Class practical

A series of simple experiments to develop the idea of doing useful work.

Apparatus and Materials

• Balloon
• Steel spring
• Long nails or doweling to hold spring
• Brick or wood block
• Pulley, single, on clamp
• Cord

Health & Safety and Technical Notes

Precautions are needed to prevent a spring flying into someone's face or a brick falling onto toes. If the class is positioned such that any member is at risk, eye protection should be worn or a box placed below the brick.

Read our standard health & safety guidance

Procedure

1. Blow up the balloon and tie the neck.
2. Put the blown-up balloon on a table and sit and watch it.
3. Hold the spring between your hands, stretch it and then release it.

4. Keep the spring stretched permanently between two nails.
5. Raise the brick (or block of wood) from the floor to a table by pulling on a string tied to the brick.

6. Use the brick on top of a pile of loose papers as a paper-weight.

7. Fix the pulley to the edge of the table and run the string over it and raise the brick by pulling the string horizontally along the table with the other end attached to the brick.

Teaching Notes

• These are all simple examples of ‘jobs' to be done, for quick discussion as to whether food or fuel is necessary for each, directly or indirectly:
• You might start by asking:
‘What does your food enable you to do besides keeping warm, breathing and generally living? Could an engine which uses petrol or a motor connected to the electricity supply do that job for you instead?’
• You can rule out some jobs that people have to do as not requiring food or fuel. You might say:
‘Yes I know that you get tired if you hold up four bricks above your head but you could just as easily put them on a shelf and you wouldn’t have to give the shelf any food or petrol or pay for an electric motor to hold up the shelf.’ ‘Yes you can use some of your food to clench your fist and grip your mobile phone, and you can go on gripping it for as long as you like. Food is not really needed to keep up the job of gripping. You could do the gripping by tightening up a gripping device with an electric motor and then stopping the motor while it, the gripping device, continued to grip. That would go on holding the phone, free of charge, as long as it stayed there, just like the ‘hands free’ device in a car.’
• In these examples, fuel-using jobs in which forces move are jobs in which energy is transferred. Invite students to give their own suggestions (e.g. cycling uphill, climbing a mountain, stirring custard or sweeping the floor). Note that energy is stored chemically in food and oxygen.
• The stretched spring in 3 and 4 has something like the energy that fuel and oxygen stores. Instead of using fuel to drive an engine or using food to keep a person going you could use a stretched spring to pull on a string and haul up a load or do some other useful job. So a stretched spring or a wound up clock spring stores energy elastically. You do work on the spring when you stretch it, but not while it is attached to the supports and is stationary.
• There is a problem when discussing the holding up of bricks, that may arise from 5 or 6. This is because muscles are not stationary objects and they keep ‘firing’ in and out of tension and in so doing become warm. You would need feeding in order to keep holding some bricks up. However the bricks could be put on a shelf and that would not need to be fed.
• A brick that has been raised up stores energy gravitationally because of work done by a moving arm or motor.

This experiment was safety-tested in March 2006.

Demonstration

How the vertical position of a brick determines how much energy is stored gravitationally.

Apparatus and Materials

• Bricks, 3
• Pulley, single, on clamp
• G-clamp
• Compression spring, large
• Cord
• Retort stand base
• Mass, 0.5 kg
• Hardboard sheet, approx 500 mm x 100 mm
• Retort stand rod, long

Health & Safety and Technical Notes

In all activities where bricks may fall on toes, precautions (such as using cardboard boxes full of waste material) should be taken.

Read our standard health & safety guidance

Procedure

1. Start with a brick on the bench and ask students to describe it. They will probably give a lot of details but not mention its position. Now place the brick on the floor and ask for a description again. This should extract the relevance of the brick's position to its ability to do a job for you.

2. Place some folded newspapers on the floor as protection and let the brick drop onto them. Make sure that the acceleration of the brick is obvious.

3. Tie the brick securely with the cord. Pass the cord over the pulley and allow the falling brick to raise a load.

4. Support the spring in a clamp from a retort stand and attach the brick, via the cord, to the spring. Allow the brick to drop gently. Let students observe the oscillations that take place.

5. Support the hardboard bridge on two bricks and place the third brick on top. Ask students to describe the arrangement. Now lift the third brick sufficiently so that when it is released the hardboard bridge cracks when it is hit.

Teaching Notes

• These simple experiments are intended to introduce the idea that energy can be stored in different amounts gravitationally, depending on their vertical position in relation to other objects and the Earth's gravitational field. Formally, this is described as their gravitational potential energy. At an introductory level, you could call it energy stored gravitationally.
• Also common to these demonstrations is the energy of a moving object, formally kinetic energy. At an introductory level, you could call this energy stored kinetically.
• What you might be saying, then, with each part of this demonstration is:
• 1, 2: "When the brick is higher up it can do a job for you that it cannot do when it is on the floor. There is more energy stored gravitationally when it is in a higher position.'
• 'When the brick is attached to another load across a pulley and is allowed to fall, it exerts a force on the other load, which rises. There is now more energy stored gravitationally of the load, and less stored gravitationally in the brick.
• "This time the raised brick is attached to a fixed spring . When the brick is allowed to fall it does work on the spring, which stretches. Afterwards, there is energy stored elastically in the extended spring. In fact, a vertically supported spring-brick system will continue to oscillate up and down, with energy shifting between two ways that it is stored."
• "When the brick is raised it stores energy gravitationally. If the brick is dropped onto the bridge it does work on the bridge, which may be enough to break it. There may also be some sound produced, and some heating. Ultimately the extra energy stored gravitationally is stored thermally in the surroundings. This is dissipated energy, which cannot be used to do another job."

This experiment was safety-tested in December 2005

Demonstration

This series of activities is designed to give students experience of energy transfers.

Apparatus and Materials

• G-clamps, 5 cm, 2
• Malvern energy transfer kit
• Variable low-voltage power supply
• Mass, 0.5 kg
• Brick, single
• Cord
• Rubber band or driving belt

Health & Safety and Technical Notes

Read our standard health & safety guidance

The Malvern energy transfer kit is also available as individual components from Beecroft & Partners and other suppliers. Individual items may also be substituted for those in the kit.

All motors can be driven by up to 12 V and can also be used as dynamos. Do not allow the dynamo to labour under too heavy a load.

Procedure

1. Using the brick to operate a dynamo and light a lamp...

2. Clamp the motor/dynamo unit to the bench with a G-clamp. Fix the line-shaft unit next to it. Tie the cord round the brick and wrap it round the axle of the line shaft. Connect the output terminals of the dynamo to the lamp unit. When the brick is released it will turn the dynamo which in turn lights the lamp, or several lamps in parallel.
3. Using a battery to drive an electric motor...

4. Connect the motor to a 4-6 volt battery.
5. Using an electric motor to lift a load which then drives a dynamo to light a lamp(s)...
6. Clamp the motor/dynamo unit and the line shaft unit next to each other. Connect the pulleys on each with a rubber band or driving belt. Secure a cord to the axle of the line shaft and attach the lower end of the cord to a 1/2 kg mass. Connect a 4-6 volt DC supply to the motor/dynamo unit via a two-way switch. The switch is thrown so that it connects the power supply to the motor and the load is raised. When the switch is thrown back and the load allowed to fall, then the dynamo lights the lamps.
7. Using the motor to drive a dynamo, which can light a lamp...
8. Clamp the motor and dynamo units next to each other and join their pulleys with the rubber band or driving belt. Connect the motor to the power supply and the dynamo unit to the lamps.

9. Using the motor to drive a flywheel and the flywheel to drive the dynamo...

10. Clamp the flywheel next to the motor/dynamo unit. Connect the pulleys on each with a rubber band or driving belt. Connect the power supply to the motor via the two-way switch so that when the switch is thrown the motor turns the flywheel. When the switch is thrown back the flywheel turns the dynamo and the lamps light.
11. Using the water turbine to drive a dynamo, which lights a lamp. This requires practice...

12. Position the turbine/pump unit next to the motor/dynamo unit and clamp both rigidly with G-clamps. Connect the pulleys on the two units with a rubber band or driving belt. Connect the output from the dynamo to a lamp unit with one lamp. The water from the mains enters the turbine at the top and the pressure drives round the turbine blades, which in turn drives the dynamo. If the water pressure is not very great, some form of force pump will be necessary to increase the pressure.
13. Using a pump to raise water...

14. Clamp the motor/dynamo unit next to the turbine/pump unit, which in turn is clamped next to the head of water unit. Connect the pulleys on the motor and pump units with a rubber band or driving belt. Apply 4-6 volts d.c. to the motor. This will drive the pump unit which takes water from the lower level to the higher one. (It is necessary to prime the pump by filling with water before use: this is achieved by sucking on the third connection to the pump unit with a finger over the output and the input under water.)

Teaching Notes

• You can set up a selection of these activities as a circus so that groups of students can progress round them. At each station students record the main energy transfers, saying where the energy is stored at the start, where it is stored at the end, and the processes by which it is transferred.
• In step 1, energy is initially stored chemically in muscles (due to food and oxygen), and at the end energy is stored gravitationally in the raised brick. When the brick is released, the force of gravity pulls it downwards. Just before it hits the floor the all (or very nearly all) the energy that was stored gravitationally is now stored kinetically. When the brick hits the floor it exerts a force on the floor warming it up. The floor and the surrounding air may be caused to vibrate and a sound will be heard too.
• When the raised brick is connected to the motor/dynamo unit and allowed to fall then the lamp lights. The dynamo produces an electric current, which flows through the filament of the lamp. The filament becomes hot and glows, radiating light to the surroundings. At the end, the energy that was stored gravitationally is now stored thermally in the surroundings.
• In 2 the energy stored chemically in the battery is transferred by the electric current to the motor, causing the motor to spin.
• In 3 the rotating motor produces a force on the line shaft, raising the load. There is less energy stored chemically in the battery, and more stored gravitationally in the load. When the load is allowed to fall, it exerts a force on the dynamo which makes it spin. The dynamo produces an electric current, which flows through the filament of the lamp. The filament becomes hot and glows, radiating light to the surroundings. At the end, the energy that was stored chemically in the battery is stored thermally in the surroundings.
• In 4 the power supply produces an electric current in the motor, making it spin. The motor exerts a force on the dynamo, via the belt, so that the dynamo spins and produces an electric current which flows through the filament, which gets hot and the lamp then radiates light to the surroundings.
• In 5 the power supply produces an electric current in the motor, making it spin. The belt attached to the motor produces a force on the flywheel, which also spins. When the belt is transferred to the dynamo it produces a force on the dynamo causing it to spin. When it spins a current is produced, which flows through the filament, which gets hot and the lamp then radiates light to the surroundings.
• In 6 water from a high reservoir (feeding the water main) or a pressurized water pump flows through the turbine, turning the turbine blades before flowing out of the system. There is more energy is stored gravitationally in the water because the water reservoir is at a higher level compared to the ground. The falling water exerts a force on the turbines, making them spin. A belt produce a force on the dynamo, which produces an electric current, which flows through the filament, which gets hot and the lamp then radiates light to the surroundings. At the end, the energy that had been stored gravitationally ends up stored thermally in the surroundings.
• This demonstration depends critically on the water pressure and the pressure may only be high enough to turn a small generator and drive a very slender elastic band. If the lamp will not light, it can be removed and replaced with an ammeter to show that there is a small current flowing.
• In 7 an electric current transfers energy from the power station to the motor, producing a force on the motor, which spins. A belt can be used to produce a force on the pump, which produces a force on the water and raises it up. If the power station burns coal, then energy has been transferred by an electric current and forces. Energy that was stored chemically is now stored gravitationally. This is the reverse of demonstration 6.
• Steps 6 and 7 model a pump-storage hydro-electric power station such as at Dinorwig. There is only enough water in the reservoir to operate for about 3 hours. When there is spare capacity on the grid the water can be pumped back up to the reservoir. Power stations like this can be brought up to generating speed in a few seconds to cope with a dramatic rise in demand for electricity.

This experiment was safety-tested in November 2005.

Demonstrations

Energy transfers using a model steam engine.

Apparatus and Materials

• Steam engine unit (A model steam engine is included in the Malvern energy transfer kit)
• Motor/dynamo unit
• G-clamp, 5 cm
• Line shaft unit
• Mounted pulley and shaft light cord, about 1 m
• Mass, 0.5 kg
• Lamp unit
• Driving belt

Health & Safety and Technical Notes

The Head of Department should ensure only staff that have been trained to use the school's steam engine(s) are allowed to do so.

Technicians also need instruction on maintenance.

All steam engines must be examined regularly as specified in the written scheme of examination (see CLEAPSS guidance).

Those models designed to use the laboratory gas supply should have been disposed of years ago. Methylated spirit burners should have been replaced with those designed for solid fuels.

Read our standard health & safety guidance

The steam engine must be operated using solid fuel, or where designed for it, liquefied petroleum gas.

When the steam pressure is high enough, turn the flywheel by hand until the condensed vapour has been expelled. The engine will now run freely.

Procedure

1. Clamp the engine to the bench with a G-clamp and clamp the line shaft next to it. Join the small pulley on the engine to the large pulley on the line shaft with a belt. Attach a length of cord to a mass on the floor (about 1/2 kg ) with the other end attached to the line shaft. The engine will raise this load, increasing the energy stored gravitationally, and reducing the energy stored chemically in the gas supply or solid fuel and oxygen.

2. Remove the load from the steam engine so that it accelerates.
3. Remove the line shaft and clamp the motor/dynamo unit next to the steam engine. Connect the two with the drive belt. Connect the output of the dynamo unit to the lamp unit. Unscrew the lamps so that 1, 2, or 3 lamps are connected. This will produce a change in the mechanical load on the steam engine.

Teaching Notes

• Avoid talking about a 'chain of energy transfers' and instead, the teacher needs to decide choose specific start and end points, and discuss the ways in which energy is stored.
• When the fuel is burnt, energy stored chemically in the fuel is transferred. There is less energy stored chemically. One end point could be when water and cylinder have heated up, so that the energy is stored thermally in the water in the pressure cylinder.
• Alternatively, the end point could be when the pistons are accelerating. The water vapour produced increases the pressure on the pistons, and the force produced makes the pistons accelerate. The energy stored chemically is now stored kinetically.
• In procedure 1, the belt and line exert a force on the load, and a 1/2 kg load will be raised. The energy stored chemically is now stored gravitationally. When heavier loads are put on the line-shaft the steam engine will slow down because has a fixed power, which is the rate at which it can do work. If the load is removed all together as in 2, the steam engine speeds up.
• In procedure 3, the dynamo produces an electric current which transfers energy to the lamps. The lamp filament warms up and light is radiated to the surroundings. The more lamps that are connected the slower the steam engine runs and the dimmer the lamps are. If too many lamps are connected the steam engine might stall. It is effective to use two or three low voltage bulbs in parallel. With all the lamps alight, the engine labours heavily; with none connected, it races.
• It is worth pointing out that the overall energy analysis of the steam engine is one where the energy stored chemically is decreasing (as fuel and oxygen are used up), and the energy stored thermally in the surroundings is increasing (as the surroundings heat up).
• This may be a good time to discuss the generation of electricity from various fuels and the reason why power cuts can happen when too many people try to run too many appliances at the same time. For example, on Christmas Day, it can take longer to cook lunch because everyone wants to do it at once. System managers must ensure that a power station does not stall and the a.c. frequency does not fall too low (this would damage sensitive equipment).

This experiment was safety-tested in November 2005

Demonstrations

Examples of apparatus and demonstrations that can be used to illustrate different energy transfers and generate discussion.

Apparatus and Materials

Various described below

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

The list below uses a wide range of apparatus and activities. Which ones you use will very much depend upon the apparatus that you have available and the interests of your students.

1. Use a battery to light a lamp.
2. Drive a bicycle dynamo by hand to light a lamp: The dynamo can be driven at different speeds and gearing to a light a lamp.
3. A thermocouple and galvanometer: Attach a piece of iron wire (about 24 SWG) about 75 cm long between two similar lengths of bare copper wire (also about 24 SWG) by twisting the ends together. Scrape the iron wire to obtain a good electrical contact with the copper. Connect the free ends of the copper wires to a demonstration galvanometer with, if necessary, a resistance box in the circuit. Keep one of the two junctions cool in a beaker of water at room temperature whilst the other is heated gently with a flame. (About 1,500 microvolts is the maximum likely to be reached.)
4. Heat a piece of platinum wire with a Bunsen burner until it is white hot.
5. Grind a handle round against a friction brake until it warms up. (An old-fashioned apparatus for measuring the mechanical equivalent of heat (J) could be used.)

6. Hit a piece of lead with a hammer: Wind a small piece of sheet lead about 3 mm thick round a thin piece of iron wire which will serve as a handle. If the sample is any more massive the temperature will not rise significantly. (A piece of lead pipe should not be used because it is too thick.) Place the lead on a sturdy base such as an iron kilogram mass to serve as an anvil. If necessary, you can place a felt pad or a newspaper under the anvil.
The hammer should be raised and allowed to fall (not hammered) on the lead a number of times. The temperature can be detected by holding the lead to the lips or cheek. It is also possible to use the thermocouple from 3 to measure the temperature rise. One junction is be inserted between the lead and its wire holder and the other connected to a sensitive galvanometer.
7. Light a match.
8. Light a firework rocket: Do it as the last thing in the lesson, outside, making sure that the class is standing well back.
9. Add acid to alkali to produce a temperature change: Use an electrical thermometer or computer sensor.

10. Coupled pendulums: Place two retort stands about 50 cm apart and fasten a light cord between them. Suspend two identical pendulums symmetrically from the cord so that they are about 25 cm apart. Set one of the pendulums swinging and let students observe the motion.

11. Releasing a weight on a spring: Hang the compression spring from a retort stand firmly clamped to the bench. Hang a simple hardboard platform about 30 cm square from the end of the spring by four strings so that it is horizontal. Drop a brick onto the platform and observe what happens.
12. Torsional pendulum: Use the equipment in the last demonstration (11) as a torsional pendulum. Stop the vertical motion and give the platform a twist.
13. Flywheel of variable inertia: This consists of a rod symmetrically mounted so that it can rotate freely about an axis as shown and carrying two equal masses whose positions on the rod can be altered. Mount the whole on a stand so that the rod can rotate in a vertical plane. Tie a 1/2 kg mass to a string and wind the other end around the axle in such a way that in falling, the weight will cause the rod to rotate.

14. Inertia operated toys, and clockwork toys: There are many fun-to-operate, cheap toys which will motivate students to engage in energy transfer discussions.
15. Photographic exposure meter in which light produces an electric current.

Teaching Notes

• In these discussions it is important to choose start and end points carefully when talking about the way or ways that energy is stored. Separate out the processes producing the transfer (an electric current flows, a force does work etc.) from the energy analysis.
• Step 1, the cell produces an electric current which heats the lamp filament. The hot filament glows, white-hot, and radiates light and infrared radiation to the surroundings. Energy stored chemically in the cell is now stored thermally in the surroundings.
• Step 2, you apply a force to rotate the dynamo, which produces an electric current. The current flows in the filament of the lamp, which warms up and radiates energy to the surroundings as electromagnetic waves. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• Step 3, when two dissimilar wires are twisted together and the junctions kept at different temperatures an e.m.f. (voltage) will be generated between the junctions, which can be measured by a sensitive galvanometer (Seebeck effect). The current produces a force on the hairspring inside the galvanometer, which moves the needle to show the current. Energy stored thermally in the junctions is now stored elastically in the hairspring.
• Step 4, the gas burns and heats the platinum and the surroundings. The platinum warms up until it is so hot that it radiates light into the surroundings. Energy stored chemically in the gas and oxygen is now stored thermally in the surroundings. (You can use cheaper iron wire.)
• Step 5, turning the handle of a friction brake or even rubbing your hand along the table top will produce a temperature increase wherever there is friction between contact materials. The temperature of the materials increases. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• Step 6, here you use a force to lift the hammer, and the force of gravity pulls the hammer down. The hammer exerts a force on the lead, heating it up. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• This experiment is modelled on one carried out by Hirn in the 1850s. He used 350 kg hammer moving at 5 m/s to smash into 3 kg block of lead held against a 1 tonne anvil. It is often said lead is used because it has a small specific thermal capacity. However, it also has a large density so that the thermal capacity per unit volume for lead is only a little less than for other metals. The real reason is that the collision between the hammer and the lead is inelastic, and most of the energy of the falling hammer is transferred to it.
• Step 7, when a match is struck, friction warms up the chemicals in the match head and the match lights. The atoms of the surrounding air move faster and spread-out away from the match. The air gets hotter. Energy stored chemically in the match head and the oxygen is now stored thermally in the surroundings.
• Step 8, when the chemicals in the firework ignite they heat the air, and produce a force on the fragments which move. The firework emits light, and the heating of the air produces sound. The air slows down the fragments of firework. Energy stored chemically in the firework head and the oxygen is now stored thermally in the surroundings.
• Step 9, adding an acid to an alkali causes a chemical reaction, and the temperature of the product increases. Energy stored chemically in the acid and alkali is now stored thermally in the product and the surroundings.
• Step 10, when one of the pendula is set oscillating (by using muscles to displace it) its amplitude gradually dies down but the string exerts a force on the other pendulum whose amplitude increases. (The pendula must be the same length, as this is a resonance effect. Note that the two pendulums are not in phase.) Eventually the pendula will be at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• Step 11, you use a force to lift the brick, and the force of gravity pulls the brick when it is dropped onto the platform. The platform exerts a force on the spring, which extends. The spring exerts a force on the platform, which moves up. The air exerts a force on the platform and brick, reducing the amplitude of the oscillation. Eventually the platform and brick are at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• Step 12, you use a force to twist the pendulum, which deforms the wire. The force produced by this deformation twists the wire back, and it continues to oscillate. The aire exerts a force on the platform and brick. Eventually the platform and brick are at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
• Step 13, compare the effects of the 1/2 kg falling when the masses are at the inner ends of the rod and at the outer ends. As the weight falls it exerts a force on the rod, which causes it to rotate. The way that it rotates depends on the distribution of the masses. Energy stored gravitationally is now stored thermally in the surroundings.
• Step 15, when light falls on the exposure meter, an electric current is produced which affects the display. The display emits light which heats the surroundings. Energy stored chemically (e.g. in the fuel in the power station and oxygen) is now stored thermally in the surroundings. If you are using sunlight, then the energy wasstored in the nuclear fuel in the Sun.
• In demonstrations (4), (5), (6), (7), (8), and (9) energy is transferred from various sources so that it ends up warming up the system. In most instances the temperature rise is very small and so the energy appears to have disappeared at the end of the process. These activities are examples of useful energy being dissipated so that it is difficult to use it. It is also difficult to reverse the process. For example, the energy stored chemically in a match head (and oxygen), when struck, warms up the surrounding air and the atoms move a little faster and spread away from the match. In order to reverse the process the faster moving atoms would need to be collected together in order to warm up the chemical constituents of the match head and so reform the original match.

This experiment was safety-tested in August 2007

Demonstration

The classic experiment to show that light can radiate across a vacuum, but sound requires a medium through which to travel.

Apparatus and Materials

• Vacuum pump
• Clapper bell in round flask
• Nichrome wire (26 SWG), reel of
• T-piece
• Hoffman clip
• Rubber bung to fit flask
• Bourdon gauge
• Voltage supply, low, (typically 12 V at 6 A)

Health & Safety and Technical Notes

Vacuum pumps are heavy and should be lifted on or off a trolley by two persons. Rotary vacuum pumps are reliable if maintained properly. See CLEAPSS Laboratory Handbook for details.

Round-bottom flasks are much less likely to implode than flat-bottomed or conical ones. However, safety screens should always be used to protect demonstrator and class.

Read our standard health & safety guidance

Wire the clapper bell to a length of rubber tube and suspend it inside the flask. Do not use wire supports – these will transmit too much sound out of the flask and make the demonstration less impressive.

Procedure

1. Shake the flask so that the bell can be heard ringing.
2. Evacuate the flask and shake the bell again to demonstrate that the sound can no longer be heard.
3. Re-admit the air and the bell can be heard once more.
4. Remove the bell and fix a coil of nichrome wire inside the flask. Connect the Bourdon gauge via a T-piece to record the pressure.

5. Connect the coil to the variable voltage supply. Turn up the voltage until the filament glows.
6. Pump out the air and show that the glow from the wire is still visible.

Teaching Notes

• The main point of the demonstration is to show students that sound waves require a material medium for their transmission whereas light can be transmitted through a vacuum. As the air is evacuated, the sound of the bell becomes gradually quieter. With a good vacuum and careful suspension of the bell (see technical note), the sound will cease, even though the bell is still seen to be ringing. By contrast, students see the nichrome wire glowing with or without the air.
• Point to the reading on the Bourdon gauge to convince students that air really is being removed from the flask. Alternatively, if you cannot obtain a Bourdon gauge, you could later open the flask with its mouth under water, and watch it fill up with water.
• Energy is tranferred from the Sun to the Earth as electromagnetic waves. Some of it is visible light, but our eyes are not sensitive to most of the radiation. All of this is transmitted across empty space at a very high speed (3 x 10⁸ m/s). There is no actual stuff moving along with the motion of the light. The energy is transferred by waves in which there is a variation in the electromagnetic field.
• Sound also travels as waves, but sound waves need a medium in which to travel. The sound wave produced by a bell is transmitted across the air in the flask, through the glass itself, and through the surrounding air, as a longitudinal wave. The wave causes vibrations in the ear. These in turn transmit signals to the brain. The energy stored due to the vibration of the bell decreases, and the surroundings heat up.
• When the air is pumped out, there can be no convection currents. The glow of the nichrome wire may be brighter than before, as the wire will be hotter.
• The sound from the bell will grow fainter even without a vacuum, not because the few remaining molecules cannot carry a sound but because the bell cannot move the molecules of air when they are so far apart. This is often referred to as a poor ‘impedance’ match between the bell and the thinner air.
• A school-quality vacuum pump cannot produce a perfect vacuum. The reason that the bell becomes inaudible is more subtle than the explanation you will want to give to most students. With air pressure inside the flask just 1% of its normal value, some sound will reach the inner surface of the flask. The boundary will reflect almost all of the sound, because of the change in density and wave speed (air to glass). At advanced level you may explain this in terms of the difference of acoustic impedance in air and glass, and the intensity reflection coefficient. This also explains why a gel is used between an ultrasound probe and the patient's skin.

For more information, see the article Warning Bells by Frank Harris in the School Science Review, September 2005

This experiment was safety-tested in November 2005.

Demonstration

A simple analogy that could be used with small groups or with individual students finding it difficult to understand the concept of energy stored gravitationally.

Apparatus and Materials

• Brick, single
• Cardboard box, filled with screwed-up paper or containing polystyrene packaging

Health & Safety and Technical Notes

Ensure that the brick catcher is in the correct place with the student's feet wide apart.

Read our standard health & safety guidance

Procedure

1. Ask a student to stand with their feet apart, holding the brick in their hands. As they raise and lower the brick they can imagine the brick to be attached to a long spring, fixed to the centre of the Earth.

Teaching Notes

• When you discuss examples involving energy stored gravitationally, students will often ask questions about where the energy resides. This demonstration can help students think of the energy stored in the gravitational field that connects the Earth with the raised load, by telling them an artificial story.
• You might introduce the story with these words:
"Imagine a spring connected between the load you are holding and the centre of the Earth. There is no real spring but the pull of that stretched spring is just an imaginary idea to help you think about the way the Earth pulls on the load."
• Ask the student holding the brick to shut their eyes and imagine the spring connecting the brick to the Earth.
"Feel how heavy the brick is. Feel its weight. Feel how the Earth pulls it down. If you don't believe the Earth pulls it down, let go and see what the Earth does to the brick.
• (Protect the floor and the student at this point!)
"Pretend to yourself that the pull of the Earth, which you can feel, is the pull of a long stretched spring that is attached to your brick and runs through a hole in the ground to the centre of the Earth. You will find it difficult to imagine that spring if you keep your eyes open and can see that there is no spring there. So now shut your eyes and think about that spring." "Raise the brick up, holding it with your two hands. As you haul the brick up, you can feel that spring s-t-r-e-t-c-h-i-n-g. Keep your eyes shut, lower the brick, and let the spring contract a little. Now pull the brick up and stretch the spring. Were you able to imagine the spring?"
• If students can visualize this long spring being stretched, some may use their knowledge of springs to expect gravity to increase with height above the Earth, (because the force of the spring grows as it stretches). If so, you need to explain that this imaginary spring, all the way to the centre of the Earth, is so long that ordinary stretches would not make it change its strength. Then, to avoid the story being misleading, you should add a warning that the real gravitational spring pull of the Earth gets weaker as you go farther out. This is not the time to go into any inverse-square story.

This experiment was safety-tested in November 2005