Energy Transferred by Working
Energy and Thermal Physics

Examples of energy going from one thing to another

for 14-16

These experiments provide further examples of energy being transferred between different energy stores.

Up next

Simple kinetic energy transfers

Kinetic Energy
Energy and Thermal Physics

Simple energy transfers involving motion

Practical Activity for 14-16

Demonstration

Simple demonstrations to introduce the concept of energy stored kinetically.

Apparatus and Materials

  • Retort stand
  • Pulley, single, on clamp
  • Mass hanger and slotted masses (100 g)
  • Thread
  • Dynamics trolley
  • Spring, expendable
  • G-clamp, 10 cm

Health & Safety and Technical Notes

In all of these experiments a student should act as a trolley catcher to ensure no trolleys land on toes.

Read our standard health & safety guidance


Procedure

  1. Put the trolley on a bench and give it a push. Energy stored chemically (in food + oxygen) is now stored kinetically.
  2. Instead of a trolley you could, more impressively, use a student on a skate board (with due consideration to student behaviour and safety).
  3. Put a trolley on the bench. Fasten the pulley to the edge of the bench, running a thread over it from the trolley to a 100 g hanger. Let the load fall a short distance to the floor so that the thread falls slack, allowing the trolley to continue moving. Energy stored gravitationally is now stored kinetically.
  4. Repeat the demonstration, but in reverse. Start the trolley moving with a push away from the pulley. Let it pull the thread taut and lift the load as it comes to rest. Energy stored kinetically is now stored gravitationally.
  5. Before you use the expendable spring, stretch it until it is clearly an open, weak spring. Anchor one end (A) to the bench by slipping the end loop over the rod of a retort stand, which is itself clamped to the bench.
  6. Fasten 1 – 2 m of thread to the other end (B) of the spring. Then stretch the spring gently by at least several centimetres. Anchor this end (B) temporarily.
  7. Straighten out the thread and attach the free end to a trolley. Position the trolley so that the thread is taut. Energy stored chemically in muscles is now stored elastically in the thread.
  8. Release the end B. You need to have a long enough piece of thread for the trolley to travel 30 cm or so at constant speed after the spring has fully contracted.
  9. Repeat the experiment, but in reverse. Give the trolley a push so that it causes the spring to stretch. Energy stored kinetically is now stored gravitationally. If you make any compensation for friction, by tilting the board, this will have to be adjusted when you reverse the transfers.

Teaching Notes

  • The concept of energy stored kinetically is very important. Energy stored in fuels can be transferred to energy stored kinetically in rockets, gas molecules and anything else which is moving.
  • Before students can tackle calculations with energy stored kinetically, they need a clear picture of what energy stored kinetically is, and a good feeling for it. These simple qualitative demonstrations invite discussion. It is useful to use them before attempts to measure and calculate values for energy stored kinetically.

This experiment was safety-tested in November 2005

Up next

Energy carried by a moving trolley

Kinetic Energy
Energy and Thermal Physics

Energy and moving trolleys

Practical Activity for 14-16

Demonstration

Forces produce a transfer of energy so it is stored kinetically.  In Demonstration A, elastic bands store energy elastically. Demonstration B introduces energy stored in a magnetic field.

Apparatus and Materials

  • Dynamics trolley
  • Runway
  • Horseshoe magnets, 2
  • Dowel rod
  • Rubber bands

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons.

Retort stands should not be used to form a catapult unless they can be clamped to the bench.

If a trolley may fall from either end, fit a buffer across the end of the runway to catch it.

Read our standard health & safety guidance


The runway needs to be adapted by stretching a catapult across it. Here are two ways of doing this.

The first way is easy, but clumsy and not very rigid. See safety notes below.

  1. Set up two massive retort stands on either side of the runway. Clamp the stands to the bench. Stretch elastic between them.
  2. Here is a better arrangement for a satisfactory and simple catapult. Drill the runway with holes to take 15 cm lengths of 1 cm wood dowel. Stretch a 10 cm elastic band (0.3 cm width is satisfactory) between the two dowels.

An alternative to the trolley and ramp in Demonstration B is a train set with the appropriate track and wagons. In addition it is possible to fit compression springs to the wagons as shown below.

Procedure

    Demonstration A: Transferring energy between elastic bands
  1. Set up a catapult across each end of the runway (about 30 cm from the ends). Do this by stretching large elastic bands between the dowel rods (or retort stands) fixed at the sides of the runway as described above.
  2. Firmly fix a single vertical dowel rod on the trolley. Adjust the height of the elastic so that the vertical rod will catch the middle of it. (It is probably better to let the vertical rod engage the rubber band, although it is possible to have the rubber lower and let it engage the actual body of the trolley.)
  3. Place the trolley on the runway. Pull it back against one of the catapults so that the rubber is stretched by a measured amount. Then release the trolley so that it is projected by the catapult along the runway and strikes the second catapult. If you have aligned the apparatus well, the trolley will oscillate backwards and forwards for a few cycles, before it comes to rest due to frictional forces and hysteresis in the rubber band (warming it up).

Demonstration B: Transferring energy from a stretched rubber band to a magnetic field

  1. Firmly attach two powerful horseshoe magnets to two trolleys. Sellotape can be used for this. The magnets should be orientated so that the trolleys will be repelled as they approach one another.
  2. Place each trolley at an end of the runway. The back of the raised front of the trolley, below the magnet, should be touching the catapult. Pull the trolleys back by equal amounts. Release them so that they run towards one another, are brought to rest and are then pushed away again by the magnets, before returning to the catapults.

Teaching Notes

  • When the catapult is pushed back by the trolley in Demonstration A, the elastic band stretches. Energy is stored elastically in the elastic band. When the trolley is released the elastic band returns to its original length, and the trolley accelerates. Energy that was stored elastically is now stored kinetically in the trolley.
  • The trolley will continue at constant speed if no energy is dissipated. (Note that energy may be transferred by means of frictional forces, warming up surfaces and wheel bearings until it strikes the elastic band at the other end of the board. The elastic band will extend and the trolley will come to a stop for a moment. Energy that was stored kinetically is now stored elastically.
  • You might ask students to look at the extensions of the rubber bands to see whether the energy stored elastically in each band is the same. Note that this will only succeed if you have a very free-running or friction compensated arrangement.
  • You will need to rehearse Demonstration B carefully to ensure that the trolleys do not slew round as they approach one another.
  • Each trolley is pushed into its respective rubber band, so that energy is stored elastically in the band. When the trolleys are released, the elastic bands return to their original length, and the trolleys accelerate. Energy stored elastically is now stored kinetically in the trolleys.
  • As the magnets on the trolleys approach each other fhe force between the magnets increases, the trolleys slow down and will come to a stop. All the energy stored kinetically is now stored in the magnetic field.
  • The trolleys then bounce back and repeat the process.

This experiment was safety-tested in November 2005

Up next

The swinging pendulum

Energy Transferred by Working
Energy and Thermal Physics

The swinging pendulum

Practical Activity for 14-16

Demonstration

The simple pendulum used as a stimulus to discuss changes in the ways that energy is stored.

Apparatus and Materials

  • Metal or wood blocks to hold the pendulum cord, 2
  • Simple pendulum bob, 1 - 2 cm lead, iron or brass
  • Cord for pendulum, at least 1 m or longer
  • Retort stand, boss, and clamp
  • G-clamp

Health & Safety and Technical Notes

Any attempt to fix the support to the ceiling requires two persons: one to hold the ladder or steps.

The use of a brick as a pendulum bob would be unwise. The brick may rotate and present a rough edge or corner to the demonstrator.

Read our standard health & safety guidance


This demonstration is most successful when the energy dissipation is kept to a minimum. This can be done by using a massive support and by ensuring that the cord is firmly clamped. Clamp the pendulum cord between a pair of blocks with the G-clamp, keeping the lower edges of the two blocks flush.

If it is possible to clamp the cord from the ceiling, the support will be even better.

Procedure

  1. Clamp the pendulum to a rigidly held retort stand.
  2. Let the pendulum swing back and forward and encourage students to discuss the transfers taking place.

Teaching Notes

  • Do not do any timing of the pendulum, nor discuss the periodicity. Changes to the ways that energy is stored are the focus of this demonstration.
  • Students should look for the ways energy is stored. Energy is stored gravitationally at the top of its swing, and kinetically (and gravitationally) at the bottom of its swing. The change in energy stored gravitationally is equal to the energy stored kinetically if there is no energy dissipated.
  • Apart from at the top and bottom of the pendulum's swing, the bob has a mixture of energy stored gravitationally and energy stored kinetically. As the bob swings one gradually increases as the other decreases.
  • If the pendulum is fixed as described, it should rise to the same height on either side for a good number of swings. As long as this continues, all the energy stored gravitationally is transferred to energy stored kinetically and back again. The total amount of energy stored kinetically plus energy stored gravitationally remains constant.
  • This is one of the few demonstrations which, for a few oscillations before energy is transferred to the support, illustrates the Principle of Conservation of Energy.
  • An entertaining extension to this experiment is to hold the pendulum bob at nose height and to leave go of the bob, taking care not to push it. The bob will come back to the same point just in front of your nose, as long as you don't flinch. If the bob is more massive the effect is more dramatic, and illustrates that energy cannot be gained from outside the system.

This experiment was safety-tested in November 2005

Up next

Galileo's rolling ball

Newton's First Law
Energy and Thermal Physics

Galileo's rolling ball

Practical Activity for 14-16

Demonstration

Roll a ball on a curved track to explain Galileo's idea, which in turn led to Newton's First Law of Motion.

Apparatus and Materials

  • Large ball bearing (or large marble)
  • Retort stands, 2
  • Flexible curtain rail
  • Boss
  • Clamp
  • G-clamps, 2

Health & Safety and Technical Notes

The risks here are trivial provided the instructions are followed. If the ball is turned into a projectile, consider the use of eye protection.

Read our standard health & safety guidance


The flexible rail should be symmetrical and not too flimsy. An alternative to a curtain-rail is the flexible track used with Hot wheels toy cars.

A good way of supporting the curtain rail is to glue a 0.5 m wooden lath (1 cm square) to each end of the underside of the curtain rail. One end can be held with a retort stand and clamp, at a height of about 30 cm above the bench. The other end can be held in another retort stand or you may prefer to hold it by hand.

It is very important for the support to be rigid if the experiment is to be effective. Avoid energy dissipated by the rail moving.

Procedure

  1. Hold the ball bearing at the top of one end of the curtain rail and release it so that it rolls down one side and then up the other. Release the ball from each end in turn to see if any difference occurs.
  2. Tilt the curtain rail to various slopes, both equal and unequal and repeat the demonstration The experiment may also be tried with a horizontal length between the two slopes. Finish with a slope on one side and the other side horizontal.

Teaching Notes

  • When the ball is at the top of the track energy is stored gravitationally. When the ball is at the bottom of the track the energy is now stored kinetically. If no energy is dissipated through friction, the ball would return to its orginal height on the far side of the track, and briefly be at rest. This remains true regardless of the shape and slope of the track. In practise, energy will be dissipated. The ball will warm up, as will the track, and also sound will be produced.
  • When the steel ball rolls down the slope and along a horizontal level it continues at uniform speed (as long as there was no frictional force) because there is no force to do work on it. There is no change to the energy stored kinetically. It would go on for ever. This idea led Galileo to what we now call Newton's First Law:
  • 'If a body is at rest it remains at rest or if it is in motion, it moves with uniform velocity until it is acted upon by a resultant force.'
  • Galileo also used this experiment as a starting point for his theory of forces and motion on an inclined plane, which gave a hint towards Newton's Second Law F = ma.
  • If students are genuinely willing to blame friction for the failure of the ball-bearing to reach its original height, the demonstration will have worked. If they accept the excuse because you tell them to, the demonstration is probably not worth pursuing.
  • See also the collection on this site:

    Inertia and Newton's First Law


This experiment was safety-tested in November 2005

Up next

Energy carried by an electric current

Energy Transferred by Working
Energy and Thermal Physics

Energy transferred by an electric current

Practical Activity for 14-16

Demonstration

An electric current transfers energy.

Apparatus and Materials

  • Lamp, 240 V 100 W
  • Lamp, 12 V 24 W
  • Lamp, 12 V 6 W
  • Lamp, 12 V 36 W and power supply
  • Stopclock
  • Demonstration meter with 0-5 amp DC and 0-5 amp AC ranges, 0-12 volt and 0-240 V AC ranges
  • Power supply, 12 V AC
  • Power supply, 240V
  • Socket complete with wattage and current meter (see Technical notes)

Health & Safety and Technical Notes

Do not build circuits for connection to the mains.

The simplest demonstration just uses a safety pattern BC batten lamp holder wired via a 5 A double-insulated flex to a 13 A plug.

If ammeter and voltmeter(s) are used, all connections must use shrouded 4 mm connectors. The connections to the mains can be bared wires using a safebloc.

Read our standard health & safety guidance


The lamps can be purchased from Beecroft and Partners or other scientific equipment suppliers.

An alternative is a device called cost plug, which is no longer on sale.

The socket with wattage and current meter is available from the supplier: Machine Mart.

Procedure

  1. Connect a simple series circuit of a 12 volt 24 watt lamp and demonstration meter to a 12 volt supply. Switch on for 15 seconds. Discuss the ways that energy is being transferred: an electric current is flowing, doing 'electrical work', and the filament is getting hot and emitting radiation.
  2. Repeat using the 12 volt 36 watt and again with the 12 volt 6 watt lamp.
  3. If an AC ammeter is available, repeat again, using the 12 volt AC terminals of the transformer.
  4. Connect the 240 volt 100 watt lamp using the commercial socket with in-built meters.

Teaching Notes

  • An electric current transfers energy stored chemically that was stored in the fuel (and oxygen) in a power station to the surroundings where it is stored thermally. When the electric current passes through the filament, the filament warms up. Its temperature rises so that it glows red or white hot. The energy is transferred from the filament as electromagnetic waves of visible light and infrared radiation which spread out into the surroundings, so warming up the environment.
  • The 5 W,24 W and 36 W lamps, operating from 12 V, glow with different brightness. The 36 W lamp is the brightest and so radiates most energy per second. The electric current in the circuit with the 36W lamp is also greatest. It appears that the energy radiated may be proportional to the current. (More experiments than this one need to be done before this can be substantiated!)
  • There is an advantage in setting up two circuits side by side e.g. 12 V 24 W bulbs, one on DC and one on AC. Students can see that AC makes very little difference.
  • The 240 V 100 W lamp and 12 V 6 W lamp carry the same current but more energy is radiated from the 240 V lamp per second, and this is indicated by its 100 W rating.
  • You have shown that the energy radiated depends not only on the current but also on the potential difference. It will also depend on how long the current is flowing:
  • Energy transferred by the electric current, E = V x I x t = VQ
  • The volt can be defined as the energy transferred per coulomb passing from a power supply to a component. This definition is frequently used when students are learning electrical concepts in the early stages.

This experiment was safety-tested in December 2005

Up next

Energy stores and carriers

Energy Transferred by Working
Energy and Thermal Physics

Storing energy and transferring energy

Practical Activity for 14-16

Demonstration

Simple demonstrations to stimulate discussion about the different ways in which energy can be stored: gravitationally, elastically, kinetically.

Apparatus and Materials

  • Large mass or block of wood (tied up with string)
  • Single pulleys on clamps, 2
  • Large spring
  • Dynamics trolley
  • G-clamps, 5 cm, 2
  • Retort stands, bosses and clamps, 2
  • Rough cloth, piece approx 30 cm x 30 cm

Health & Safety and Technical Notes

The retort stand should be clamped to the bench. Any attempt to attach a pulley to the ceiling requires two persons: one to hold the ladder or steps. Ensure no one walks beneath the suspended object.

Steps 2 and 3 require a trolley catcher.

Read our standard health & safety guidance


For demonstration 2, a better alternative to the pulley attached to a retort stand is a hook fixed in the ceiling. A single pulley can be attached to the hook.

Procedure

  1. Hang the spring from a retort stand and attach the large mass or block of wood to the lower end. Support the mass or block with your hand raised from the equilibrium position, release it and then catch it at its lowest point. Discuss the changes in the way that energy is stored - as suggested in the teaching notes.
  2. Clamp one end of the spring to the end of the table with a G-clamp. The two pulleys are clamped to the retort stand, which itself will need clamping to the bench. The cord is run round the two pulleys as shown in the diagram, but without attaching the free end to the spring. Stretch the spring horizontally and hold it in the stretched position while you discuss the way in which energy is stored. (See teaching notes.) Now attach the free end of the cord to the stretched spring. When the spring is released the unbalanced force from the spring accelerates the mass upwards; in energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored gravitationally increases (as the mass rises).
  3. Keep the spring clamped as in 2 and attach the other end to the dynamics trolley. Stretch the spring and hold it still. Then release the trolley. The unbalanced force will accelerate the trolley. In energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored kinetically (kinetic energy) increases (as the trolley speeds up). What is more, the increase in energy stored kinetically is the same as the decrease in the energy stored elastically. Energy is conserved.
  4. Lay the cloth on the bench as in the diagram. Give the trolley a push so that when it runs over the rough patch of cloth it slows down and stops. You can discuss the transfer of energy to the cloth as suggested in the Teaching notes.

Teaching Notes

  • Demonstrations 1 and 2 involve energy stored gravitationally.
  • Take care in defining where is the zero of gravitational energy. It is best to refer to changes in energy stored gravitationally rather than absolute values. Students are often mystified when they discover that, if you raise a mass from the floor the energy stored gravitationally will increase, but that if a trap door is opened in the floor then the mass could fall further and the energy stored gravitationally can decrease. When the mass is on the floor, the system stores more energy gravitationally with respect to the basement but less with respect to the ceiling. Before you start, think carefully and avoid confusing statements regarding the gain or loss of gravitational potential energy.
  • In 1 the system is storing energy gravitationally at the start. Although the spring is pulling up on the mass, to begin with the gravitational force downwards is bigger than the upwards force from the spring. The net downwards force causes the mass to accelerate downwards. As it moves downwards, the spring will begin to stretch.
  • In energy terms, the energy stored gravitationally decreases; the energy stored kinetically increases and the energy stored elastically increases.
  • At the equilibrium point, the spring's force balances the gravitational force. Beyond that point, the upwards force from the spring is greater than the downwards gravitaitonal force. So the mass slows down (it is accelerating upwards whilst moving downwards). As it does so, the spring is stretched more. In energy terms, the energy stored kinetically is decreasing (as is the energy stored gravitaionally) as the energy stored elastically is increasing. When it reaches the bottom, the mass is no longer moving and the system is storing energy elastically.
  • The stretched spring is still pulling upwards on the mass with a force bigger than the gravtiational force. So there is still a net upwards force on the mass. Therefore it begins to accelerate upwards. It overshoots the equilibrium position, and the process repeats.
  • In each cycle, the stretching and compressing of bonds in the spring will result in its temperature going up a little. And, in turn, the spring will raise the temperature of the surroundings (by heating). Therefore, in each cycle, it does not go quite so high because some of the energy has been transferred to the surroundings and there is less energy to be stored gravitationally.
  • Over time, the spring will come to rest. At this point, the energy stored thermally by the surroundings has increased by an amount equal to the energy stored gravitationally by the system before the mass was released.
  • Initially, you might say: "I have lifted the mass from its rest position so the energy stored gravitationally has increased. What will happen to this energy when I let it drop?"
  • When you catch the mass at its lowest point you might ask: "How is the energy stored now? Will the mass stay here when I let it go? If not, why not?" You should then allow the mass to oscillate for a few cycles. Ask again for an explanation of the changes in the way in which energy is stored in the system.
  • When you stretch the spring horizontally the energy stored elastically increases. Pause at this stage for students to consider where and how that energy was stored before (stored chemically (food + oxygen) in your muscles).
  • When the stretched spring is attached to the mass, around the pulley, the force from the spring lifts the mass. In energy terms, the energy stored elastically has gone down and the energy stored gravitationally has increased.
  • In demonstration 3, the energy stored elastically decreases and the energy stored kinetically (due to the moving trolley) increases.
  • In demonstration 4 when the trolley is given a push, the energy stored chemically (food and oxygen) in muscles decreases and the energy stored kinetically increases.
  • When the trolley moves over a rough surface it is slowed down by friction. There is a very slight rise in temperature as the sufaces rub over each other. When the trolley comes to rest the energy stored thermally by the surroundings has increased and the energy stored kinetically - due to the moving trolley - has decreased.

This experiment was safety-tested in November 2005

Up next

Looping the loop

Circular Motion
Forces and Motion | Energy and Thermal Physics

Looping the loop

Practical Activity for 14-16

Demonstration

A fun demonstration of the relationship between energy stored gravitationally, and energy stored kinetically; it can also be used to consider the forces involved during a complete loop through a vertical circle.

Apparatus and Materials

  • Flexible curtain rail, 3m to 4m long
  • Ball bearing
  • Toy hot wheels kit (OPTIONAL)

Health & Safety and Technical Notes

Be very careful over working at heights when setting up this apparatus and when using it. No one should climb on stools or benches.

Read our standard health & safety guidance


Toys such as 'Hot Wheels' allow these demonstrations to be set up easily at floor level.

It is rewarding to construct a version of this apparatus, very well mounted with the top section detachable. A convenient method of mounting is to glue blocks of wood 3 cm x 3 cm x 3 cm at 30 cm intervals around the curtain rail. The blocks are screwed to the track with counter-sunk screws (so that the steel ball does not hit the screw-heads as it goes round the rail). The blocks could be drilled so that the holes fit the ends of clamps attached to retort stands with bosses (see illustration). Alternatively, 10 cm nails can be put through the holes in the blocks.

Bend the curtain rail so that the ball can 'loop the loop'. This is more effective with a 3.5 m length or even a 4 m one. The initial fall should be as steep as possible and the loop needs to be tight.

If you plan to discuss circular motion: the size of the gap should be such that while the ball is in the gap the parabolic motion it gains is approximately equal to the circular motion it would have had if the track was there.

A "Roller Coaster Physics" kit is now (October 2011) available from the supplier: Data Harvest. This allows the user to build a variety of models of roller coaster and investigate several related concepts. It includes a 62-page teachers' guide, with lesson plans and design briefs. Order number KX78880.

Procedure

  1. Use a tray of sand, or a good wicket keeper, to catch the ball.
  2. Use the apparatus to explore what happens when the ball is released from different positions on the track. (See teaching notes.)
  3. Extension for circular motion discussion: remove the top piece and release the ball from different heights until it completes the circle.

Teaching Notes

  • It is great fun to challenge students to set the steel ball off at the right height, h (assuming that no energy is dissipated, the ball bearing just goes round the loop when h = 5R/2). You might ask them why you need to store more energy gravitationally at the start than is needed to lift the ball to the height of the top of the loop.
  • Ask: "What makes the ball go round a circle? What pushes or pulls the ball with a real force to make it do that?"
  • Point out that there must be some inward force towards the centre of the loop
  • Ask: "What provides the force at the top of the loop B?" (the track and gravity). After removing a bit of the track in step 3, the ball will still go round the loop, provided the speed is right. Too fast and it flies upwards along a projectile (parabolic) path and too slow and it falls downwards along a projectile (parabolic) path.
  • Ask: "What provides the force at the sides halfway up the loop (A and C)?" (The track.) The effect of gravity pulls vertically and only slows the ball down a bit.
  • Ask: "What provides the force at the bottom of the track (D)?" (The resultant between the inward push of the track and the downward force of gravity.)

Up next

Helpful language for energy talk

Conservation of Energy
Energy and Thermal Physics

Helpful language for energy talk

Teaching Guidance for 14-16

Some ways of talking about energy are clearer and more helpful than others.

Energy stores

It is helpful to talk about energy stores. A spring, or a rubber band, can rather obviously store energy. You do work to stretch them (or to squash the spring), and you can get back pretty much the same amount of energy when they relax. These then are two of the best iconic examples for grasping what ‘potential energy’ is all about. It is energy in a mechanical store.

Many students find the term ‘potential’ confusing. They think ‘potential energy’ is somehow different from actual energy. Talking about energy stores offers a way of deferring the term ‘potential energy’ until later, for students who choose to continue studying physics.

You can similarly feel energy being stored when magnets are pushed together or pulled apart.

The example nearly all textbooks give of potential energy is perhaps the most difficult of all. It is the gravitational energy of a lifted mass. Now the energy is said to be ‘in’ the lifted object – as for a spring it is said to be ‘in’ the spring. If you have the courage, you could say that the energy is stored between the Earth and the lifted object (in the gravitational field). The trouble is of course that an external examiner might score that truthful answer as wrong because specialist understanding is not required at this level.

Another kind of energy store is a mixture of fuel and oxygen. In this case bonds between carbon and oxygen atoms can snap shut, releasing energy in a fire or explosion. It is common to talk about just the fuel – for example petrol – as the energy store, but do remember that for this chemical spring to snap shut, there must be oxygen too.

There are a limited number of energy stores:

  • chemical (e.g. fuel + oxygen)
  • kinetic (in a moving object)
  • gravitational (due to the position of an object in a gravitational field)
  • elastic (e.g. in a stretched or compressed spring)
  • thermal (in a warm object)
  • magnetic (in two separated magnets that are attracting, or repelling)
  • electrostatic (in two separated electric charges that are attracting, or repelling)
  • nuclear (released through radioactive decay, fission or fusion)

Energy carriers (or pathways) and energy transfers

It is often helpful to think of energy being carried from one place to another. For example, light carries energy from the Sun to the Earth. Light is not itself ‘energy’ – it is after all an electromagnetic wave, or a stream of photons (however you care to look at it). But energy does travel with the light. The same is true of radio waves. In a microwave oven microwaves carry energy from the microwave generator to the interior of the food. Other kinds of waves carry energy too, for example ocean waves.

Electric current in a circuit is another energy carrier. It is helpful to think about a power circuit as a way of moving energy from one place to another. The National Grid distributes energy from a number of power stations, via the wires and cables, to homes and factories.

It is often handy to think of moving matter as carrying energy, too. A strong wind delivers energy to a wind turbine. But, equally often, it is better to think of the moving mass as storing energy. A train has to be given energy to get it moving, and energy has to be taken from the train to stop it. This is what we call kinetic energy.

Energy carriers (or pathways, or transfers)

  • mechanically (when a force moves through a distance)
  • electrically (when a charge moves through a potential difference)
  • by heating (because of a temperature difference)
  • by radiation (e.g. light, microwaves, sound)

With all of these, we are interested in the rate at which energy is being transferred and not the amount stored anywhere.

You can use flow diagram representations to strengthen the distinction between energy stores and carriers, for example:

There are some very important scientific ideas in this way of looking at things. Among them are:

  • that energy tends, in most cases, to spread from a more concentrated store to more dispersed stores; and that this makes it less useful for doing anything more
  • that the energy often ends up warming the environment

Visit School Science Review for two useful papers: Richard Boohan Making sense of energy and Robin Millar 'Teaching about energy: from everyday to scientific understandings':

School Science Review


Up next

What’s wrong with ‘forms of energy’?

Energy Transferred by Working
Energy and Thermal Physics

What’s wrong with ‘forms of energy’?

Teaching Guidance for 14-16

Many textbooks and teaching schemes talk of ‘transforming’ energy, or of ‘converting energy from one form to another’.

This is a very common way of talking, but it has its problems. Particularly, it is in danger of saying nothing at all. For example, “A torch converts chemical energy in the battery to light energy”. All this says is that a chemical reaction happens and light comes out.

It is easy to teach and learn the language of ‘transforming’ energy. Students can translate throwing a ball into ‘muscle energy is changed into kinetic energy’. But it is dangerously close to being no more than a game of words.

Describing chains of energy transfers, for example with a steam engine or electric motor lifting a load, particularly tends towards this kind of talk. Yet the energy in the motion of the moving parts is irrelevant to an understanding of the overall process. If, for example, a drive belt was replaced with a lighter one that otherwise had the same mechanical properties, it would have less energy – but the process would continue as before. The energy of the moving parts is not a useful quantity to know.

It is more useful to focus on the initial and final energy stores. This puts the emphasis on where the energy is and why, not on renaming it once it goes from one thing to another.

In some situations, the ‘forms of energy’ approach can easily lead to incorrect analyses of processes. For example, many textbooks discuss energy transfers in a car moving along a level road. It is common to show stored energy of the petrol being transferred to the car (kinetic energy) and heating parts of the car and the surroundings. This is correct for the period when the car is speeding up. But once the car is going at a steady speed (and so has constant kinetic energy), all of the stored energy is ultimately heating parts of the car and the surroundings, in part by pushing air out of the way.

The really important thing is to work from very early on with actual quantities of energy, to do plenty of simple sums about amounts of energy and rates of delivery. This is where there is real payoff; where something is actually being said, and understanding has something to get a grip on.

Up next

Measuring energy transfers

Conservation of Energy
Energy and Thermal Physics

Measuring energy transfers

Teaching Guidance for 14-16

In physics, there is a standard way to work out how much energy has been transferred. It is to calculate the work done.

Work is done when an applied force causes something to move in the direction of the force.

ΔE = work done = force x distance moved in the direction of the force.

Notice that no energy is shifted in the two situations below:

  • when an object rests on a shelf – although the object has weight, there is no movement.
  • if the force is perpendicular to the direction of movement - e.g. a satellite in orbit around the Earth.

This equation leads to the definition of the SI unit for energy, the joule: 1 joule is the work done when a 1 N force moves through a distance of 1 m.

For example, a motor or a human arm might raise kilogram masses onto different height shelves. The change in energy stored gravitationally can be calculated using the formula,

ΔE = weight x Δh =mgΔh, where Δh is the vertical distance a mass m has been raised, and g is the gravitational field strength.

Energy and the human body

However, there is more than this to working out how much energy has been transferred. When you lift bricks your body also gets warmer, due to the energy from digested food. It does not look as if there is any “force x distance” here. But the energy that is transferred by heating to make it warmer can be calculated in this way, and can be measured in the same unit, joules. (See food packets, labelling portions in kJ.)

Human beings are only about 25% efficient for doing mechanical jobs. For every 1,000 joules of energy which are transferred from fuel stored in muscles, only 250 joules are transferred to raising a load or doing some other kind of job. 750 joules are stored thermally (the body warms up). Thermodynamics shows that muscles could be more than 70% efficient in transferring their energy to do useful jobs, but only if the action was conducted infinitely slowly. So when estimating the useful energy transferred from energy stored in food to muscles in order to climb the stairs, for an eight hour day, then the answer needs to be multiplied by four to find the demand on food.

When a 1kg mass is raised by a height of 1 metre, then 10 J of energy is now stored gravitationally . This can be obtained from four grains of sugar, a mini-snack. One grain of sugar is for doing work to raise the load, and three grains are for heating the room. If you raise 1 kg through a height of 1 m every second requiring 1 mini-snack per second then this is about 10 grams of sugar per hour. Not enough to allow you to eat a cream-cake or a bar of chocolate without putting on weight (i.e. mass)!

Transferring energy electrically

Energy transferred electrically is calculated using the equation ΔE = IVt , where I is the current, V is the potential difference and t is time.

Heating with friction

In frictional rubbing, a force moves over a surface, but just makes it hot. You measure the rise in temperature of the material, and how much of it is warmed up. Then, next time something gets warmer, you know what amount of “force x distance” or work would have been needed if the warming up had been done in this way.

Sooner or later you’ll need to tell a story about what “getting hotter” means, in energy terms. It just means that the invisible atoms or molecules are moving about faster. Energy is stored kinetically by a large number of molecules. And it isn’t easy to claim it back again, because they have shared it out randomly amongst a huge number of particles.

There are plenty of practical examples of friction making something hotter. Car (or bicycle) brakes are a case where we want to transfer the energy of a moving car stored kinetically as speedily as possible. Exercise bicycles let students feel how what seems a large amount of mechanical work done produces only what seems like a modest heating effect.

A key teaching point is not to let ‘friction’ become a kind of excuse for things not working properly. It’s the way that the work done by forces ‘gets inside’ matter.

Up next

The law of conservation of energy

Conservation of Energy
Energy and Thermal Physics

The law of conservation of energy

Teaching Guidance for 14-16

Energy is conserved. What does this really mean, and why is it true?

Water in a reservoir is more or less conserved. So the amount of water can always be calculated from the amount that was there some time ago, plus the amount that has come in, minus the amount that has gone out (you may have to take account of evaporation as well as water drawn off).

Another way of saying the same thing is that water can’t be made or destroyed. For there to be more, it has to come in; for there to be less it has to go out.

Energy is similar. If you take any volume of space, then the total energy inside that volume at a given time is always the amount that was there earlier, plus the total amount that has come in through the surface, minus the total amount that has gone out through the surface.

Another way of saying the same thing is that energy can’t be made or destroyed. For there to be more, it must have come from somewhere; for there to be less it must have gone somewhere else. This also means that energy is a calculable quantity. The practical teaching implication here is that it is important to do sums about energy changes – how much in, how much out – and not just to talk generally about it.

The conservation laws, such as the conservation of energy, give physics its backbone. They are not really statements of knowledge but they contain implicit assumptions and definitions. They are however tied to the natural world, and they contain experimental knowledge.

The emergence of energy physics

By the early 19th century, steam engines were widely used. Both physicist and engineers sought to understand them by developing a ‘theory of steam engines’. Through the 1840s, as part of this process, several key people developed the concept of energy and its conservation : Mayer, Joule, Helmholtz and Thomson.

Julius Mayer, a German physicist, was the first person to state the law of the conservation of energy, in an 1842 scientific paper. Mayer experimentally determined the mechanical equivalent of heat from the heat evolved in the compression of a gas (without appreciating that heat could be explained in terms of kinetic theory).

In 1847 another German physicist, Hermann von Helmholtz, formulated the same principle in a book titled On the Conservation of Force. By contrast with Mayer, Helmholtz did view heat as matter in motion. The idea of conservation arose from his interest in animal (body) heat. He may not have known about Mayer’s prior work.

Between 1839 and 1850 the English brewer James Joule conducted a remarkable series of experiments, seeking to unify electrical, chemical and thermal phenomena by demonstrating their inter-convertibility and their quantitative equivalence. His numerical results and conclusion were published in the Philosophical Transactions of the Royal Society with the title On the mechanical equivalent of heat.

William Thomson (later Lord Kelvin) took the next step, considering the problem of irreversible thermal processes, until that time simply a contradiction between Carnot and Joule. Carnot, in his 1824 theory of heat engines, had argued that heat could be lost; more recently Joule argued that energy was convertible from one form to another but could be destroyed. In Thomson’s 1851 scientific paper The Dynamical Equivalent of Heat, he contended that energy was "lost to man irrecoverably; but not lost in the material world". Thomson was thus the first person to understand that all energy changes involve energy dissipation.

From energy to thermodynamics

In the second half of the 19th century Thomson and other scientists (including Clausius, Rankine, Maxwell and, later, Boltzmann) continued to develop these ideas. Kinetic theory and the science of thermodynamics gradually became established, with energy conservation as its First law and energy dissipation as its Second law.

ONLINE COMMUNITY FORUM

Have a Physics Teaching Question?

Want to ask it in a safe, friendly, knowledgeable environment? TalkPhysics is an online community for anyone involved in the teaching of pre-19 physics.

Visit TalkPhysics