School Science Lessons
UNPhysics1 Physics experiments
Conduction, convection, current electricity, density, heat and temperature, heat radiation, static electricity
Please send comments to: J.Elfick@uq.edu.au
2014-07-23

Table of contents
4.16.0 Conduction of heat
4.24.0 Convection
4.51.0 Current electricity
4.12.0 Density
4.5.0 Expansion
4.1.0 Heat and temperature, heat as energy
4.35.0 Heat radiation
4.39.0 Static electricity
4.36.0 Thermometers

4.16.0 Conduction of heat
4.3.0 Ball and ring, plug and ring
4.22 Conduction in a metal bar
4.21 Conduction of heat by a coin on paper
4.17 Conduction of heat by metals, Davy lamp
23.7.12 Cook an egg on a piece of paper
4.20 Copper coil snuffer
4.16 Reduce heat loss with insulation
4.19 Liquids that conduct electricity
4.18 Solids that conduct electricity
4.23 Water is a poor conductor of heat, boil water in a paper cup

4.24.0 Convection
4.29 Convection disc, heat snake, convection wheel
4.30 Convection currents and ventilation
4.26 Convection currents from an ink bottle
4.25 Convection currents in a container
4.27 Convection disc, heat snake
4.29 Convection box, mirage
4.24 Feel convection currents in a test-tube
4.31 Temperature of water at maximum density, 4ºC
4.28 Trace convection currents

4.51.0 Current electricity
4.61 Cells in parallel
4.60 Cells in series
4.59 Circuit board
4.58 Conductors and non-conductors of electricity
4.54 Dry cells in an electric circuit
4.62 Electric light bulbs (lamps) in series and parallel, resistors in series and parallel
4.66 Electric current detector
4.57 Electric torch (flashlight)
4.63 Electrical fuses
4.51 Electricity from two coins
4.52 Electricity from a lemon
4.53 Examine a dry cell, electric torch (flashlight) battery, Leclanché cell
4.65 Incandescent lamp, electric light bulb, filament lamp, light globe
4.59 Make a circuit board
4.63 Make a fuse
4.159 Resistors in series and paralle
4.55 Simple switch
4.56 Switches in a circuit

4.12.0 Density
4.12 Density of solids
4.13 Density of liquids, relative density

4.5.0 Expansion
4.6.0 Expansion and contraction of liquids
4.8.1 Expansion of air in a balloon
4.3 Plug and ring experiment
4.4 Expansion of a solid when heated
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

4.1.0 Heat and temperature, heat as energy
4.4 Expansion of a solid when heated
4.36 Newton's law of cooling
4.3 Plug and ring experiment
4.38 Rate of heat transfer
4.1 Temperature rise and quantity of heat intake
4.2 Transfer kinetic energy to heat energy

4.35.0 Heat radiation
4.32 Transfer heat by radiation
4.33 Focus radiant heat waves
4.34 Reflect radiant heat waves
4.35 Feel radiation through glass
4.36 Different surfaces affect heat radiation and absorption
4.36.0 Thermometers
2.105 Ring and plug experiment
2.106 Expansion of a solid when heated
2.107 Bimetallic strip
2.108 Expansion of liquids
2.109 Expansion and contraction of a liquid
2.110 Expansion of air
2.111 Expansion of air with a balloon
2.112 Is your temperature sense reliable?
2.113 How a thermometer works
2.114 Make a spirit thermometer
2.115 Test a thermometer
2.116 Thermoscope

4.39.0 Static electricity
4.138 Attract water to a comb
4.42 Balloon stays in place
4.141.1 Coin stays on the cupboard door
4.44 Cottrell smoke precipitator
4.42 Electric pinwheel, a simple electrostatic motor
4.145 Electroscope, Gold leaf electroscope
4.145 Gold leaf electroscope
4.144 Electroscope, metal foil ball electroscope
4.43 Franklin's bell, lightning warning device
4.50 Many charges from one source
4.141 Newspaper stays on the wall
4.143 Pith ball indicator
4.140 Repulsing balloons
4.142 Static electricity detector
4.137 Static electricity from rubbing
4.146 Two kinds of static charge
32.1.1 Voltage produced by friction, Van de Graaff generator
4.40 Use a van de Graaff generator

2.105 Ring and plug experiment
See diagram 2.105
Use a large wood screw and a screw eye through which the head of the screw will just pass (the eye may be fashioned out of heavy wire). Screw each part into the end of a stick, but leave at least 2.5 cm of metal protruding (see diagram). Heat the head of the screw in a flame for a while and then try to put it through the screw eye. Keep the screw hot and heat the screw eye in the flame at the same time. Now try to put the screw head through the screw eye. Keep the screw head in the flame. Cool the screw eye in cold water. Again try to put them together. Next cool the screw head and try again.
2.106 The expansion of a solid when heated
See diagram 2.106
Use a piece of stout copper tubing about 2 m long. Lay it on a table and fix one end by a clamp. Underneath the other end put a piece of knitting needle or bicycle spoke with a right angle bend at one end to act as a roller. A thin strip of balsa wood about 1 m long fixSee diagramed to the roller by sealing wax will show any movement of the rod resting on it (see diagram). Blow steadily down the tube at the fixed end, and the expansion of the tube caused by the hot breath will be detected by this arrangement. Now pass steam through, and note the motion of the pointer. Damage to the table top can be avoided by placing a sheet of asbestos under the copper tubing. Try the experiment with different types of tubing.[A copper tubing B clamp c knitting needle or bicycle spoke D balsa wood pointer]
2.107 Bimetallic strip
See diagram 2.107
A pair of iron and brass strips, riveted together, will bend when heated because of the difference of expansion. Make the holes with a nail and fix small tacks as rivets (see figure). Another way of fastening the strips together is to cut them with projections at equal intervals and bend the projections over to interlock.
2.108 Expansion of liquids
See diagram 2.108
Fit two or three similar medicine bottles with corks and tubes. Fill them with liquids of different viscosity and immerse them in a pan of hot water (see diagram). The rise inside the tubes will indicate the difference in expansion rates.
2.109 Expansion and contraction of a liquid
See diagram 2.109
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 about 30 to 60 cm (see diagram). Pour warm water over the flask and the water rises in the tube. Pour cold water and the water drops inside the tube.
2.110 Qualitative examination of expansion of air
See diagram 2.110
Air is trapped in the flask by means of a small bead of oil in the glass tubing (see diagram). Gentle heating with the hand will produce a sufficient temperature rise to cause the oil drop to move up the tubing. Then plunge the flask first into cold and then into warm (not hot) water. In place of the flasks, glass test-tubes and corks with capillary tubing could be used.
2.111 Expansion of air with a balloon
1. Stretch a rubber balloon over the neck of a flask. Heat the flask gently with a candle or an alcohol flame.
2. Partially inflate a balloon. Hold it over a hot plate or place it in the warm sun for a while, and observe the result.
2.112 Is your temperature sense reliable?
Fill three pans with water. Have one at the highest temperature you can bear your hand in. Fill a second one with ice-cold water. The third should be lukewarm. Put both hands in the lukewarm water and hold them there for about half a minute. Does the water seem to be the same temperature for both hands? Does it feel hot, cold or neither? Next place your left hand in the hot water and your right hand in the icy water for a minute. Quickly dry your hands and plunge both into the lukewarm water again. How does the right hand feel? Do they feel the same as when in the lukewarm water before? What do you think about your temperature sense?
2.113 How a thermometer works
Fill a flask with water coloured with ink. Insert a one-hole stopper carrying a 30 cm length of glass tubing until the water rises about 5 or 6 cm in the tube. Place the flask on a tripod over a heat source and observe the water level as you heat it. The water expands more rapidly than the glass and rises up the tube. Careful observation will reveal that the water level drops at the moment heating is begun, and then it begins to rise. This is because the glass bulb starts to expand before the water inside reaches the temperature of the glass.
2.114 Make a spirit thermometer
To make a simple alcohol thermometer accurate enough for most purposes, use 20-30 cm of glass tubing of about 5 mm external diameter with about 1 mm bore. A bulb of about 1.5 cm external diameter is first blown in one end of the tubing. The tube is inverted and the open end is placed in alcohol. The bulb is alternately heated and cooled. After each cooling the alcohol drawn into the tube is shaken into the bulb. In this way the thermometer is filled with alcohol, care being taken to remove any air bubbles. The bulb of the thermometer is then placed in water at 60oC, which is slightly below the boiling point of alcohol. The excess alcohol is removed from the top of the tube as it oozes out. Now seal the open end with a hot flame. Caution great care must be taken when sealing the tube. Calibrate the thermometer by placing it in water at different known temperatures. 
2.115 Test a thermometer
1. Thermometer scales are marked at two fixed points, i.e. boiling-water temperature and the temperature of melting ice. Use a thermometer and place it in steam immediately above the surface of water boiling in a flask. Leave it there for several minutes and notice how closely it reads 100oC.
2. Remove the thermometer from the steam, allow it to cool for a few moments, and then place it in a jar of melting ice. Now observe how nearly it reads 0oC.
3. If you live at a high altitude, the temperature of boiling water may be below 100oC, because of the reduced atmospheric pressure. The thermometer will read exactly 100oC only at sea level or where the barometer reading is 760 mm of mercury.
2.116 Thermoscope
See diagram 2.116
Flasks, or cut-off light bulbs, can be used to construct this apparatus. Fit both bulbs with corks and tubes about 15 cm in length. Pass the lower ends of the tubes through flat corks and, having made holes about 22 cm apart in a suitable baseboard, glue the tubes in a vertical position and connect the open ends by rubber tubing. Remove one bulb and blacken the other in a candle flame. Pour liquid into the U-tube so formed until the level is about 7 cm above the baseboard. Replace the clear bulb and slide the tube in or out a little so that the liquid remains level. Place a candle equidistant between the bulbs and wait for results.

4.3.0 Ball and ring, plug and ring
Ball and ring, "Scientrific", (commercial website) | Bar and gauge, "Scientrific", (commercial website)
See diagram 23.105: Ball and ring
1. Use an iron ring with a set of two balls one over size and one under size. Heat the ring and slip over both metal balls. A ball passes through a ring only when the ring is heated.
2. 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 due to 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. If 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. 3. Leave the over size ball in liquid nitrogen for ten minutes then try to pass the ring around it.

4.6.0 Expansion and contraction of liquids
See diagram 23.108 Expansion and contraction of liquids
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.
3. 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.

4.8.1 Expansion of air in a balloon
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.

4.12 Density of solids
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: 2.70, carbon (graphite): 2.25, 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.
Experiment
Measure the density of examples of different metals then decide whether they are pure substances.
In SI units, measure density in kg m-3, e.g. density of dry air at sea level = 1.29 kg / m3.

4.13 Density of liquids, 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. 2.50, 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.

2. The density of water is close to 1 g per cc, (1g cm-3), so you can compare the density of substance with the density of water as relative density, RD. Relative density (formerly specific gravity, G), 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. Examples of relative density include the following:
e.g. petrol (RD 0.70), ethanol (RD 0.79), ice (RD 0.90), olive oil (RD 0.92), water (RD 1.00), sea water (RD 1.03), glass (RD 4.50), mercury (RD 13.60), gold (RD 19.30).
A special bottle, a density bottle, gives an accurate measure of relative density.
Mass of empty density bottle = A, Mass of bottle + liquid = B, Mass of liquid = B – A, Mass of bottle + water = C, Mass of water = C – A, RD = B – A / C – A
However, a more convenient way to measure the density of a liquid is to use a hydrometer.
3. 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. Density = weight of unopened can of cola - 13 g / volume of cola mL. Repeat the experiment with "diet" cola where sugar is substituted by a chemical sweetener, e.g. phenylalanine, aspartame.

4.21 Conduction of heat by 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.

4.24 Convection currents in a test-tube
See diagram 23.1.8
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.
See diagram 23.1.8
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.

4.25 Convection currents in a container
1. 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 nearer the source of heat displaces the heavier cold water and convection currents occur. Hot water is less dense than cold water. This is the cause of convection currents.
2. Fill two identical containers with water near 100oC and near 0oC. Drop 5 drops of food colouring into the water in different places in the containers containers. Observe the spread of the food colouring. The food colouring mixes more quickly with the hot water because its molecules are moving faster around each other. In the cold water, the food colouring may just sink to the bottom to displace water by its own weight.

4.26 Convection currents from an ink bottle
See diagram 23.126: Convection currents from an ink bottle
Use a small ink bottle, fitted with a two hole 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 box, convection currents in air
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. Another way of showing air current is by making use of the difference in refractive indexes of warm and cold air. A car headlight bulb without a reflector will cast “shadows” of convection current from an electric heater. 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.
4.28 Trace convection currents
See diagram 4.28: Trace convection currents
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 23.127: 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.
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 23.128: 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 following different 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.

4.34 Reflection of 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.

4.36 Newton's law of cooling
Newton's Law of Cooling states that the rate of loss of heat from a body both by radiation and convection is proportional to the difference between the temperature of the body and the temperature of the surroundings. It applies only to small ranges in temperature. Test whether a hot cup of coffee cools faster than a warm cup of coffee. Record the room temperature, e.g. 17.5oC. Use identical coffee cups. Put the same volume of hot coffee or warm coffee in the coffee cups. Insert a thermometer and use it to keep stirring gently. Record the temperature every two minutes for twenty minutes while still stirring. In the second column, record the temperature of the cooling water every two minutes. In column D, record the difference between the temperatures every two minutes and the room temperature. Calculate F / D for each two minute interval. The mass of coffee in the coffee cup is constant so the rate of heat loss of the coffee is proportional to the fall in temperature. The rate of fall of temperature is proportional to the mean difference of temperature between the coffee and the surroundings. So the fall in temperature during time interval / mean difference in temperature between the coffee and surroundings = constant. As the temperature of the body is higher and the temperature of surroundings is lower, the difference of the two temperatures is greater, so the rate of heat loss of the body is faster.
Table 4.36
Time in
minutes
Temp. of coffee F = fall in temp. in the last 2 minutes Mean temp. in last 2 minutes
(to nearest 0.1oC)
D = difference between
water temp.
and room temp.
F / D
(constant)
0 44.7oC .
.
. .
2 41.4oC 3.3oC 43.1oC 25.6oC 0.13
4 38.7oC 2.9oC 40.1oC 22.6oC 0.13
6 36.1oC 2.6oC 37.4oC 19.9oC 0.13
8 33.7oC 2.4oC 34.9oC 17.4oC 0.13

4.38 Rate of heat transfer
1. Use two identical thermos flasks containing (a) 700 g water at 40oC and (b) 100 g ice + 200 g water at 0oC. Which reaches room temperature 20oC first? The loss or gain of heat is greatest when the difference in temperature between the contents of the flask and the surroundings is greatest. In thermos flask (a), the temperature difference with room temperature is continually decreasing from its original temperature difference of 20oC. In thermos flask (b) the temperature difference remains the same at 20oC when latent heat is absorbed to convert the ice to water. More than half the total heat is received when the temperature difference remains at 20oC. So (a) reaches room temperature first.
2. Add water at 90oC to tea in a teapot and leave it to stand for 5 minutes. If you want the tea to be as cool as possible before drinking it, do you add milk immediately after pouring out the tea or just before drinking it? The rate of loss of heat depends on the temperature difference between the body and the surroundings. If the milk were added immediately after pouring the temperature of the tea would fall and the rate of loss of heat would be less than if the milk had not been added. So the milk should be added just before drinking the tea.
4.42 Electric pinwheel, a simple electrostatic motor
See diagram 4.42: Electric pinwheel
Place the electric pinwheel on top of a Van de Graaff generator dome. Start the generator then the wheel rotates. A bluish light and a hissing sound may come from the points of the pinwheel. The air is ionized in the high electrical field of the points. The ions and the points have the same sign of charge and thus repel each other.
4.43 Franklin's bell, lightning warning device
See diagram 4.43: Franklin's bell
Place the bell apparatus beneath the Van de Graaff generator dome. Start the generator to simulate a storm passing overhead. The high voltage generated present rings the bell.

4.44 Cottrell smoke precipitator
See diagram 44: Smoke precipitator
Clearly visible smoke, e.g. from a burning incense stick, passes through a glass tube "chimney" containing a central electrode and an outer earthing mesh. Apply high voltage to the central electrode to reduce the smoke coming from the top of the chimney.

4.54 Dry cells in an electric circuit
See diagram 32.3.9.1d: Cells in a circuit | See diagram 32.151.2: Torch battery electrical experiments
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 32.152: 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 32.154: Electric torch (flashlight) | See diagram 32.154.1: Electric torch
A. Glass screen in front protects 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 (e.g. one with a current of 0.5A at 2.4V) to see the different parts. Draw a circuit diagram. Note the directions of insertion of batteries.

1. Trace the circuit in an electric torch. Use a torch with metal sides and a torch battery opened with the back of an axe. Take a torch to pieces and put it together again. Show the class the torch, turn it on and off with the sliding switch. Dismantle the torch. Show the class the different parts: switch, metal case. Use the arrows to show them how these parts are part of a circuit. Show the children the opened battery. The electricity comes from the zinc case when some zinc dissolves in the black chemical. In the very old batteries so much zinc dissolves that holes in the zinc case let some chemical leak out. The carbon rod does not dissolve. Why should the two batteries be in the same direction in the circuit? [Otherwise they would push electric currents against each other.] Take out the batteries in a radio. Are they all in the same direction in the circuit? [Yes.] The electrical strength of a battery is measured in volts. How many volts in one battery? [1. 5 volts.] If the batteries are put end to end in the circuit, how many batteries do you need for a total of six volts? [Four.]

2. 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.

3. The flashlight is an electrical device that makes use of a switch, insulators and conductors, dry cells and a bulb. Examine various kinds of flashlights and take them apart. Connect the bulb to the dry cell without using the flashlight case. Reassemble the flashlight. Find the circuit in a flashlight and to determine where the circuit is completed and broken. In metal flashlights, the case is part of the circuit. In a two cell flashlight, the cells must be inserted so that the bottom of one cell touches the top of the other to provide the proper electrical circuit. Place the cells in various positions to discover which way works best.

3. Observe its interior structure and the position of each component (bulb, switch, and cell), its circuit and how the switch operates. Secondly, install cells, operate the switch and observe if the bulb works normally. Note the installation of the cells' polarity and the electrical source in series. Draw the circuit diagram of the electric torch. Start from one battery connection or terminal and trace the conducting path to the other terminal. Make sure that you include the switch and element of the globe. Using the following standard symbols as used for radio and other electrical circuits, draw the circuit for the torch. Take apart an electric torch, e.g. electric torch, 2.4V, 0.5A, to see the different parts. Draw a circuit diagram. Note the directions of insertion of batteries.

4.58 Conductors and non-conductors of electricity
See diagram 32.155: Test conductivity
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, a finger and water. Test these in a circuit across an open knife switch. Materials that carry electricity are electrical conductors, or simply conductors. Materials that do not carry electricity are non-conductors, or 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 measuring 30 × 30 cm as a base. Fix clips on it to hold the cells, and sprung metal strips to provide 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 32.5.1.1: Series circuits (Motor vehicles)
See diagram 32.4.6.5: Cells in series |
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 1.5V cells, then two cells give 3V, and three cells give 4.5V, four cells give 6V. The current will change.

4.61 Cells in parallel
See diagram 32.4.6.6: Cells in parallel | See diagram 32.4.6.1: Cells in parallel
See 32.5.1.2: Parallel circuits (Motor vehicles)
Connect two or three fresh dry cells or lead cell accumulators so that their positive terminals are joined and their negative terminals are joined. 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 one, two or three cells are used. The total current is unchanged. If four cells in the circuit, the total current is 0.125 × 4 = 0.5 amps.

4.62 Electric light bulbs (lamps) in series and parallel, resistors in series and parallel
Electric light bulbs in series and parallel
See diagram 32.4.6.0: Series and parallel | See diagram 32.4.6.2: Series and parallel | See diagram 32.4.6.4: Dry cells in series and parallel
See diagram 32.157: Resistors in series | See diagram 32.158: 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. When bulbs are connected in series, the total voltage is divided between them, e.g. if three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt. When lamps are connected in parallel, each bulb receives the full voltage of the supply.

4.63 Electrical fuses
See diagram 32.159: Fuses | See diagram 32.160: How a fuse works | See diagram 32.161: Fuse with increasing load
A fuse is a safety device that protects electrical appliances by preventing too much electricity flowing into them. The fuse is a thin wire of easily melted metal as part of an electric circuit inside a protective case. If the flow of electricity becomes too powerful, the wire melts and stops the current flowing and so it interrupts the circuit. The fuse wire is a length of wire with a given current rating at which the wire would melt if that current is exceeded. A fuse is a wire that melts at a certain temperature and so breaks the circuit preventing damage to other components of the circuit due to excessive current. The choice of fuse is restricted by the electrical source and conducting wire used in the circuit. In the installed circuit, the allowable current is fixed, so it is very dangerous to use a large capacity fuse that allows more than the allowable current to pass. Any device that opens a circuit because of abnormal electric current is called a circuit breaker. A fuse wire will eventually fail when the load on the circuit is increased.. Use mains operated circuit breakers (MOCBs) instead of fuses to eliminate the possible use of inappropriate fuse wire. Be aware, MOCBs do not act in the same way as safety switches and should not be confused with them.

Experiments
1. Check the fuse box
Know where the fuse box for your premises is and know how to turn off the power supply in case of an emergency. Check that each switch, circuit breaker and fuse is correctly labelled and call a licensed electrical contractor if there is any confusion. Use only the correct size fuse wire to rewire fuses.
The fuse box is the equivalent of the circuit breaker's electrical service panel in that it is a metal box with a hinged cover that houses and controls the incoming electrical service and distribution to branch circuits within the house. It provides overcurrent protection through the use of fuses. The fuse box will have threaded sockets into which the fuses will be screwed. These large threaded sockets look like light bulb sockets and are called Edison sockets. They are named after Thomas Edison who, like everything else we take for granted, invented them. The types of fuses that go into these sockets however are several. There are fuses that have Edison bases, and fuses that have a socket adapter that screws into the Edison base, but the fuse itself screws into the adapter base. These are called "S" fuses and are also called "tamper-proof" fuses with Rejection bases.
Open the fuse box at your school or home. Note the different kinds of fuses, how to “trip” a fuse and how 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. Be careful! Putting a coin behind a fuse to allow more current to flow is a very dangerous practice. Use the correct fuse wire. A 30A fuse in a circuit designed for a 15A fuse is unsafe.
Do not repair a fuse in a fuse box by wrapping a burnt out fuse with a metal foil gum wrapper where the metal in the shiny part of the gum wrapper acts as a replacement conduit for the burned out fuse. It may not burn out under excess current as a proper fuse does and result in a house fire. Do not use any wire, pins, staples or hair pins to replace burnt out fudes. Always replace burnt out fuses with the correct fuse wire.

2. Normal and burnt out fuses
Examine normal and burnt out fuses. 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.
3. Place a model fuse 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.

4.65 Incandescent lamp, electric light bulb, filament lamp, light globe
Light bulb IEC Light Source 2 Wires 2.5V MES "Scientrific", (commercial website)
See diagram 32.162: Heat and light from electricity | See 4.116: Incandescent lamp
A substance is incandescent if it emits light as a result of its temperature being raised. Hot solids or liquids emit wavelengths of radiation depending on the temperature as a continuous spectrum. At lower temperatures they emit red wavelengths, so the metal appears to be "red hot". At higher temperatures, they emit the full visible spectrum as white light, so the metal appears to be "white hot" or "incandescent". The incandescent filament in an electric light globe, a filament lamp, is "white hot". 

1. Heat source.
1.1 Remove the shade from a bed lamp containing a 100 W incandescent electric light bulb. Cover the bulb with very thin aluminium sheet, e.g. aluminium cooking paper.
1.2 Insert a 100 watt pearl electric light bulb in a holder so that you can dangle the light bulb down.
2. 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, nd 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.
3. Examine a manufactured electric light bulb. It contains a mixture of argon and nitrogen, 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 than incandescent lamps.
4. Examine an electric light bulb for any greying of the inner surface. The grey layer comes from the evaporation of the tungsten filament that becomes thin with time. Other reactive metals, e.g. tantalum and titanium, may be placed near the filament to to attract the tungsten vapour away from the glass bulb. At the time of burnout, an electric light bulb may have dimmed by 15%.

4.137 Static electricity from rubbing
See diagram 31.137: Static electricity from rubbing
1. Static electricity is electricity not flowing as a current. It is a form of energy caused by charged particles, e.g. protons, electrons, accumulating statically.
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.
2. Make a pile of finely divided cork particles by filing a cork. Cut up some thin paper into small pieces. Use very clean objects such as a plastic comb, a plastic pencil, a plastic fountain pen, a piece of wax, a rubber balloon, a glass or china dish and any other non-metallic objects you may find. Rub each of these things briskly with your dry hair or a piece of fur and then bring near the pile of cork particles. Rub again and bring near the pile of thin paper. Observe what happens. Repeat the experiment, rubbing each article in turn with a silk cloth. Repeat using a piece of flannel.

4.138 Attract water to a comb
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. Repeat the experiment using "Golden Syrup" or treacle or thin honey instead of water.

4.140 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.

4.141 Newspaper stays on the wall
See diagram 31.141: 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.
4.141.1 Coin stays on the cupboard door
This experiment can work on unpainted wood but it works best on waxed surfaces. Rub a coin up and down on the cupboard door with a brisk action. Remove you hand and the coin stays on the vertical door. The rubbing action causes increased pressure and friction so that the air under the coin becomes hotter and expands. Some of this hotter air leaks out from under the coin as it moves across the surface of the cupboard door. When you stop rubbing, the remaining air under the coin cools and contracts causing a partial vacuum. So the atmospheric pressure on the outside of the coin is greater than the pressure of air under the coin and the coin stays pressed against the cupboard door.

4.142 Static electricity detector
See diagram 31.142: 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.

4.143 Pith ball indicator
Pith balls, "Scientrific", (commercial website)
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.

4.144 Electroscope, metal foil ball electroscope
See diagram 31.144: Metal foil ball electroscope
An electroscope is used to detect electricity in the air by ionization of air molecules.
1. 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.
2. Metal leaf electroscope
Use the equipment above 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.
4.145 Gold leaf electroscope
See diagram 31.5.3: Gold leaf electroscope
The gold leaf electroscope is used for detecting and measuring electric charges. It consists of two gold leaves attached to a brass rod in a glass vessel. A metal cap is attached to the end of the rod. When the leaves have charges of the same sign they repel each another like an inverted V and separate more widely if the charge is increased or fall together if the charge is decreased. The gold leaves may be charged by induction or by contact. Suppose that they are charged negatively. The sign of the charge on any charged body may be found by bringing it up to the terminal of the electroscope. If a positively charged body is held close to the terminal, electrons from the leaves will be attracted up to the terminal by the positive charge, and the charge on the leaves will be decreased and they will fall together. If a negatively charged body is held close to the terminal, electrons will leave the terminal and go down to the gold leaves to increase their charges and so the leaves will separate more than before. So if the gold leaves are charged together negatively, they fall together for a positively charged body, and separate for a negatively charged body.
To use a negatively charged body to charge the gold leaf electroscope positively, place the negatively charged body near the terminal so that electrons will flow down to the gold leaves, leaving the terminal positively charged. When the rod is earthed, grounded by touching with the finger, electrons flow from the leaves to the earth, leaving the leaves neutral. When the earth connection is broken, and the charging body then removed, the positive charge from the terminal distributes over the terminal and leaves, leaving the electroscope with a net charge of opposite polarity to the charged body
To use a negatively charged body to charge the gold leaf electroscope negatively, place the negatively charged body on the terminal to make direct contact between the terminal and charged body
An electroscope used for measuring electric charges may have only one leaf. The case is metal and is also charged. The movement of the leaf indicating the magnitude of the charge or potential measured.

4.146 Two kinds of static charge
See diagram 31.146A: Positive and negative charges attract each another. Negative charges repel each another.
See diagram 31.146B: Use an uncharged pith ball electroscope.
See diagram 31.146C: 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.

4.159 Resistors in series and parallel
See diagram 32.2.2.1: Resistors in series and parallel
Connect one, two and three identical bulbs in series. Record the brightness of the bulbs. When bulbs are connected in series, the total voltage is divided between them, e.g. if three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt. Connect one one, two and three bulbs in parallel. Record the brightness of the bulbs. When lamps are connected in parallel, each bulb receives the full voltage of the supply.