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
2014-08-11

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

4.16.0 Conduction of heat
4.22 Conduction in a metal bar
4.21 Conduction of heat by a coin on paper
4.19 Liquids that conduct electricity
4.16 Reduce heat loss with insulation
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 box, convection currents in air, mirage
4.30 Convection currents and ventilation
4.26 Convection currents from an ink bottle
4.25 Convection currents in a container
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
4.66 Electric current detector
4.62 Electric light bulbs (lamps) in series and parallel, resistors in series and parallel
4.57 Electric torch (flashlight)
4.63 Electrical fuses
4.52 Electricity from a lemon
4.51 Electricity from two coins
4.53 Examine a dry cell, electric torch (flashlight) battery, Leclanché cell
4.59 Make a circuit board
4.63 Make a fuse
4.159 Resistors in series and parallel
4.55 Simple switch
4.56 Switches in a circuit

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

4.5.0 Expansion
4.9 Burning candles over water
4.6 Expansion and contraction of liquids
4.4 Expansion of a solid when heated
4.8 Expansion of air
4.3 Plug and ring experiment

4.1.0 Heat and temperature, heat as energy
4.4 Expansion of a solid when heated
4.36 Newton's law of cooling
4.38 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
4.36 Different surfaces affect heat radiation and absorption
4.35 Feel radiation through glass
4.33 Focus radiant heat waves
4.34 Reflect radiant heat waves
4.32 Transfer heat by radiation

4.39.0 Static electricity
4.41 Attract water to a comb
4.42 Balloon stays in place
4.141.1 Coin stays on the cupboard door
4.42 Electric pinwheel, a simple electrostatic motor
4.43 Franklin's bell, lightning warning device
4.145 Gold leaf electroscope
4.50 Many charges from one source
4.141 Newspaper stays on the wall
4.143 Pith ball indicator, electroscope
4.140 Repulsing balloons
4.142 Static electricity detector
4.137 Static electricity from rubbing
4.146 Two kinds of static charge, electroscope

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 solids
See diagram 4.4: Expansion of solids
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.

4.6 Expansion and contraction of liquids
See diagram 4.6: Expansion and contraction of liquids 1 | See diagram 4.7: Expansion and contraction of liquids 2
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 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.

4.9 Burning candles over water
See diagram 4.9: Burning candle over water
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

4.10 Heat energy to change solid to liquid, melting point, latent heat of fusion, naphthalene, ethanamide (acetamide)
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 2.26 MJ kg-1.

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.

4.14 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.

4.18 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 distilled 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 distilled water.

4.20 Copper coil snuffer conducts heat
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 of heat 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
See diagram 4.24: Feel a heated 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
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. 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.

4.28 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 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.

4.30 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.

4.32 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
See diagram 23.3.7: Focus sun's rays
1. Use a magnifying glass to focus the rays of the sun on a piece of paper tissue. The paper chars and catches fire.
2. Repeat the experiment with paper tissue soaked in black ink. The black paper catches fire sooner than the white paper.
3. 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 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.
4.35 Feel radiation through glass
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 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

4.37 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 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.

4.41 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.

4.42 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 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
See diagram 4.47: Metal foil ball electroscope
Hang a strip of aluminium foil, or folded piece of tissue paper, 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. B 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.
1. The basic observations of electrostatics are as follows:
1.1 Rub a plastic comb with fur. The plastic comb becomes -ve and the fur becomes +ve.
1.2. Rub a glass rod with silk. The glass rod becomes +ve and silk becomes -ve.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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
See diagram 4.50: Produce many charges of static electricity from one source, electrophorus
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.
4.51.0 Current electricity

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 lemons
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 battery, electric torch (flashlight) battery, Leclanché cell
See diagram 4.53: Investigating a dry cell: A Plastic wrapper, B Zinc case (cathode) (-ve) C Ammonium chloride paste (electrolyte) D Manganese dioxide + carbon particles, E Carbon rod (anode) (+ve)
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
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 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
See diagram 32.154.1: Electric torch
Take apart an electric torch to see the following different parts. Note the directions of insertion of batteries. The batteries must be inserted in series. Note the rating on the side of the light bulb, e.g. 2.4 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.
Draw a circuit diagram.

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 fuses. 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.

4.64 Electricity fuse
See diagram 4.63: How a fuse works | See diagram 4.64: Uses of fuses
1. 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.
2. Place the model fuse from experiment 2.160 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.
3. 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 Make a model electric light bulb (incandescent filament lamp)
See diagram 4.65: Getting heat and light from electricity
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 than incandescent lamps.

4.66 Make an 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.

4.137 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.

4.140 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, electroscope
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.

4.145 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, electroscope
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.