Energy Transferred by Heating
Energy and Thermal Physics

Thermal transfers

for 14-16

This collection is concerned with the transfer of energy from a hot body to a cooler one, known as heating. The approach we use is to stress how the energy is stored, how it is being transferred, and where it ends up.

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Comparing the thermal conductivities of different materials

Energy Transferred by Heating
Energy and Thermal Physics

Comparing the thermal conductivities of different materials

Practical Activity for 14-16

Demonstration

This experiment uses a commercially available apparatus to compare the thermal conductivity of different metals.

Apparatus and Materials

  • Ingenhousz's apparatus, or similar (see diagram below)
  • Paraffin wax

Health & Safety and Technical Notes

Read our standard health & safety guidance


The apparatus has a number of rods, each made of a different metal. Accompanying information will identify them.

The rods must be coated with wax. This can be done by one of the following methods:

  • Take the rods out of the water box and lay them in a chilled tin tray containing molten paraffin-wax. Remove quickly, hold vertically to allow the excess wax to drain off, and push them back into the water box.
  • Keep the rods in the water box. Paint each rod with a paint brush dipped in very hot molten wax. This produces an uneven, thick coating of wax, which must then be thinned by blowing a Bunsen flame up and down the rod. (This is a poor method, only successful in very skilful hands.)

A simple and easy-to-use set of conductivity bars is now available from:

Timstar


Four bars made of different metals are mounted on the same plastic support. Each bar has a liquid crystal strip showing temperature changes along the bar.

Timstar Laboratory Suppliers, Marshfield Bank, Crewe, Cheshire. CW2 8UY Tel: 01270 250459 Email: sales@timstar.co.uk

Procedure

  1. Fill the water bath with hot water. Note how far along the rods the wax has melted when the apparatus reaches a steady state.

Different designs are available. Generally they use a bath of hot water to heat the rods.

Teaching Notes

  • Note that the speed at which a particular temperature (such as the melting point of wax) travels along the bar when one end is heated is essentially the speed of the temperature waves. This involves specific heat capacity and density as well as conductivity. Thus a rod of lead makes a quick start in the race although it is a poor conductor, but the wax-melting will not have travelled far when a steady state is attained.
  • Energy is dissipated (stored thermally in the surroundings) from the surface of each rod. If, for a steady state, the distance from the heated end to the melting point of the wax is twice as great for rod A as for rod B, then rod A has only half the temperature gradient but twice the surface area for heat losses. So rod A must have four times the conductivity of rod B.
  • The versions of this apparatus currently available are of the static warm water type. The preferred form has steam passing continuously through the apparatus. This is left to attain a steady state. Slider rings indicate the progress of the melting wax.

This experiment was safety-tested in January 2007

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Convection currents in a beaker of liquid

Convective Heating
Energy and Thermal Physics

Convection currents in a beaker of liquid

Practical Activity for 14-16

Class practical

Observing convection currents in a beaker of water.

Apparatus and Materials

For each group of students

  • Bunsen burner
  • Tripod
  • Glass tube, approximately 150 mm long x 3 mm internal diameter
  • Pyrex beaker, 600 ml
  • Potassium manganate VII crystals

Health & Safety and Technical Notes

Read our standard health & safety guidance


You need to get the potassium manganate VII crystals into the beaker of water without colouring it unduly. Put a glass tube into the beaker so that it rests on the base of the beaker. Then drop the crystal through the tube.

Procedure

  1. Fill the beaker with water. Put individual crystals of potassium manganate VII on the bottom of the beaker, using the method suggested above.
  2. Heat the water gently over the Bunsen burner and observe the motion of the coloured water. For a clear effect, use a small flame and no gauze between Bunsen and beaker (Pyrex beakers do stand this).
  3. Note the path that the coloured water takes from the heater to the top of the water and back down again.
  4. When repeating the experiment you should always start with a new batch of cold water.

Teaching Notes

This experiment was safety-tested in August 2007

Up next

Convection in a test-tube of liquid

Convective Heating
Energy and Thermal Physics

Convection in a test-tube of liquid

Practical Activity for 14-16

Class practical

Watching convection currents in water.

Apparatus and Materials

For each student or group of students

  • Bunsen burner
  • Test-tube (hard glass)
  • Potassium manganate VII crystals
  • Glass tube, approximately 150 mm long x 3 mm internal diameter

Health & Safety and Technical Notes

This experiment is quite safe so long as the Bunsen flame is small.

Read our standard health & safety guidance


Procedure

  1. Fill the hard glass test-tube with cold water. When the water is still, put the glass tube into the test-tube so that it rests on the base of the test-tube. Drop a single, very small, crystal of potassium manganate VII through the tube so that it falls to the bottom, leaving little colour.
  2. Hold the test-tube near the top but not above water level. Heat it gently with a Bunsen flame at the bottom of the tube to see the streamers of dye begin to circulate. As soon as the tube is felt to be warmed by the hot water, stop heating.
  3. Empty the test-tube, allow it to cool, wash it and fill again with cold water. When the water is at rest, add another very small crystal without stirring.
  4. This time hold the tube at the bottom and heat with the Bunsen flame near the top of the tube, just below the water surface.
  5. Continue until the water at the top of the tube just begins to boil, but the bottom remains cool. No coloured streamers are observed.

Teaching Notes

  • If the test-tube is held at the top of the water and heated from the bottom, convection trails can be seen in the water. If the test-tube is held at the bottom and the water heated at the top, there are no convection trails. The water at the bottom stays cool. A piece of ice fixed (with gauze) at the bottom of the test-tube will not melt even when the water is boiling at the top. Melting has to await conduction of energy through the water, which is a slow process in comparison with convection.
  • The hot water transfers energy with it when it moves. That is the process of convection. It happens in a fluid whether it is a liquid or a gas. On a large scale it happens in the oceans and in the atmosphere, in hot water heating systems and in ventilation. Winds are just convection currents.

This experiment was safety-tested in March 2006

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Convection in a Bunsen flame

Convective Heating
Energy and Thermal Physics

Convection in a Bunsen flame

Practical Activity for 14-16

Class practical

Visualizing convection currents in air.

Apparatus and Materials

  • Compact light source (slide projector or other high intensity lamp can be used but needs dim-out), 100 W
  • Bunsen burner
  • Translucent screen
  • LT variable voltage supply (capable of 8 A at 12 V)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Light the Bunsen and put it about 1/2 metre in front of the compact light source. Cast the shadow either on to the wall or on to a translucent screen.
  2. Students should look at the shadow and discuss what they see.

Teaching Notes

The convection currents from the Bunsen flame can be clearly seen. The warm air rises up above the flame and falls down away from it.

This experiment was safety-tested in March 2006

Up next

Transmitting and absorbing radiant energy

Energy Transferred by Radiation
Energy and Thermal Physics

Transmitting and absorbing radiation

Practical Activity for 14-16

Class practical

Simple observations on transmitting and absorbing radiation.

Apparatus and Materials

  • Radiant heater, connected to the mains
  • Insulating sheet approximately 250 x 250 mm with hole
  • Glass plate approximately 250 x 250 mm
  • Retort stand, boss, and clamp
  • Slab of rock salt, approximately 250 x 250 mm

Health & Safety and Technical Notes

Supervise this experiment closely. Watch that students do not get too close to the heater.

Read our standard health & safety guidance


Procedure

  1. Set up the heater on the bench and connect it to the a.c. mains.
  2. Fix the insulating sheet in front of the heater with a retort stand, boss, and clamp. Adjust it so that the hole is level with the glowing element.
  3. Ask students to look at the element through the hole in the insulating sheet. They should hold the back of their hand near the hole to feel the radiation.
  4. Students should now use their cheek as a detector, about 25 cm away from the hole.
  5. Insert a book between cheek and hole to see what happens; move the book in and out several times.
  6. Move farther away from hole until the heating effect can only just be detected on the cheek.
  7. Repeat the experiment of inserting a book. The book can be put in by a partner either near the cheek or near the source. See if there is any difference due to the time taken for the radiation to travel.
  8. Insert the glass plate between the cheek and the hole. Move the plate in and out several times.
  9. This can be repeated using two sheets of the glass plate held together to give a thicker sheet. (Be ready to move out of the way quickly if it is too hot when the glass plate is removed.)
  10. If a slab of rock salt is available, replace the glass plates with the rock salt.

Teaching Notes

  • In these experiments, special detecting instruments such as thermopiles are deliberately avoided. Students use their cheeks or the backs of their hands as the detecting device.
  • Students should notice that the infra-red radiation appears to travel in straight lines through the hole. As they move their head from side to side, the heating effect is cut off and so is the light. Both heating effect and the light appear to travel together.
  • Also the intensity of the infra-red radiation varies with distance from the source, and the infra-red radiation does not pass through thick materials such as a book. The infra-red radiation appears to travel instantaneously when the heater is aligned with the hole because it has a high speed (the same as that of light).
  • Although visible light (also radiation) gets through the glass plate, the more abundant longer wavelength infra-red radiation does not appear to pass through the glass. Rock salt is translucent to visible light and transparent to infra-red.
  • Links can be made to greenhouses and sealed-up cars on a hot day. Radiation from the Sun reaches us across empty Space.

This experiment was safety-tested in April 2006

Up next

Absorbing radiant energy with different surfaces

Energy Transferred by Radiation
Energy and Thermal Physics

Absorbing radiation with different surfaces

Practical Activity for 14-16

Class practical

Using the hand as a detector to compare the absorption of infra-red radiation.

Apparatus and Materials

  • Radiant heater
  • Insulating sheet with hole
  • Retort stand, boss, and clamp
  • Crystallizing dishAluminium leaf (not kitchen foil)
  • Vegetable black
  • Paint brush
  • Methylated spirit

Health & Safety and Technical Notes

Only aluminium leaf should be used. Do not use the thicker aluminium kitchen foil, which has a higher thermal capacity.

Some pupils may burn their hands without realizing the danger, unless an appropriate warning is given. Appropriate words are given in the "Procedure" below.

Read our standard health & safety guidance


Procedure

  1. Set up the heating element with the insulating sheet in front of it. Ask a student to place the back of one hand near the hole for about 5 seconds. You might say:
  2. "Hold your hand close to the hole in the screen, with the back of your hand towards the red-hot heater. Notice what you feel. Only do this for a short time to decide what you feel: one . . . two . . . three . . . four . . . five ... about that long. Do not hold your hand longer than this in case you burn it."
  3. Cover the back of the student's hand with aluminium leaf and ask them to repeat this for a further 5 seconds. Now coat the aluminium leaf with a vegetable black paint. When the paint is dry, ask the student to place the back of the hand again near the hole for about 5 seconds.

Teaching Notes

  • To put the leaf on the student's hand, get them to clench their fist and moisten the back of their hand with clean water until wet all over. Lay a sheet of aluminium leaf gently on the wet skin, blowing on it to push it on to the skin. Ask the student to unclench their hand a little to stop the leaf from cracking.
  • To blacken the aluminium leaf, mix some vegetable black with methylated spirit to the consistency of thick soup. Apply with a soft paint brush on top of the leaf.
  • To get rid of the paint and leaf, students should hold their hand under a running tap. Students should not try to rub off the paint or leaf. This will rub soot into the hands which will be difficult to remove.
  • When infra-red radiation is absorbed by any surface, the surface is warmed up.
  • The hand absorbs most infra-red radiation when it is painted black, a medium amount when it is not coated with anything, and the least when coated with a shiny surface. In the last case the infra-red radiation is being reflected away.
  • White paper rather than aluminium foil stuck on the back of the hand and brought up to the heater may give a surprising result. The white paper and blackened foil absorb nearly the same amount of infra-red radiation, even though the white paper reflects most of the infra-red radiation. What is it about the respective properties of paper and aluminium foil?
  • How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and the absorption of infra-red radiation. You could:
    • Use the experiment to illustrate the concept of valid evidence.
    • Point out that, in a comparative analysis, numerical data is not always necessary.
    • Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
    • Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in January 2007

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Radiation from Bunsen flames

Energy Transferred by Radiation
Energy and Thermal Physics

Radiation from Bunsen flames

Practical Activity for 14-16

Class practical

A simple comparison of the infra-red radiation from two Bunsen flames.

Apparatus and Materials

For each group of students

  • Bunsen burners, 2
  • Insulating screens

Health & Safety and Technical Notes

Set up insulating screens with central apertures 1 metre from each flame. Then there is no danger of students putting their hands too close.

Read our standard health & safety guidance


Procedure

Light the two Bunsen burners and adjust them so that one flame is clear blue and the other yellow. Ask students to hold the back of their hand about 1 metre from the flames so that they can compare the energy radiating from each.

Teaching Notes

  • The object of the experiment is to decide which flame is hotter.
  • If the air-hole collars of the burners are easy to adjust, students may use a single burner and change the flame from clear to yellow.

This experiment was safety-tested in March 2006

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Radiation from black and shiny surfaces

Energy Transferred by Radiation
Energy and Thermal Physics

Radiation from black and shiny surfaces

Practical Activity for 14-16

Class practical

Comparing the infra-red radiation from a dull black and a shiny surface.

Apparatus and Materials

  • Copper sheet, mounted, (or use Leslie's cube, with mains immersion heater)
  • Thermopile
  • Light-spot galvanometer
  • Retort stand, boss, and clamp
  • Bunsen burners, 4
  • Methylated spirit
  • Vegetable black

Health & Safety and Technical Notes

Supervise this experiment closely. Watch that students do not get too close to the hot plate.

Read our standard health & safety guidance


The mounted copper sheet is made from a sheet of 5-mm copper, blackened on one side, with 1-cm iron rod handle secured rigidly with two nuts and bolts.

Give one side of the copper sheet a coat of vegetable black mixed with methylated spirit. Allow it to dry so that it has a dull black surface. Polish the other side of the plate so that it is bright. (Tarnishing in the flame is inevitable.)

Procedure

  1. Secure the copper sheet rigidly to the retort stand, using a boss. The sheet should be horizontal with the bright side downwards.
  2. Heat it vigorously with four Bunsen burners underneath until it is as hot as possible.
  3. Remove the Bunsen burners and turn the plate so that it is vertical. Avoid burning hand when doing this.
  4. As quickly as possible ask students to hold the back of their hand first near but not touching the bright side of the plate, then the black side, and then back near the bright side. (Teacher supervision essential).
  5. Alternatively, use a thermopile connected to a light-spot galvanometer as a detector of infra-red radiation.

Teaching Notes

  • The plate will need to be re-heated after every 6 to 8 students have tried it.
  • Radiation appears to come from hot surfaces. Copper is a good conductor and the two sides of the plate will be at the same temperature. You can bring your hand much closer to the shiny side than to the black side without burning it. More radiation is coming from the black side even though it is at the same temperature as the shiny side.
  • An alternative experiment is the Leslie cube, made of copper, whose four vertical faces are finished differently. Boiling water is placed in the cube and the radiation detected with the cheek as the sensitive detector. The temperature is too low for this to be impressive using the back of the hand: a thermopile or other detector is better. (The water may be kept boiling with a mains immersion heater.)
  • To make the distance comparison clear, both hands are brought up to either side of the plate. The hand on the shiny side can approach closer to the plate.
  • How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and infra-red radiation. You could:
    • Use the experiment to illustrate the concept of valid evidence.
    • Point out that, in a comparative analysis, numerical data is not always necessary.
    • Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
    • Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in August 2007

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Further experiments on radiation

Energy Transferred by Radiation
Energy and Thermal Physics

Further experiments on radiation

Practical Activity for 14-16

Class practical

Shows the properties of infra-red radiation.

Apparatus and Materials

For each group of students

  • Mains lamps and holders, gas-filled and vacuum-filled (60 watt). Please note this piece of apparatus is very difficult to find.
  • Copper calorimeter, large or a steam chest
  • Immersion heater (mains powered)
  • Vegetable black
  • Thermometer (0°-100°C)
  • Paper, white

Health & Safety and Technical Notes

If mains lamps are used, the holders should be the safety pattern where the contacts are isolated when the lamp is removed.

Read our standard health & safety guidance


Pre-focus (P13.5s) torch bulbs (2.37W) are available both vacuum and krypton filled. Distinguishing between these will require a sensitive detector.

Procedure

  1. Keep some water boiling inside a copper box using an immersion heater. Alternatively the box can be kept at 100'C by passing steam through it. One face is shiny; one face is dull black having been coated with vegetable black, one face is covered with white paper. Use the back of the hand to compare the radiation.
  2. Put a thermometer (0°-100°C) in a metal container filled with boiling water, and observe the rate of cooling. Do this first with a well-polished container. Then with a layer of vegetable black painted on the outside.
  3. Switch on a 60 watt gas-filled mains lamp and a 60 watt vacuum-filled mains lamp near each other. Ask students to decide, as a detective problem, which of the two has gas inside.
  4. Put a cheek near a mains lamp and switch it on and off to feel how promptly the radiation reaches the face.

Teaching Notes

  • Step 1 is a version of Leslie's cube and demonstrates the differing amounts of radiation emitted from differently coloured surfaces.
  • In step 2, the matt black can cools down most quickly because more radiation is emitted from it. Cooling curves could be plotted.
  • In step 3 the surface of the gas-filled lamp will be hotter. This is because of the energy transferred through the gas by conduction, although the energy transferred by infra-red radiation will be similar.
  • In step 4 the time lag is too short to distinguish because the radiation travels at a very high speed. (The speed of light.)

This experiment was safety-tested in April 2006

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Heating and cooling curves

Cooling Curves
Energy and Thermal Physics

Heating and cooling curves

Practical Activity for 14-16

Class practical

To introduce ideas of energy transfer by heating and thermal capacity.

Apparatus and Materials

For each student group

  • Datalogger with temperature sensor
  • 1 litre beaker
  • 250 ml beaker
  • Insulating jacket
  • Immersion heater
  • 1 kg metal block (e.g. aluminium) with bores drilled for heater and temperature sensor
  • Electric kettle or Bunsen burner to heat water rapidly
  • Mug(s), ceramic OPTIONAL
  • Cup(s), paper, polystyrene and plastic, with lids if possible OPTIONAL
  • Different insulating materials (e.g. expanded polystyrene, newspaper, wool) OPTIONAL
  • Instant coffee and tea bags OPTIONAL

Health & Safety and Technical Notes

An electric kettle is a much safer source of hot water than a Bunsen burner, tripod and gauze. However, immersion heaters also get hot and must be handled with care.

Read our standard health & safety guidance


Procedure

There are a number of things you can do with just temperature sensors.

  1. Cooling curves. Fill a beaker with hot water from a kettle. Record its temperature once a second for a few minutes. If possible, produce a graph directly.
  2. Compare cooling curves for beakers with different insulation, lids etc. Start each with water at the same temperature and record information from several sensors on the same graph.
  3. Heating curves. Place sensors and heaters in beakers with 1 litre water and 250 ml water, and a 1 kg metal block. Start the heaters at the same time and with the same voltage and record the temperature-time graphs, all on the same display.

Teaching Notes

  • These activities are excellent to emphasize the value of datalogging as the display is much easier to read than normal thermometers. Readings can be taken more often and with less chance of recording errors. Suitable software can produce an immediate graphical display to confirm that the data are being collected correctly.
  • Specific teaching points:
  • This experiment can be used to calculate cooling rates in °C per second. The flattening curve shows that the rate of decrease of temperature is lower as the temperature falls.
  • Without being quantitative, cooling curves which are produced live provide at-a-glance evidence for the effectiveness of different insulations.
  • Comparing different masses of the same material (water is easiest) shows how the same amount of energy transferred causes different changes in temperature that depends on the mass. This is an introduction to thermal capacity and to the difference between energy transferred and temperature.
  • Comparing the different materials (but same mass) is a further step on this road. The temperature of the aluminium will rise much more quickly than the 1 kg of water (1 litre). This is also partly because it will dissipate energy more slowly: it will take longer for energy to be transferred to the surface of the aluminium by conduction, and then be transferred to the surroundings by radiation, compared with time for convection currents to be set up in water. Hence the ratio of the rate of temperature rises is not the same as the ratio of the specific thermal capacities.
  • If you want to use these methods to measure specific thermal capacities, then you need to ensure that you minimize energy dissipated to the surroundings with good thermal insulation.
  • How Science Works extension: You could either set students a structured investigation and then follow with questions based on this or offer an open-ended investigation.
  • Students could:
    • identify and select the variables that they wish to measure and control
    • produce their own experimental procedure, including the selection of appropriate time intervals.
  • The amount of guidance given will very much depend on your students’ level of confidence and skills with designing their own experiments.
  • Some groups could be set a very open-ended brief, ‘investigate cooling’. With others you might set the investigation in a real world context, suggesting some of the possible variables e.g. you could tell them that they are to investigate which is better to keep a cup of hot coffee warm for longest - a ceramic mug, a paper or a polystyrene cup? Most takeaway coffee cups have a lid, so this could be extended to investigating how effective the lid is at reducing energy dissipation. More advanced students could investigate whether tea and coffee behave in exactly the same way as water.
  • Collecting data for cooling curves for cups of different materials is relatively straightforward, so students need only minimal guidance in the specifics of what they are to do.

Heating and cooling worksheet (Word, 58 KB)


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The Sun’s luminosity

Orbits
Earth and Space

The Sun’s luminosity

Practical Activity for 14-16

Demonstration:

Students collect data and gain experience in using the inverse-square law for intensity of radiation. They use simple but ingenious apparatus to deduce a value that cannot be measured directly.

Apparatus and Materials

  • Lamp, 240 V 150 W or 100 W
  • Mains extension cable fed through earth-leakage circuit-breaker (ELCB)
  • A4 paper, plain white, one sheet
  • Optical pin or similar pointed object (e.g. drawing compass)
  • Cooking oil, a few ml, in a small beaker or cup
  • Tape measure or metre ruler

Health & Safety and Technical Notes

Make sure that participants do not look directly at the Sun. Ensure that the extension cable is safely positioned so as not to trip up passers-by, and that connections to the lamp and power supply are protected from moisture. Check that the ELCB is operating (by using its Test button) before use.

Read our standard health & safety guidance


The lamp should ideally be clear glass and held in a standard batten holder.

Procedure

This activity needs to be performed outdoors on a clear sunny day.

  1. Use the pin to place a very small drop of oil on the paper – it should spread to form a translucent patch no more than 5 mm diameter and ideally smaller.
  2. Hold the paper so that it is illuminated by the Sun on one side and by the lamp on the other, as shown in the diagram.
  3. Viewing the paper from the ‘Sun side’, adjust its distance from the lamp so that the oil spot appears to merge with the surrounding paper.
  4. Record the lamp distance d.
  5. Assume that the light from the lamp and from the Sun varies in intensity according to an inverse-square law, and using the Earth-Sun distance of 1.50 x 1011m, obtain a value for the Sun’s luminosity, Lsun.
  6. Discuss factors that might affect the result.

Teaching Notes

  • This activity can be carried out in a few minutes as a quick demonstration, followed by calculation and discussion.
  • Alternatively, the demonstration could be followed by setting a challenge to students: how can they design their own experimental set-up so as to reduce uncertainties in measurement?
  • When the spot appears to merge into the surrounding paper, the intensity of illumination due to the lamp (seen through the translucent spot) is the same as that due to the Sun on the surrounding paper.
  • With D the Earth-Sun distance, d the lamp distance and L lamp the luminosity (power) of the lamp (150 W or 100 W):
  • Lsun /(4π D 2) = Llamp /(4π d 2 ) which can be rearranged to obtain a value for Lsun.
  • The calculated value generally lies within an order of magnitude of the accepted value for the Sun’s luminosity: Lsun = 3.9 x 1026W.
  • In addition to uncertainties in judging and measuring the correct position of the paper, the result is affected by two sources of systematic error.
    • Solar radiation that reaches the Earth’s surface is absorbed by the atmosphere. The amount of absorption depends on the elevation of the Sun above the horizon, and atmospheric conditions. Near midday in the UK in summer, on a clear day, about 30% of the radiation may be absorbed.
    • The judgement of the correct position for the paper depends on sampling only the fraction of radiation to which human eyes are sensitive. As the Sun and the lamp have very different temperatures, they do not emit the same fraction of visible radiation.

This experiment comes from University of York Science Education Group:

Salters Horners Advanced Physics


Diagrams are reproduced by permission of the copyright holders, Heinemann.

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Thermal radiation from the human body

Energy Transferred by Radiation
Energy and Thermal Physics

Infra-red radiation from the human body

Practical Activity for 14-16

Demonstration

This experiment shows that electromagnetic radiation in the infrared region is emitted from warm objects such as the human body.

Apparatus and Materials

  • Mirror galvanometer, with sensitivity of about 20 mm per μA
  • X-band microwave detector with its horn

Health & Safety and Technical Notes

Do not use any source of power on the diode detector. Do not use a Gunn diode source.

Read our standard health & safety guidance


Do not use a transmitter. Do not apply any source of power.

Note: even under good conditions the galvanometer, with a sensitivity in μV, will have a deflection of only about 5% fsd.

David Sumner says: "I used a diode detector, Unilab 045.674, which comes complete with a horn. This detector has enormous bandwidth. Any similar X-band receiver can be used."

Procedure

  1. Set up the apparatus
  2. Cover the horn window with metal foil. Zero the galvanometer and carefully switch it to the most sensitive range.
  3. Remove the foil and point the horn at the body, at a distance of a few centimetres. There will be a noticeable deflection.

Teaching Notes

  • Students may be surprised to discover that they emit infra-red radiation. Thermal imaging systems used by the military and by emergency workers (e.g. seeking people trapped in burning or collapsed buildings) detect this infra-red radiation.
  • You can show that the detector is responding to infra-red radiation by placing a simple aluminium reflector, painted black, between the radiation source (human body) and detector. The detector will show no effect. Infra-red photons are absorbed by the black coating; any microwaves noise will be reflected without any loss.
  • The experiment can also be used when discussing radio telescopes. While gathering radio waves emitted from astronomical objects, radio telescopes also detect ‘noise’ in the form of infra-red radiation from Earth’s horizon, the atmosphere and the antenna itself.
  • The operation of a radio telescope involves identifying noise power and improving the signal-to-noise ratio. Radio astronomers think of the various contributions to noise in terms of system noise ‘temperature’. Nobel prize-winners Wilson and Penzias were studying just such effects when they identified cosmic microwave background radiation, corresponding to a black body radiator at a temperature of 3 K.
  • Electromagnetic radiation will be detected from the head, body, limbs, etc. and also from a plastic bucket of hot water. This will mainly be infra-red radiation but may also include some from the microwave region (depending on the detector used). Radiation will not be detected from a metal container, since reflective surfaces are poor radiators of infra-red radiation.
  • The long wavelength portion of the electromagnetic spectrum gathered by a radio telescope is referred to as the Rayleigh-Jeans region. In this region, as wavelength increases, the solid angle of the beam that an antenna collects also increases, meaning it sees a greater surface emitting noise consisting of infra-red radiation. But as wavelength increases, the surface brightness decreases. These two effects counteract each other, so the noise power per bandwidth interval is uniform across the...

    Rayleigh-Jeans region


  • Electromagnetic radiation gathered will warm the telescope’s detector, producing ‘Johnson noise’, random motions of electrons in a metal conductor. Johnson noise power, P , in watts, given by P = 4 kT Δ f , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δ f is the bandwidth in hertz.
  • Some astronomical detectors are cooled by liquid helium to reduce Johnson noise.

This experiment was originally submitted by David Sumner, a Science Technician at Glebelands School in Surrey. It now incorporates improvements suggested by microwave engineer Jiri Polivka, of Santa Barbara, California.

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Conduction, convection and radiation

Energy Transferred by Conduction
Energy and Thermal Physics

Conduction, convection and radiation

Teaching Guidance for 11-14 14-16

Thermal conduction

Conduction is the way in which energy is transferred (through heating by contact) from a hot body to a cooler one (or from the hot part of an object to a cooler part). It is the result of particle motion: fast or vigorously moving particles bumping into less energetic particles and making them move faster or vibrate more vigorously.

Before beginning any other experiments, students could touch a number of objects around the room and classify them into those which feel warm to the touch and those which feel cool to the touch. Unless sunlight is falling on them or they are near to a heater:

  • all the materials are likely to be at the same temperature
  • that temperature is likely to be lower than the temperature of the body.

Because the objects are at a lower temperature, energy will be transferred from the students' hands to the object. However, even though they are all at the same temperature, some of the materials will feel colder. These are the ones that are better conductors. The reason that they feel colder is because, being good conductors, they will transfer the energy quickly across the whole object. Or, put differently, the good conductor cannot maintain a temperature difference between the piece that the students is holding and the rest of the object. Therefore, the student has to raise the temperature of the whole object, not just the bit that they are holding.

Thermal convection

Energy can be carried from one place to another by wholesale movement of the medium: a warmer fluid moves, displacing a colder fluid and thus transfers energy in convection currents. This is rather like a student carrying a message in a letter to others rather than just passing it on down the line, as is the case with conduction.

Radiation

Radiation is quite different from conduction and convection. It is not a matter of something hot carrying the energy itself, or of atoms transferring the energy on from one to the next. Hot things produce electromagnetic waves and so they cool down, unless we keep them hot. When electromagnetic waves hit something, they are absorbed and can raise its temperature.

The energy transferred by each photon of electromagnetic radiation is given by {hf} (Planck’s constant multiplied by the frequency of the radiation). All frequencies transfer quanta of energy. The energy transferred by a quantum of ultra-violet radiation is greater than for a quantum of infra-red radiation. However, there is more infra-red radiation emitted from a hot body than a cooler body. One watt of green light gives just as much heating as one watt of infra-red light. There are no special kinds of heat rays or heat radiation. The electromagnetic waves only increase the energy stored thermally by an object when they are absorbed; they transfer no energy as they travel through a completely transparent medium or when being reflected from a perfectly reflecting mirror.

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Heat and temperature

Energy Transferred by Heating
Energy and Thermal Physics

Heat and temperature

Teaching Guidance for 14-16

One important aspect of students’ growing understanding of energy ideas involves sorting out the ideas of heat and temperature (hotness or coldness).

Kinetic theory describes the energy of an object as being due to the random motion of its molecules. If you give more energy to be shared out amongst the atoms and molecules of some piece of matter, it usually gets hotter. But ‘hotness’ is not energy. Something hot (like the surface of the Sun, or a flame in a gas cooker, rather easily gives up energy to cooler things (energy goes without help from hotter to cooler).

What counts is the average energy per particle, not the total energy stored. So hot objects have, as it were, very concentrated energy that easily spreads out and dilutes, warming other things. This is what lies behind talk about “heat is a form of energy”. It is best to refer, as soon as possible, to the sharing out of energy amongst the motion of all the particles.

Students will learn that all energy transfers involve some losses through energy dissipation. They deserve (and will understand) an explanation of how this happens, and not simply a dramatic conclusion about the Universe warming in some mysterious way.

In describing ways in which energy goes from one place to another, physicists distinguish between ‘heating’ and 'working’. Heating is the process whereby energy moves from one object to another, which is in contact with it, as a result of their temperature difference.

Compressing and expanding gases

If a gas is compressed by pushing a piston quickly into a cylinder, the gas grows hotter: all the energy transferred to the gas goes into energy of molecular motion. If the gas then cools back to the original temperature, it transfers energy to the surroundings until they reach the same temperature. This will make its pressure fall slightly too, but still the pressure will remain higher than it was before compression.

The compressed gas, back at room temperature, can still transfer energy to other things by pushing the piston out. But the energy which it now supplies will be taken from the gas by cooling it down below room temperature.

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Cooling corrections

Newtons Law of Cooling
Energy and Thermal Physics

Cooling corrections

Teaching Guidance for 14-16

Experiments that involve changing the temperature of a material and measuring that change are necessarily subject to energy transfers between that material or materials and the surrounding environment. These transfers will often not be accounted for and can cause inaccuracies. If the temperatures used are within 10°C or so of the surroundings, the inaccuracy is unlikely to be significant compared to other school laboratory errors. However, if you really want to make the correction, a number of methods can be used, all based upon Newton's law of cooling.

1

In some cases it is possible to cool an object before starting the experiment. You can arrange this so that its temperature difference with the surroundings is equal (but opposite in sign) after heating. It is then reasonable to assume that any energy transfer away from the object when it is above the temperature of its surroundings is countered by a energy transfer into the object when its temperature is below. This technique can be employed when mixing liquids, or when measuring the specific thermal capacity of metal blocks.

2

The formal Newton's law method assumes that the rate of loss of heat to the surroundings is proportional to the temperature excess above the surroundings, i.e.

dQdt = k(T-Troom)

  • Where Q is the quantity of energy transferred in a time t,
  • T Troom are the temperatures of the cooling object and the surroundings respectively,
  • and k is a constant of proportionality.

Measure the temperature of the object (block, calorimeter, etc.) at the time of start of the heating, t0. Read the temperature at about 30-second intervals until the maximum temperature has been passed and for a significant time after. The longer this time, the more accurate the correction.

Plot the temperature against time on graph paper. On the graph (indicated in diagram below), select times t1 and t3, equal times either side of the maximum temperature at t2. The energy transfer between t2 and t3 is given by integrating the equation above between these values to give:

Q = kk(T-Troom)dt

The right-hand side of this equation is proportional to the area under the curve of k(T-Troom) versus t, denoted by A2 in the diagram below.

The left-hand side, Q, the energy transferred to the surrounding in the interval (t3-t2, is proportional to ΔT3, the drop in temperature during this time interval.

Remember that Q = mcΔθ, where m is the mass of the cooling body, c is its specific thermal capacity, and Δθ is the drop in temperature.

Therefore Δt3 = KA2, where K is another constant.

Similarly, the drop in temperature due to cooling in the time interval between t1 and t2, is given by Δt2 = KA1. (Note that, since the mechanism by which cooling takes place is the same for times between t1 and t2 and between t2 and t3, the constant of proportionality will be the same for both regions.)

So ΔT2ΔT3 = A1A2

If T2 is the temperature observed at time t2, the temperature which the object would have reached had there been no thermal transfer to the surroundings is:

T2 + ΔT2 = T2 + ΔT3 (A1A2)

A1 and A2 can be measured by counting squares on graph paper.

Image courtesy of www.upscale.utoronto.ca/IYearLab/heatcap.pdf

3

If you are using a heater, a simpler method is as follows (courtesy of Frank Grenfell on the CAPT email discussion list):

  • Observe, (there is no need to record) the temperature as it rises, starting a tt0. Turn off the heater and record the time t1. You need this anyway to find the energy transferred to the object.
  • Keep the clock running.
  • Observe the temperature as it continues to rise, and reaches its maximum value (temperature Tmax) at time t2. Keep the clock running.
  • Record the temperature ( T ) after another 0.5 t2 (i.e. half as long again as it took to reach the maximum temperature).
  • The cooling correction to be added is (Tmax-T).
  • Reasoning. The rate at which energy is transferred to the surrounding while the block is being heated is roughly half what it is at Tmax. So if you observe the temperature drop from Tmax in a time interval equal to half t2, that should be about right.

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Note on “warming things up” and thermal energy

Heating
Energy and Thermal Physics

Note on “warming things up” and "thermal energy"

Teaching Guidance for 11-14 14-16

All the energy transfers that you demonstrate will ultimately end up warming the atmosphere, or surroundings. There has been much debate about what is happening when a body is warmed up, at both the scientific and the education level. One of the teaching routes is suggested here.

A hot body has a high temperature and can store a lot of energy thermally. This hot body can be put into contact with a cooler body and some of the energy of the hotter body is transferred to the cooler body to increase the energy stored thermally, and the temperature, of the cooler body.The word ‘heat’ has been used as a similar word in physics to the word ‘work’. However, a better approach is to say that heating and working both transfer energy, and can change the temperature of a body or system.

When a body falls from a height, energy stored gravitationally (when the object is high up) is then stored kinetically (when it is at a lower height). Again, it is often said that 'gravitational potential energy changes into kinetic energy". A better approach is to describe how the energy is stored (e.g. gravitationally, or due to its position in a gravitational field, and then kinetically because it is moving). Energy is transferred due to the work done by a force, and it is driven by the difference in height through which the body falls.

There are many other processes in which energy is transferred. When a cell is used to light a light bulb:

  • there is a transfer of energy from energy stored chemically in an electrical cell to energy stored thermally in the hot filament of a lamp
  • this is called working (electrically) by means of an electric current.

When a light bulb is on:

  • there is a transfer of energy from energy stored thermally in a hot filament to energy stored thermally in the surroundings
  • this is called heating (by radiation).

In the Malvern energy kit energy is often transferred from one component to another by an elastic belt. The belt is used to do work.

While it is tempting to use terms such as "thermal energy" or "internal energy" for the energy stored in a hot body, it is misleading. A better approach is to use energy without an adjective.

The concept of "heat" also achieves ‘notoriety’ because when the energy transfer is not 100% efficient then it is attributed to being transferred to the Universe, so warming it up in some mysterious way. The model is one in which the kinetic theory of gases ‘explains’ the energy in a gas as being due to the random motion of its molecules. (Not only the translational kinetic energy of the molecules but also the rotational and vibrational energies as well. There is no potential energy stored up in a gas except the potential energy of the (P.E. + K.E.) of vibrational motion.)

Students sometimes think of a gas at high pressure as being like a compressed spring. Springs store energy elastically. If a gas is compressed by pushing a piston quickly into a cylinder, the gas grows hotter, and all the work done on the gas goes into the energy stored thermally in the gas. The molecules of the gas move faster. If the gas cools back to the original temperature, energy is transferred to the surroundings;the energy transferred to the gas has now 'escaped' to the outside world. The compressed gas, back at room temperature, has no extra energy by virtue of being compressed, yet it can transfer energy to other things by pushing the piston out with its high pressure. But the energy which it now transfers will be taken from the gas by cooling it down below room temperature.

If you transfer energy to a gas then you might want to calculate how much the energy stored kinetically by the molecules increases. An indication of that energy stored kinetically is the temperature of the gas. You can measure the energy experimentally by heating it up with an electrical heater. You might expect the energy transferred from an electrical supply to warm up a sample of gas agrees with the calculated increase of average energy stored kinetically by the gas molecules. You will find that to be true for a gas such as helium or neon, in which the molecules are single atoms that do not indulge in rotational or vibrational motion that can also be increased by heating. However, for other gases, such as ‘air’ or carbon dioxide, you will find that the electrical supply has to deliver more energy than goes simply in the energy stored kinetically by the molecules flying about in the gas. The extra energy goes to provide for these extra (rotational and vibrational) motions of the molecules.

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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.

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Cannons, steam engines and ‘caloric’

Energy Transferred by Heating
Energy and Thermal Physics

Cannons, steam engines and ‘caloric’

Teaching Guidance for 14-16

The idea of 'heat' is an everyday phenomenon, familiar even before fire-making. Aristotle discussed it as one quality among others, such as colour or smell. Mediaeval scholars discussed ‘degrees of heat’ but only with the development of thermometers during the 17th century did it become possible to quantify the study of 'heat'. The Scottish professor Joseph Black (1728 – 99) was the first to distinguish between temperature and 'heat', or, as we would now say, energy stored thermally. 

By the 18th century it was generally thought that 'heat' was an invisible and weightless fluid, called ‘caloric’. In 1760 Black had conducted sufficient experiments to conclude that there was a different heat ‘capacity’ for each substance. In 1781 the Swedish scientist Johann Carl Wilcke independently came to the same conclusion. Black went on to measure water’s latent heats of fusion and of vaporisation.

The first person to seriously challenge the caloric idea was Benjamin Thompson, a founder the Royal Institution who in 1791 became Count Rumford. As director of the Munich arsenal, Rumford noticed that boring cannons produces a great heating effect, especially if the boring tool is dull. Rumford argued that the supply of 'heat' was limitless, showing that a boring drill would continue to boil water so long as the horses driving it kept moving. This is more easily explained by a mechanical theory of 'heat' than the caloric (fluid) theory.

But the fluid theory was still needed to explain 'heat' transfers, and so it prevailed for many decades. In France the publication of Joseph Fourier’s mathematical theory of heat conduction in 1822 did not rely on caloric theory yet Sadi Carnot’s 1824 theory of steam engines did. When explaining how heat engines did mechanical work, Carnot mistakenly assumed that caloric ('heat') is a conserved quantity.

Finally in the 1850s William Thomson (later Lord Kelvin) and Rudolf Clausius modified the Carnot theory and began to convince others that energy is conserved (not 'heat'). As kinetic theory became established, so caloric theory withered and died.

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Fundamentals

Conservation of Energy
Energy and Thermal Physics

Fundamentals: energy

Practical Activity for 14-16

A discussion by Jon Ogborn, emeritus professor of science education at the Institute of Education, London.

An intrinsic problem of teaching about energy at secondary level is that school science is obliged to try to run before it can walk. School biology and chemistry need to use the idea of energy before its physical meaning or its measurement in terms of force multiplied by displacement can be taught.

Teachers want and need to talk about the role energy plays in changes, but the idea that energy is conserved (first law of thermodynamics) is simply not enough to do the job. What they need are some ideas from the second law of thermodynamics.

It is no real surprise that the world is richer and more complicated than science textbooks make it appear. And it is no surprise that it takes a lot of skill, knowledge and creativity to find good ways to explain things simply to young people.

In the resource downloadable below, I offer a rough guide to the fundamental physics, using these subtitles:

  • What is energy?
  • Energy is conserved
  • Energy amongst the molecules
  • Free energy
  • Is energy needed for a change to happen?

and concluding with

  • Is there a better way to teach energy?

Resource

IOP DOMAINS Physics CPD programme

Energy CPD videos

Our new set of videos gives teachers and coaches of physics a preview of the training we offer ahead of this term's live support sessions.

Find out more