. Use an iron bolt 5 cm long with a nut and two washers. Put a washer
at
each end and screw the nut on to the bolt. Leave 30 cm of wire then
wind three layers of bell wire on the bolt between the washers. Leave
another 30 cm of wire then cut the wire. Twist together the two ends of
the wire. Wind insulating tape around the ends of the bolt to prevent
the wire unwinding. Remove insulation from the two ends of the wire to
link the electromagnet in a circuit with two dry cells or lead cell
accumulators in series. Use a headlight bulb in series with the
electromagnet.
2. Connect the circuit and then pick up pins and nails. Disconnect the
circuit and see the iron objects fall. The magnetic force
exists only when you turn on the current.
Use a plotting compass to
test the poles at each end of the electromagnet. Reverse the
connections to the source of electricity and test the poles
again.
4.79 Horseshoe electromagnet
See diagram 4.79: Horseshoe electromagnet
Do NOT use a lead cell accumulator, car battery, for this experiment
because the resistance of these coils is low and the current will be
too large with a significant fire risk. If you use horseshoe magnets or
C-shape magnets, wind the coil in opposite directions on each arm of
the magnet. Use an U-shape piece of iron. Wind a coil of three layers
of bell wire on each straight arm of the iron, but not on the curving
part. Leave 30 cm of wire before you start winding the coil from the
end of one arm. Cross to the other arm. Wind a coil of three layers and
leave 30 cm of wire at the end. Wind three layers of wire on this pole
then wind insulating tape around the wires so they cannot unwind.
Remove the insulation from the ends of the coil, connect the horseshoe
magnet in series with a car headlight bulb, connect to two dry cells or
lead cell accumulators, and test the poles of the electromagnet. One
pole should be a north pole and the other pole should be a south pole.
If each pole has the same polarity, you have wound the second coil in
the wrong direction so you must unwind the coil and rewind it in the
opposite direction. Use the magnet to attract different things. Compare
the strength of this electromagnet with the cylindrical electromagnet
of 4.175.
4.80 Test the strength of electromagnets
Do not use lead cell accumulators for this experiment because the
resistance of these coils is low and the current will be large with a
significant fire risk.
1. Wind 25 turns of bell wire on a straight iron bolt and connect one
dry cell or lead cell accumulator to the ends of the wire. Record the
number of pins or paper-clips you can pick up with the electromagnet.
2. Repeat the experiment with two dry cells or lead cell accumulators
connected in series.
3. Wind on 25 more turns of wire in the same direction. Join them to
the first 25 turns. Repeat the experiment.
4. Repeat the experiment with two dry cells or lead cell accumulators
connected in series.
5. Wind on another 50 turns. Join them to the first 50 turns. Repeat
the experiment.
6. Repeat the experiment with two dry cells or lead cell accumulators
connected in series. Remove 50 turns and rewind them on the bolt in the
opposite direction.
7. With 100 turns so wound, repeat the experiment with two dry cells
or lead cell accumulators connected in series.
4.81 Magnetic field from electric current in
a
wire
Pull 25 cm of insulated copper wire through a hole in the centre of a
small white piece card. Connect the ends of the wire to a battery
through a car headlight bulb. Fix the card in a horizontal position.
Fix the wire in a vertical position. Sprinkle iron filings evenly on
the card. Switch on the current. Tap the card gently with the end of a
pencil. The iron filings move into a pattern showing the magnetic
field. Switch off the current. Repeat the experiment using a small
plotting compass instead of iron filings. Compare the directions of the
compass needle to the patterns of iron filings on the card. Repeat the
experiment with the direction of current reversed.
4.82 Magnetic field inside an open coil,
open
solenoid
See diagram 4.82: Open
solenoid
Wind five evenly spaced turns of bell wire around a wooden cylinder.
Slide the coil off the cylinder. Fit the cylinder into slots in a piece
of cardboard so that the cardboard appears to cut the coil in half
lengthways. Connect the coil to the terminals of a dry cell or lead
cell accumulator or low voltage power supply using a car headlight bulb
in series. Sprinkle iron filings evenly on the card. Switch on the
current. Tap the card gently with the end of a pencil. The iron filings
move into a pattern showing the magnetic field. Note the pattern inside
the coil and outside the coil. Switch off the current. Repeat the
experiment using a plotting compass instead of iron filings.
4.83 Electricity from a magnet and a coil
See diagram 4.83: Producing electricity with a
magnet
and a coil
Connect a coil of fifty turns of bell wire to a current detector. Use
long connecting wires so that the coil, and the magnet are away from
the compass in the current detector. Hold the horseshoe magnet or bar
magnet in your left hand and the coil of bell wire in your right hand.
Hold the coil vertically. Pass one pole of the magnet through the soil
while observing the compass needle in the current detector. When the
coil moves through the magnetic lines of force, an electric current
moves through the circuit.
4.84 Make a simple electric motor
See diagram 4.84: Simple electric motor
1. horseshoe magnet, 2. axle, 3. commutator, 4. coil, 5. brass strip,
6.
electric motor with 3 coils, A Contact (brush), Aw Wire from
contact to coil, B Contact (brush), Bw wire from contact to
coil
Fix a simple coil, mounted on an axle, between the poles of a horseshoe
magnet. Two wires from the coil connect to the commutator. The
commutator is a cylindrical insulator revolving on the axle with two
strips of brass attached. The commutator rotates with the coil. Each
brass strip is joined to one wire from
the coil. Two carbon contacts, brushes, touch the side of the
commutator and allow electric current to pass from the battery to the
commutator. Electric current goes from the battery to brass strip
A then along wire Aw, through the coil then back through wire Bw
and brass strip B then back to the battery to complete the circuit.
When the commutator and coil make one half turn, the current enters
through brass strip B and returns through brass strip A, reversing the
current
in
the coil. The electric motor runs more smoothly if more than one coil
is used. This electric motor uses a permanent magnet but most electric
motors use a field coil that forms a more powerful electromagnet.
Using Fleming's left hand rule, direction of thumb is thrust, first
finger is magnetic field and second finger is current. In the diagram,
side 7 to 8 of the coil has upward force on it and side 9 to 10 has
downward force on it. So the coil turns until it it vertical and the
brushes no longer touch the brass strips because of the gaps between
them, and no current flows. However, due to inertia of the commutator,
the coil keeps turning so side 7 to 8 is now on the right side and side
9 to 10 is on the left side. The brushes touch the brass strips again
and the coil keeps turning clockwise.