Power
Energy and Thermal Physics

Power

for 14-16

James Watt devised the concept of the rate of doing work to help market his improved steam engine. And so the unit of power (rate of transferring energy) is the watt. In industrial societies, measuring power continues to be a useful way of comparing machines of all types.

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Measuring the power of a lamp

Power
Energy and Thermal Physics

Measuring the power of a lamp

Practical Activity for 14-16

Class practical

Calculating the energy transferred per second from a lamp.

Apparatus and Materials

For each group of students...

  • Power supply, LV
  • Lamp 12V 6W
  • Lamp holder on base
  • Ammeter (0 - 1 amp), DC
  • DC voltmeter (0 -15 volt)
  • Variable resistor, optional

Health & Safety and Technical Notes

Read our standard health & safety guidance


The rating of the lamp is chosen so as to provide reasonable current and voltage readings. Any lamp that produces similar values to a (12 V 6 W) lamp is suitable. Remember that, on switch-on, a lamp draws several times the rated current: the power supply must be able to supply this.

Procedure

  1. Connect the circuit shown and take readings of the ammeter and voltmeter. Calculate the energy transferred electrically each second.

Teaching Notes

  • To give more practice in making calculations of power, a variable resistor can be included in the circuit. Students take a series of readings and compare them with the brightness of the lamp.
  • The table could be labelled as shown:
    • Current in amps (charge flowing in coulombs per second)
    • Potential difference in volts (energy transferred in joules by each coulomb)
    • Power (= energy transferred electrically from the power supply per second, in joules/second or watts (Power = VI)
  • Working out the units is a useful check on what is happening in the circuit in terms of the physics.
  • How Science Works extension: Students could be asked to design an experiment whereby they calculate the efficiency of the energy transferred electrically to light. Discussion will likely identify the difficulty in quantifying the amount of light produced. What should also emerge is that the amount of light radiated can be inferred by calculating the energy stored thermally. A possible approach is to put the lamp in a sealed polystyrene cup filled with air (or even water) and measure the temperature rise.
  • The specific thermal capacity of air at constant pressure is about 1,000 J/kg 'C, and that of water is 4,200 J/kg 'C

This experiment was safety-checked in January 2007

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Comparing the powers of lamps

Power
Energy and Thermal Physics

Comparing the powers of lamps

Practical Activity for 14-16

Demonstration

The intensity of illumination from different lamps is compared.

Apparatus and Materials

  • Lamp holders (SBC) on base, 2 (Safety pattern batten lamp holders are best)
  • Mains lamp, 230V 25W, pearl
  • Mains lamp, 230V 60W, pearl
  • Screen and post

Health & Safety and Technical Notes

Safety pattern lamp holders incorporate a switch so that when the bulb is removed the contacts become dead. The lamp holders must use a double-insulated mains lead fitted with a proper (13 A) mains plug, and a suitable fuse.

Read our standard health & safety guidance


Procedure

  1. Switch the lamps on and compare the brightness of their illumination.
  2. Stand the lamps in front of the post so that the shadow of the post falls on the screen. If the lamps are lit one at a time, the apparent intensity of the shadow gives a good comparison of the lamp intensities.

Teaching Notes

  • The two lamps are rated at different wattages. This means that they will transfer energy at different rates. A different number of joules per second are carried from the power supply to the lamp.
  • Students may wish to see the markings on the lamps to confirm the power ratings.

This experiment was safety-tested in February 2006

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Investigating light intensity from a lamp

Power
Energy and Thermal Physics

Investigating light intensity from a lamp

Practical Activity for 14-16

Demonstration

Measuring how the light intensity of a lamp varies with power input.

Apparatus and Materials

  • Power supply, LV
  • Lamp, 12 V 24 W
  • Lamp holder (SBC for LV use)
  • Rheostat (10 - 15 ohms)
  • Demonstration meter (0 - 15 volts DC)
  • Demonstration meter, 0 - 5 amp DC
  • Light meter (or photographic exposure meter)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Digital light meters suitable for education use can be obtained from several electrical equipment suppliers for £30 or so.

Procedure

  1. Set up the circuit shown with the light meter some 10 - 15 cm from the lamp.
  2. Record values from the light meter, together with ammeter and voltmeter readings.
  3. Calculations of the power input (energy transferred electrically per second to the lamp) should range from 10 to 30 watts (corresponding to a change of 7-14 volts).
  4. Plot a graph of light intensity against power input.

Teaching Notes

  • The graph of light meter readings against power input is a very important one when the efficiency of lamps is being investigated. This graph should give approximately a straight line, not passing through the origin.
  • This is an opportunity to discuss the power rating of devices. This is the power input when the device is connected to the potential difference for which it has been designed. It will not transfer energy at the same rate if connected to a lower (or higher) potential difference.
  • The light intensity is a proxy for the power output (the energy transferred per second by the light).
  • The light meter may need to be placed further from the lamp. For a 100-watt lamp, meters will be needed recording up to 250 volts AC and 500 mA. AC.
  • Energy efficient lamps could also be tried but if high voltages are needed, then suitable precautions need to be taken.

This experiment was safety-tested in January 2006

Up next

Measuring the power of a motor

Power
Energy and Thermal Physics

Measuring the power of a motor

Practical Activity for 14-16

Class experiment

Measuring the power input to a motor.

Apparatus and Materials

  • For each group of students
  • LV power supply, variable DC
  • Motor/generator unit
  • Ammeter (0 - 1 amp), DC
  • DC voltmeter (0 - 5 volt)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

Power input

  1. Set up the circuit as shown in the diagram with no load on the motor other than friction.
  2. Adjust the voltage supply to give the appropriate voltage for the motor.
  3. Note the readings of the ammeter and voltmeter.
  4. Calculate the number of joules transferred each second.
  5. Efficiency of the motor
  6. The experiment can be extended by using the motor to drive a generator which lights 1, 2, or 3 lamps in a lamp unit. Connect the voltmeter across the voltage supply and use the ammeter to measure the current to the motor. Connect 1, 2 or 3 of the lamps into the circuit. Note the change in input power. Now place the voltmeter across the generator terminals, and the ammeter in series with the lamps. Record the results in a table and calculate output power.

Teaching Notes

  • The power transferred to the motor is calculated by measuring the current in the circuit for various potential differences across the motor. You might wish to use the heading of ‘charge flowing per second, in coulombs per second’ as well as ‘current in amps’. Similarly you might want to use ‘energy transferred in joules by each coulomb’ as well as ‘potential difference in volts’.
  • Electrical power is calculated from P = potential difference x current = VI
  • The power, transferred to the motor from the power supply, is then expressed in joules/second or watts.
  • In step 3, the efficiency of the energy transfer is calculated from

This experiment was safety-tested in January 2006

Up next

Comparing the powers of electric motors 1

Power
Energy and Thermal Physics

Comparing the powers of electric motors 1

Practical Activity for 14-16

Demonstration

Comparing the power output of two motors.

Apparatus and Materials

  • Electric motor, small
  • Switch unit
  • Mass, 1 kg
  • Line shaft unit
  • Power supply, 0-12 V
  • Fractional horse-power motor (or the largest DC motor available)
  • G-clamps
  • Cord
  • Rubber band or driving belt
  • Knife switch
  • Stopwatch
  • Metre rule

Health & Safety and Technical Notes

The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)

To stop the motor, the current must stop flowing in both field and armature coils. The power supply may incorporate a smoothing capacitor which discharges slowly when the mains switch is operated.

Read our standard health & safety guidance


Procedure

Small electric motor

  1. Connect the small electric motor to the line shaft using the drive belt. Attach the mass to the line shaft with the cord.
  2. Connect the motor to the variable DC supply using a switch and set at the appropriate voltage for the motor so that the load rises slowly. (Although the knife switch is not essential, it allows the power to be connected for a more precise time.)
  3. Time the load being lifted a fixed distance, say 1 metre.
  4. Connect the field and armature terminals of the fractional horse-power motor in parallel to the DC terminals of the power supply set at 6 volts (or the rated voltage for the motor).
  5. Attach the mass to the shaft by cord, so that it is lifted when the motor is switched on, as before.
  6. Note the time for raising the load through 1 metre and compare this with time taken for the small motor.

Teaching Notes

  • The time taken to raise the load is shorter when a higher potential difference is used. The motor is said to be ‘more powerful’. The loads can be changed and the potential difference kept constant; larger loads take longer to raise. There is only so much energy per second that a motor can transfer for a given potential difference, and so it works more slowly with heavier loads.
  • Change in energy stored gravitationally = m x g x Δh joules
  • Power output to raise the load = m x g x Δh / t watts
  • Where m is the mass in kilograms, Δh is the vertical distance in metres, g is the gravitational field strength in newtons per kilogram, and t is time taken in seconds.
  • Energy transferred from a power supply to a component, such as a motor = V x Q joules
  • Power input due to current in motor = V x Q / t = V x I watts
  • Where V is potential difference in volts, Q is charge in coulombs, I is current in amps and t is the time taken in seconds.
  • The field coils of the fractional horse-power motor also have energy transferred to them. For a more accurate comparison with other motors, measure the potential difference across the armature and the current passing through it.
  • If you compare the small motor and the fractional horse-power motor, both running on 6 volts and hauling up the same load, the larger motor is faster. Measuring the current through the armature will show that the current is also greater through the heavier motor.
  • Another variation is to run the two motors at the same voltage, but with different loads, so that the speed with which the load is raised is the same for both motors. The bigger motor will have a larger current passing through it, and will haul up a larger load.
  • The motors can also be compared by measuring the braking force to stall the motor. Increase the load until the motor stops turning. (Do not keep it long in a stalled position.)

This experiment was safety-tested in January 2006

Up next

Comparing the powers of electric motors 2

Power
Energy and Thermal Physics

Comparing the powers of electric motors 2

Practical Activity for 14-16

Demonstration

Comparing the power output of two motors.

Apparatus and Materials

  • Small electric motors of different powers, 2
  • Line shaft unit
  • Knife switch
  • Mass, 1 kg
  • Power supply, LV
  • Rubber band or driving belt
  • Cord
  • Stopwatch & metre rule

Health & Safety and Technical Notes

The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)

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

Read our standard health & safety guidance


The motors suggested are part of the Malvern Energy Conversion Kit, now obtainable from the supplier: Beecroft and Partners.

Procedure

  1. Clamp the two electric motors to the bench on either side of the line shaft unit, so that the driving belt can be connected to either of them. The line shaft is used for lifting the load of 1 kg.
  2. Link one of the motors to the line shaft and connect it to the power supply.
  3. Measure the time taken to raise the load from the floor to the bench.
  4. Repeat for the second motor.

Teaching Notes

  • The same amount of work has to be done in both cases. The same amount of energy will be stored gravitationally in the raised weight in both cases. The energy is transferred electrically from the power station by the power supply and motor to the raised weight, but the faster motor is the more powerful.
  • Labels on the motors may indicate the horse-power or wattage. The motor does not always supply this power, (the maximum recommended). Instead the motor adjusts itself to the load. If more power than that is demanded the motor will overheat and perhaps burn out. Its efficiency is also likely to be lower.
  • It may be worth indicating that animals, including humans, adjust to the load as well; watch how slowly someone staggers when they have to lift something heavy. Safety rules try to prevent people hurting themselves. (Injuries to backs may not manifest themselves for many years.)

This experiment was safety-tested in January 2006

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Climbing stairs

Power
Energy and Thermal Physics

Climbing stairs

Practical Activity for 14-16

Class practical

Estimating the change in energy stored gravitationally in climbing a flight of stairs.

Apparatus and Materials

  • Bathroom scales
  • Tape measure, 5 m

Health & Safety and Technical Notes

Since this activity does not require the time taken to be measured, a delegation could measure the height while the class remains in the laboratory or classroom.

Read our standard health & safety guidance


Procedure

  1. Each student must know their mass in kilograms, use the scales to determine it, or use an 'mass of an average person'.
  2. Measure the height of the stairs from the ground floor to the top of the highest step.
  3. Each student climbs the flight of stairs and calculates the increase in the energy stored gravitationally.

Teaching Notes

  • When someone climbs the stairs energy is transferred from their muscles (energy stored chemically in food and oxygen) to be stored gravitationally. The change to the energy stored gravitationally can be calculated using the equation: change in energy stored gravitationally = mgΔh.
  • Where mg is the weight of the climber and QuantitySymbol{Δh} is the vertical distance climbed.
  • You may need skills in crowd control if other classes are not to be disturbed and students are not to get hurt. Students may try to turn this experiment into a demonstration of power, by timing how fast they can get to the top of the stairs!
  • Students may also ask why they feel fatigued when they hold up a load but the load doesn't move. In order to grip the load, muscles are kept tensed; they squeeze the blood vessels and restrict blood flow. As a result the chemical products of muscular activity accumulate and are not washed away so quickly by the blood. This accumulation of chemical products makes the nerves give a sense of fatigue. So the feeling of fatigue is chiefly an indirect result of the muscle tension.
  • The continuing demand while you hold a load at rest arises from the muscle fibres developing tension very rapidly. In a large muscle, fibre after fibre is fired into tension as a nerve impulse arrives but each fibre soon relaxes. Later on it renews its tension again. So the steady pull of a muscle is really the result of many brief tugs, a dynamic force. The sum total of these pulses of force shows tiny statistical fluctuations like a trembling effect.
  • There is a transfer from energy stored chemically to energy stored thermally. Releasing the tension does not transfer energy back to the muscles.

This experiment was safety-tested in January 2006

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Student power

Power
Energy and Thermal Physics

Student power

Practical Activity for 14-16

Class practical

Students measure their personal power by running up a flight of stairs.

Apparatus and Materials

  • Stopwatch
  • Metre rules, 2
  • Bathroom scales, calibrated in kg (or N)
  • Flight of stairs, not less than 3m in height, and preferably much more

Health & Safety and Technical Notes

Those with health difficulties may need to be given other jobs to do. Check on students in advance of the lesson.

Before using this activity, the teacher must assess carefully the likelihood of the runner slipping or any other accident on stairs. If the risk is serious, consider substituting a properly-supervised climbing activity in the gym.

Read our standard health & safety guidance


Procedure

  1. Measure the height of the staircase in metres.
  2. One student runs from the bottom to the top of the stairs, whilst another student times them with the stop-watch.
  3. Students who do not know their weight in newtons can use the scales to find their mass in kilograms and hence calculate their weight.
  4. Repeat the experiment, but with students walking up the stairs at a speed which they guess they could keep up for an 8-hour day.

Teaching Notes

  • This experiment will generate noise and discipline problems unless it is well organized. Students will wish to compete against one another to find the most powerful person. Plan in advance where students will stand and how other staff and/or students can pass by whilst the experiment is in progress.
  • Students measure their own useful power from:
    • Power in watts = (weight in newtons x height of stairs in metres) / time in seconds
    • Where the weight in newtons = mass in kilograms x g (take g to be 10 N kg-1)
  • A very short staircase and a flying start can give astounding values of power which are quite misleading. A short burst of effort cannot be sustained. Adult males may come close to one kW for a short spurt to about 75 watts for continuous labour.
  • Look at dietary charts (or packets of crisps or whatever) for energy stored in food and calculate how many cream cakes it takes to climb the stairs or do a manual job for 8 hours. Or how many stairs can be climbed on one cream cake.

This experiment was safety-tested in January 2006

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Power cycling up a hill

Power
Energy and Thermal Physics

Power cycling up a hill

Practical Activity for 14-16

Class practical

Measuring the power used in cycling uphill.

Apparatus and Materials

None listed

Health & Safety and Technical Notes

This must be done in a safe place, such as the school grounds.

Read our standard health & safety guidance


Procedure

  1. Students time themselves cycling up a rise.
  2. Determine the vertical distance climbed from an appropriate map or using a handheld GPS device. Alternatively, estimate the height with a sighting pole, a plumb line, and a large cardboard protractor.
  3. Knowing their own weight and estimating that of their bicycle, students calculate the output of useful power.

Teaching Notes

  • Useful power is calculated from:
    • Useful power = energy transferred/time taken = (total weight in newtons x height rise)/time taken
    • The mass of a bicycle is very unlikely to be less than 5 kg nor more than 20 kg . 10 kg would be an acceptable estimate, particularly as the mass of the student is probably of the order of 70 kg.
  • This experiment could be set as a class activity if a suitable site is available.

This experiment was safety-tested in January 2006

Up next

What affects the output of a solar panel?

Power
Energy and Thermal Physics

What affects the output of a solar panel?

Practical Activity for 14-16

Class practical

This activity explores factors which affect the power output of a solar panel, and may lead to an investigation of quantitative aspects of some factors.

Apparatus and Materials

Each group will need

  • Solar panel unit
  • Small motor unit
  • Desk lamp (40 or 60 W tungsten lamp)
  • Digital multimeter (or voltmeter)
  • Plug-plug 4 mm leads (red), 2
  • Plug-plug 4 mm leads (black), 2
  • Metre rule
  • Piece of cardboard
  • Translucent sheets (e.g. tracing paper) cut a suitable size to cover the solar panel
  • Coloured filters
  • Clamp stand

Health & Safety and Technical Notes

Desk lamps with metal shades can get very hot. Take care when moving them.

Place the colour filters far enough away from the lamp so they don’t burn.

Be careful if considering other types of lamp such as halogen lamp and fluorescents because they can emit significant UV.

Read our standard health & safety guidance


The activity works best if done in a room with blackout or low light levels. Otherwise ambient light may overwhelm the variations in light levels being controlled.

If students are to measure the distance of the lamp from the panel, make a reference mark on the lamp casing level with the centre of the light bulb.

You can obtain solar panels and a small motor unit from:

Middlesex University Teaching Resources


  • SEP 050 Solar panel unit
  • SEP 051 Large solar panel unit
  • SEP 052 Small solar panel unit
  • SEP 054 Small motor unit

Procedure

  1. Connect a solar panel to an electric motor. Shine a desk lamp on the panel so that the motor turns.
  2. Connect a voltmeter across the solar panel.
  3. The voltmeter can be used to get an idea about the output of the solar panel. A voltmeter does not measure power (power = voltage x current), but the voltage can be used to make comparisons.
  4. Explore the effects of:
    • Moving the lamp closer and further away
    • Partially covering and uncovering the solar panel
    • Tilting the panel backwards and forwards
    • Putting a translucent sheet between the lamp and the panel
    • Putting different coloured filters between the lamp and the panel.

Teaching Notes

  • A solar power system for a house is not always going to give the same power output. It will depend on the time of day, the season, the weather, and so on.
  • In this activity, students use a range of investigative skills, including the identification of different kinds of variable, the tabulation of data and drawing different kinds of graphs and charts.
  • This can be done in small groups as a qualitative activity to get a feel for the factors that affect the output of a solar panel, in which case the activity may only take up part of a lesson.
  • Students should find that the maximum output is given with a high light intensity and the biggest surface area, and with no tilt in relation to the light source. The translucent sheet and coloured filters reduce the power output.
  • Student could go on to a quantitative investigation of particular factors, done in pairs and taking longer. Different groups could each look at a different factor, and then report back to the whole class. Possible relationships to investigate:
    • How does the distance of the lamp affect the voltage across the panel? A suitable range of distances is from 10 to 25 cm (if using just the motor); with the voltmeter, a suitable range of distances would be from 10 to 50 cm.
    • How does the area of the panel exposed affect the output voltage?
    • How do different filters affect the output voltage? for example: glass, clear plastic sheet, white paper
  • Ask students to explain how their results relate to real conditions with the Sun. You could have students take their panel outside and look at the effect of tilting it, of partially covering it or of using different kinds of filters.

This experiment is copyright of Gatsby Science Enhancement Programme, and is reproduced with permission.

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Power and energy

Power
Energy and Thermal Physics

Power and energy

Teaching Guidance for 14-16

There are many occasions when you might want to know how fast energy is being transferred:

  • an electric motor driving a sewing machine or a lathe
  • an immersion heater in a water tank warming up the bath water
  • sunlight concentrated by mirrors on a boiler of water to produce steam
  • a loudspeaker emitting sound waves
  • your own body lifting itself, or a weight.

You may want to know how much energy is transferred in a day so that you know how much fuel has been used, and so calculate the size of a fuel bill.

The rate that energy is being transferred is called power.

Efficiency

The efficiency of a machine is a measure of how much energy is transferred to the machine (from, for example, energy stored chemically in fuel and oxygen) and how much is then transferred to do a useful job (to, for example, energy stored gravitationally when a lift full of people is lifted up).

Efficiency = useful energy transferred from the machineenergy transferred to the machine x100%

Machines are not 100% efficient because energy is dissipated to the surroundings; warming it up. These energy ‘losses’ can be reduced but never eliminated.

‘Wasted’ energy

Cars and power stations need cooling systems; the energy dissipated needs to go somewhere. There is a tendency for energy transfers to be lopsided. Energy stored thermally in a high temperature furnace can be used to do work. Energy stored thermally in the surroundings (at a lower temperature) cannot. A kettle of boiling water can run a model steam engine; but emptied into a bath of cold water it will only provide a tepid bath which could not run a steam engine. The same amount of energy is there but it is less available, less useful.

Power range of an electric motor

Machines have a maximum power at which they operate, which is a trade-off between the load and the time they take to do the job. If a motor is spinning without any load being raised then the useful output power is zero; all the input power is being used to fan the air and warm it up a little. If the motor is stalled, by too heavy a load, its useful power is again zero. Between these two extremes the motor has a wide range of adjustable power transfer behaviour.

The watt and its origins

The SI unit of power is the watt. A watt is not just an electrical unit even though we come across it most frequently applied to electrical devices. Car engines can be rated in watts too.

Before the age of steam engines, machinery used to pump water from mines was driven by horses. The business partnership between Matthew Boulton and James Watt, in the late 18th century, has been described as follows:

“Boulton’s idea was that he would sell something that no one had ever sold before – power. He actually used those words; he wrote to Empress Catherine of Russia saying, ‘I am selling what the whole world wants: power’. And this is how he did it. He sent his people down to Cornwall to say: ‘We are offering engines on these terms. Our firm, Boulton & Watt, will set up the engines, free, gratis and for nothing, at your mine. We will service them for the first five years and all we are asking in return is one-third of the difference between the cost of coals and the cost of hay for the horses that would have to do the same amount of work.’ Well, the mine owners thought he was obviously crazy but they accepted the offer.”

“Now of course came the disputed question of how much work a horse could do. …Watt measured the amount of work a horse could do by making a horse pull something lifted over a pulley. He conceived of the idea of work being the product of force and distance and of power being the rate of doing work.” [J D Bernal (1973) The Extension of Man: A history of physics before 1900 . Paladin pp 270, 271]

In modern values, 1 horsepower = 746 watts. To give a ‘feeling’ for the size of a watt, it is about the amount of energy transferred per second by a rat. So a watt is about 1 rat-power.

The kilowatt hour

A common energy unit is used by power companies to measure the amount of energy transferred by the electrical devices that consumers use. This unit is the kilowatt hour. This means that energy is being transferred at a rate of one kilowatt for an hour. (The power unit is multiplied by time to give an energy unit.)

1 kWhour = 1,000 x 60 x 60 = 3,600,000 joules.

Humans can work steadily at a rate of about 100 W. We pay about 8p for a kilowatt hour of energy transferred to us by electrical companies. If we were paid the same amount for labouring, that would be only 0.8p for an hour.

You could not live on a wage like that in countries where push-button controlled motors are in abundance. But in the developing world, where subsistence farming depends on manual labour, then this represents a real ‘currency exchange rate’. The industrialized world has created ‘power stations’ which act like slaves working for each of its citizens. A 1GW power station provides the power of 10 million slaves working at a rate of 100 W.

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

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

Energy sources

Power
Energy and Thermal Physics

Energy and generating electricity

Teaching Guidance for 14-16

In discussing energy in everyday life, mains electricity is almost certain to come up. Mains electricity transfers energy from power stations to devices we use every day. 

Electricity is generated using an resource such as a fossil fuel or uranium (a nuclear fuel). This is important to any discussion of the greenhouse effect, global warming, or climate change

Discussion of ‘resources’ for generating electricity might lead on to so-called ‘renewable’ resources such as wind, sunlight and waves. ‘Wind energy’ and ‘solar energy’ are in everyday use. However, it is more useful to refer to 'wind power' and 'solar power', as the 'power' designation more accurately refers to the process of transferring energy per second.

The process of generating electricity, or using any device that needs an electric current, will dissipate energy, and heat up the surroundings. The fuels used to generate electricity are used up in the process, and are used up more quickly if a lot of energy is dissipated.

‘Save it’

The approach described above provides a good platform for later discussions of 'energy saving', especially domestic energy saving. The idea of 'energy saving' can seem strange to students who understand that 'energy is conserved'. If it is conserved, why is there a need to save it?

It is best to link the discussion of 'saving energy' to energy dissipation. Using 'energy saving' devices actually means that less fuel is needed to achieve the same end. Less energy is dissipated.

There are good resources for making simple estimates of, for example, the rate of energy loss through insulated and non-insulated roofs. Also useful would be comparisons of the energy needed to heat water for a bath or a shower.

Electricity is the one case where quantities are known by common knowledge through powers of appliances. And calculations are easy, energy transferred = power x time.

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