Demonstration

This demonstration shows, against intuition, that an ice cube melts more quickly when in contact with a metal block than a plastic block.

Apparatus and Materials

• Metal and plastic blocks of identical dimensions
• (approx 5 cm square by 1 cm thick)
• Ice cubes at 0°C
• Clock
• 2 temperature probes with displays (optional)

Health & Safety and Technical Notes

Read our standard health & safety guidance

There are no safety problems with this demonstration.

A suitable ‘ice melting kit’ is available from the supplier: Timstar.

Procedure

1. Pass the metal and plastic blocks around the class; ask your students what differences they observe. They are likely to comment that the metal block feels colder to the touch than the plastic one.
2. Explain that you are going to place identical ice cubes on each block. Ask for predictions as to what will happen.
3. Place one ice cubes on each block. Observe the ice melting over a few minutes. The film below shows how to carry out this demonstration, together with typical results.

Thermal conductivity

4. You could use a timer to determine the time for each cube to melt completely. Alternatively, attach a temperature probe to each block and observe how their temperatures change.

Teaching Notes

• This demonstration can form the introduction to a structured development of ideas about energy transfers between objects at different temperatures. Ice cubes are placed on metal and plastic blocks; the cube placed on metal melts much more quickly than the cube placed on plastic. This is counterintuitive (for many students) because metals feel cold while plastics feel warm.
• Energy is transferred to the ice cubes by conduction from the blocks on which they have been placed. A metal block is a better conductor and so energy is transferred more quickly to that ice cube.
• Why isn’t this obvious? Metals feel cold to the touch. This is because, when you touch a piece of metal, energy conducts away from your fingers into the metal, lowering the temperature of your fingers. Plastics are good insulators so, even though the plastic is at a lower temperature than your fingers, little energy conducts to the plastic and it feels warm.
• Hence it is best to start the demonstration by asking your students to feel the two blocks so that they may be misled by this experience. Then show that the ice on the metal block melts more quickly, and discuss the reasons.

• You may then wish to take the discussion to a deeper level. Students may think that some materials (metals, water) are intrinsically cold, while others (plastic, wood) are intrinsically warm. (We talk about ‘warm clothing’). So you could use thermometers to test the temperatures of different objects and materials in the room.
• Then repeat the demonstration with electronic thermometers monitoring the temperatures of the blocks as the ice cubes melt. Show that the two blocks are both at room temperature at the start, and observe the rapid drop in temperature of the metal block.
• You could ask your students to explain why the temperature-time graphs for the two blocks are curved (they are roughly exponential). The reason for this is that the rate of transfer of energy from the block to the ice decreases as the temperature difference between them decreases.
• Note that there is a complication to this analysis which we have avoided mentioning so far. The rate at which energy is transferred to the ice depends on both the conductivity of the block and its heat capacity. It might be that the ice on the plastic block melts very slowly because the temperature of the plastic block drops very rapidly to that of the ice. This would happen if plastic had a low specific heat capacity. This is shown not to be the case by when a temperature probe is used.
• Some general notes on teaching about:

  conduction, convection and radiation

• An experiment to compare the...

  thermal conductivities of different materials

Demonstration

Using a band or friction brake and wheel to show the transfer of work into energy stored thermally.

Apparatus and Materials

• Demonstration forcemeters, 0-20N, 2
• Wheel with band brake
• Stopwatch
• Rod and two bosses
• Retort stands, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

You can improvise the wheel with band brake if you have a belt-driven table or disc. Slip off the belt and substitute a webbing belt or length of thick string. Alternatively, a bicycle ergometer could be used.

Procedure

1. Secure the belt firmly at the ends of the two spring balances, which you have supported from above. Adjust the belt tension by raising or lowering the support so that both forcemeters read about half full scale.
2. Turn the wheel by hand at a steady rate for at least 30 seconds and record the steady readings of the two forcemeters. Note the time taken to turn the wheel and count the number of turns taken.

Teaching Notes

• As the wheel is turned, energy stored chemically (in food and oxygen) in the operator’s muscles is transferred to the band brake so warming it up and increasing the energy stored thermally.
• If you want to calculate the energy transferred, the logic is quite tricky, but explained as follows.

• Read the forces F₁ and F₂ from the spring balances while allowing the wheel to slip past the fixed belt, as in diagram I. This is equivalent to diagram II in which the spring balances are replaced by two loads suspended by pulleys with the tension in each side of the belt being F₁ and F₂ as before. Now the belt does not slip but rather the loads move. Finally this arrangement can be transformed into diagram III, in which one end of the belt is fixed to the wheel and the other carries a load equal to the difference between F₁ and F₂ . As the band brake is turned the belt does not slip but the load rises.
• The energy transferred to the system, E= (F ₁ F₂ ) x distance moved.
• So the energy transferred to band brake is E = (F ₁ F₂ ) x (circumference of wheel) x (number of turns)
• This energy of the band brake stored thermally increases , and so its temperature rises.

This experiment was safety-tested in November 2005

Demonstration

Mixing two masses of water at different temperatures to discuss energy transfer.

Apparatus and Materials

• Plastic buckets, 2
• Thermometer (demonstration or digital display)
• Domestic balance
• Supply of hot water and supply of cold water

Health & Safety and Technical Notes

Read our standard health & safety guidance

The temperatures could be read with a mercury thermometer, but this would not allow the class to see the reading. Digital thermometers with large displays are now reasonably priced.

Procedure

1. Weigh 3 kg of hot water into one of the plastic buckets.
2. Weigh 2 kg of cold water into the other.
3. Note the temperature of each.
4. Pour the cold water into the hot water and stir. Take the final temperature.

Teaching Notes

• Discuss with students what happens when hot water is mixed with cold water. Light containers are used so that the thermal capacity of the container itself can be ignored. When the waters are mixed the temperature ends up somewhere between the two initial temperatures. You might ask: "Has anything stayed the same during the mixing?"
• Because the masses of water are different the temperature changes should not be equal. "What if we multiply the temperature change by the mass of water?"
• This product does stay the same (approximately), and does so in many exchanges. It is directly proportional to the change in energy stored thermally.
• The energy transfer between the two lots of water is not 100% because some energy comes from the bucket and some is transferred to the environment. But in this experiment the energy dissipated in these ways are minimized.
• Therefore energy stored thermally in the warm water is transferred to the cold water, until they arrive at a common temperature.

This experiment was safety-tested in November 2005

Class practical

Quantitative measurement of changes to energy stored thermally as a result of heating from a range of sources.

Apparatus and Materials

For each student group

• Immersion heater, 12 V 100 W (older, 60 W types will do)
• Thermometer -10°C to 110°C
• Aluminium container
• Lever-arm or domestic balance (+/- 2g)
• Stopwatch or stopclock
• Low voltage power supply or transformer (to supply 8A)
The following apparatus is required if you wish to do the extension experiments...
• Bunsen burner, with heatproof mat and tripod
• Heatproof gloves
• Small evaporating basin
• Ethanol (methylated spirit)
• Matches

Health & Safety and Technical Notes

Although the temperature rise should not be high, heat-proof gloves should be available for those members of the class who heat their containers up to 45 °C.

Take extreme care with using ethanol. All flames must be extinguished before ethanol stock is brought into the room. The teacher (or assistant) must dispense 1 ml into each evaporating basin (e.g. using teat pipette or pump dispenser).

Finally, matches are issued to the class. No one should repeat the experiment because of the danger of adding ethanol to a hot container.

Read our standard health & safety guidance

In procedure step 5, a convenient metal support for the container can be made from a strip of aluminium 20 cm x 7.5 cm bent to form both a wind-shield and a support.

Procedure

1. Measure out approximately one kilogram of water and place it in the aluminium container. Measure the temperature of the water with the thermometer. Place the immersion heater in the water and connect it to the 12 V supply. Switch on and start the clock at the same moment. Stir the water constantly - this is essential for good results.
2. After 5 minutes switch off the supply, continue to stir and note the highest temperature reached. REMOVE the immersion heater from the water and allow it to cool on the bench.
3. Repeat the experiment using 1/2 kg of water.
4. Empty the container and put another kilogram of water into it. Place a Bunsen burner underneath and heat the water for one minute. From the rise in temperature, calculate how much energy is transferred by the burner in one minute.
5. Burn 1 ml of alcohol (methylated spirit) under another 1 kg of water. From the rise in temperature, calculate the energy transferred when the ethanol is burned.

Teaching Notes

• Energy is transferred to the water by the electric current. The change of energy stored thermally in the water is calculated (see 4 below) from (mass of water) x (temperature rise). The temperature rise with half the mass of water should be about twice as much. Between experiments the apparatus should be cooled down to room temperature so that each experiment is carried out under the same conditions.

• Students can be asked to measure the temperature of the water at regular intervals of time, of about 5 minutes, and then to plot a graph of temperature against time for the water. This will show that the temperature increases fairly uniformly with time as long as the temperature does not rise too much. If the temperature rise is too high the energy dissipated (stored thermally in the surroundings) increases because of the higher temperature difference between the container and its surroundings.
• For experiment 4 the temperature rise of the water is noted and its mass measured. The change of energy stored thermally in the water is again calculated from (mass of water) x (temperature rise). Ignoring energy dissipated to the surroundings, this is the energy transferred from the Bunsen burner. A similar calculation in 5 gives the energy transferred from 1 cm³ of alcohol.
• The units may cause concern to some teachers. They can be called kilocalories (if the mass is in kilograms, which they are), or thermal units. It is not necessary to introduce the concept of specific thermal capacity.
• It is worth taking care to allow the heaters to cool in air, not water. If the heater has a crack in its seal, as it cools it will then draw in air not water. The problem with allowing water to enter is that when the heater is next used, the water boils. The heater then explodes with a bang, which is frightening if not very hazardous.

This experiment was safety-tested in March 2006 fe

Class practical

Using an aluminium block and immersion heater to estimate the specific thermal capacity (also called the specific heat capacity) of aluminium.

Apparatus and Materials

For each group of students

• Aluminium calorimeter with holes for heater and thermometer (see discussion below)
• Thermometer -10°C to 110°C
• Stopwatch or stopclock
• Immersion heater, 12 V 100 W (older, 60 W types will do)
• Low voltage power supply or transformer (to supply 8A)
• Lever-arm or domestic balance (+/- 2g)
• Insulation/cladding for the metal block OPTIONAL

Health & Safety and Technical Notes

The immersion heaters should have been allowed to cool in air after heating water, to eliminate the (small) risk that water has been drawn inside through a cracked seal.

Read our standard health & safety guidance

If bespoke insulation is not available, then scraps of material and or newspaper can be held on with string/elastic bands to provide a thick insulating jacket for the block.

If you drop some paraffin-oil into the thermometer hole it will ensure good thermal contact between the block and the thermometer. It is not necessary to use oil with the immersion heater. In fact, as there is a danger of cracking any oil which is left on the heater when it is removed from the block, it is wiser not to use it.

Procedure

1. Find the mass of the aluminium block on the balance. Place a small drop of oil in the thermometer hole. (This will provide good thermal contact between the block and the thermometer bulb.) Insert the thermometer and immersion heater in the appropriate holes. Read the thermometer. Connect the heater to the 12 volt supply and switch it on for 5 minutes. Note the maximum temperature rise obtained after the supply has been switched off.
2. Many suppliers can provide similar 1 kg blocks made of steel, copper, brass etc. If these are all set up at the same time they will show that different materials of the same mass will achieve different temperature rises when the same amount of energy is transferred to them.

Teaching Notes

• Change in energy stored thermally (due to the temperature rise) = mass x specific thermal capacity x temperature rise
• The temperature of I kilogram of aluminium rises about four times that of a kilogram of water. If the heater does not behave differently in aluminium compared to water there must be another factor which is peculiar to the aluminium. This is the specific thermal capacity (also called specific heat capacity) of the aluminium.
• The specific thermal capacity of aluminium is 900 J/kg °C
• The specific thermal capacity of water is 4200 J/kg °C
• It takes more energy to raise the same temperature of water by each °C than it does to raise the temperature of the same mass of aluminium.
• How Science Works extension: After collecting data, students calculate the specific thermal capacity of the aluminium (or other material) used. To assess the accuracy of their measured data, they can compare their value of specific thermal capacity with its accepted (true) value from data tables. You can also ask them to calculate the percentage difference between the two values, to show how the accuracy of measurements can be expressed quantitatively. Differences between the two values can also be used to prompt a discussion about errors and uncertainties in their measurements, identifying the main sources.
• Energy dissipated so that it is stored thermally in the surroundings is something that students can investigate further, to obtain a more accurate value of the specific thermal capacity. The Guidance note:

  Cooling corrections

  suggests a procedure for controlling such transfers. A less sophisticated, but equally valid, approach is to repeat the experiment (the block needs time to cool}, using insulation around the block. Their second set of data will enable them to assess whether this gives a more accurate result for specific thermal capacity.
• Before you make this comparison remember that power supplies may only give unidirectional potential differences and not fully smoothed values. The power measured is on DC meters as VI is only 0.8 of what it should be. See the guidance note...

  Explaining rms voltage and current

This experiment was safety-tested in December 2006

Class practical

Measuring the specific thermal capacity (also called the specific heat capacity) of aluminium, including the use of a cooling correction.

Apparatus and Materials

For each group of students

• Immersion heater, 12 V 100 W (older, 60 W types will do)
• Aluminium calorimeter with holes for heater and thermometer (see discussion below)
• Ammeter (0-5 amp)
• Stopwatch or stopclock
• Thermometer
• Voltmeter, 0-15 volt (see discussion below)
• Rheostat (10-15 ohms, rated at 5A or more)
• Top pan balance
• 12 volt supply, i.e. LV power supply with high current smoothing unit

Health & Safety and Technical Notes

Read our standard health & safety guidance

If you drop some paraffin-oil into the thermometer hole it will ensure good thermal contact between the block and the thermometer. It is not necessary to use oil with the immersion heater. In fact, as there is a danger of cracking any oil which is left on the heater when it is removed from the block, it is wiser not to use it.

Procedure

1. Find the mass of the aluminium block using the top pan balance. Connect the immersion heater to the 12-volt supply in series with an ammeter and a rheostat. The immersion heaters are 12 volt, 60 or 100 watt, so adjust the rheostat to give a current of about 4 amps. Switch the heater off.
2. Insert the immersion heater in the aluminium block and place the thermometer into its hole. Before switching on for the experimental run, wait for five minutes before taking the temperature of the block. Switch on the heater and start the clock.
3. The easiest way to measure the temperature rise is to leave the heater switched on until a rise of about 10°C is achieved. Switch off the heater and continue to monitor the temperature until it begins to fall. Note the maximum temperature reached by the block.
4. A more accurate method is to take temperature readings every half minute and to plot a graph of the results both whilst the heater is on and for approximately the same time after the heater is switched off. A cooling correction can be applied to the temperature rise measured, using a standard technique.

Teaching Notes

• The specific thermal capacity can be determined from the relationship: mass x specific thermal capacity x rise in temperature/ time = current x p.d.
• The ratio temperature rise / time can be obtained from the slope of a graph of temperature plotted against time.
• The aluminium blocks can be lagged by enclosing them in foamed polystyrene.
• You can attach the immersion heater to a joulemeter and so measure directly the energy transferred from the electrical supply to the material.
• The choice of power supply makes a lot of difference in this experiment. You will get an accurate result with AC supplies and AC meters because the AC meters have been constructed to read correct values.
• However, most school power supplies providing currents high enough to warm up immersion heaters are not smoothed at all and the DC terminals give voltages which are extremely bumpy. External smoothing units will help.
• If you use moving coil meters, students will need to make a correction for the electrical power calculation.
• A moving coil meter reads time-averaged values, which are (2/π) x peak value.
• So actual power = 1.2 x power calculated from meter readings.
• For further information, see the guidance note:

  Explaining rms voltage and current

This experiment was safety-tested in August 2007

Demonstration

Working mechanically to raise the temperature of different components.

Apparatus and Materials

• Blunt drill bit
• Electric drill
• Brick and G-clamp
• Piece of sheet lead
• Hammer and hardwood anvil
• Iron wire to hold piping
• Bicycle pump

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Clamp the brick over a scrap of softwood to the bench.
Bore a hole in the brick with the blunt drill, then pass the drill bit round so that it can be felt.
2. Push the stiff iron wire through the lead piping and bend the end so as to hold the lead.
Hammer the lead violently so that its temperature rises. A piece of lead sheet may be used instead of the piping, as shown in the diagram.
3. Ask students to push in the piston of a bicycle pump quickly whilst holding a finger on the outlet so that they can feel the heating.

Teaching Notes

• In all these demonstrations the components warm up. When the action stops, all the energy of the moving parts stored kinetically has been transferred to the components, so increasing the energy stored thermally.
• The bicycle pump warms up because the speed of the air molecules inside the cylinder has increased. Momentum is transferred from the moving piston to the air molecules in the same way as hitting a ball with a moving bat transfers momentum. An increase in the speed indicates and increase in energy stored thermally. The temperature rises.
• There are occasions when energy is transferred to a body but its temperature does not rise. If a beaker of crushed ice is melted with a Bunsen flame, the resulting slush remains at freezing point. In this case the heating increases potential energy in the force-fields of the molecular structure, as the molecules are pulled apart against the forces of mutual attraction that hold them in a solid crystal. The energy needed to do this is known as latent heat. (The term is too well established to think of calling it anything else - but beware of the word heat.)

This experiment was safety-tested in November 2005

Class practical

Transferring energy mechanically to lead shot and measuring its temperature rise.

Apparatus and Materials

For each group of students

• Lever-arm or top pan balance (+/- 10g is sufficiently accurate)
• Cardboard tube (approx 50 - 100 cm long)
• Lead shot, 500 g
• Plastic or cardboard cups
• Thermometer
• Metre rule
• Corks or bungs to fit tube, 2

Health & Safety and Technical Notes

There is no need to handle the lead shot. However, if it is spilt and is collected by hand (for example), the hands must be washed thoroughly before eating.

Read our standard health & safety guidance

Procedure

1. Measure out about 500 g of lead shot in one of the plastic or cardboard cups.
2. Put a thermometer into the lead shot to find the temperature.
3. Seal one end of the cardboard tube with a cork or bung. Put the lead shot into the tube and seal the open end.
4. Measure the length of the tube, Δh.
5. Turn the tube over 20, 40 or 50 times, counting the number. Quickly pour the shot into the cup and measure the maximum temperature again.

Teaching Notes

• When the lead is falling take care that it falls vertically rather than sliding along the tube, when friction will come into play. Also a hand should not cushion the bottom of the tube when the shot hits the bottom, otherwise energy will be transferred into the hand.
• Turning the tube over more times in order to achieve higher temperature rises is not advisable. The longer time and higher temperature differences will allow more thermal transfer to the surroundings.
• The energy transferred is equal to the number of falls (n) x mgΔh and therefore n x mg x Δh = m x specific thermal capacity x temperature rise.
• Note that there is no need to record the mass of the lead, m.
• Lead is used because its specific thermal capacity is about 1/30 that of water. It is the thermal capacity per unit mass which is important. (Most metals have approximately the same thermal capacity per unit volume.) It is also an inelastic metal so that the energy of the shot stored gravitationally is then stored thermally. The temperature rises.
• When Joule was investigating energy conservation he is said to have measured the temperature at the top and bottom of a waterfall on his honeymoon in Switzerland. This is a useful model of his experiment.

This experiment was safety-tested in January 2006

Class practical

Using the specific thermal capacity of water to check the reading of a voltmeter.

Apparatus and Materials

• Stopwatch or stopclock
• Thermometer
• Immersion heater, 12 V
• Measuring cylinder, 100 ml
• DC ammeter (0-5 amp)
• Plastic cup
• Battery, 12 V
• Retort stand and clamp
• Glass rod stirrer
• DC voltmeter (0 -15 volt)

Health & Safety and Technical Notes

Do not use a mains immersion heater which would involve unsafe mains connections.

Read our standard health & safety guidance

You can replace the battery by a 12 volt a.c. supply if appropriate a.c. meters are available.

Cheaper, cup-top immersion heaters are available. These remove the need for a separate support.

Procedure

1. Place 200 ml (grams) of cold water in the plastic cup. Support the immersion heater in the cup so that it does not touch the base. Place the immersion heater in the water and connect it into the circuit shown. Record the temperature.
2. Switch on the circuit and start the watch at the same instant. Stir the water and allow the current to flow for 2 minutes. Note the ammeter and voltmeter readings during this time.
3. At the end of 2 minutes switch off the current, stir the water again, and note the maximum temperature. (A rise of about 7° C can be expected.)

Teaching Notes

• This experiment makes use of the principle of conservation of energy in order to calibrate a voltmeter, rather than an experiment to measure the energy stored thermally by the water.
• Knowing that all energy is measured in the same unit, joules, then the energy transferred electrically from the power supply can be equated to the gain in energy stored thermally by the water.
• Energy transferred electrically, E = potential difference x current x time = VIt
• Energy stored thermally, E = mass x specific thermal capacity x temperature rise
• So, VIt = mass x specific thermal capacity x temperature rise
• As all the values can be measured, you can compare the calculated value for V with the reading on the voltmeter. The specific thermal capacity of water is 4,200 J/kg °C.
• If you are using d.c. meters, this experiment is best carried out using car batteries. The potential difference is ‘pure’ d.c. If rectified, unsmoothed d.c. is used from a power supply, then the calculated product of the readings on the voltmeter and ammeter will be 20% less than it should be. This is because the meters are measuring a time average not an r.m.s. average. If an a.c. supply and a.c. meters are used, r.m.s.values will be recorded, and the product will be accurate.
• It may be helpful to remind students that the definition of potential difference is the energy supplied to each coulomb of charge flowing. Therefore the basic unit of the volt can be expressed as joules/coulomb.
• In many textbooks, specific thermal capacity is also referred to as specific heat capacity.

This experiment was safety-tested in January 2006