# Measuring thermal energy

##### Class practical

Quantitative measurement of thermal energy transferred 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.

In procedure step d, 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

a Weigh approximately one kilogram of water into 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.

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.

b Repeat the experiment using 1/2 kg of water.

c 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 heat is given out by the burner in one minute.

d Burn 1 ml of alcohol (methylated spirit) under another 1 kg of water. From the rise in temperature, calculate the energy released by burning the ethanol.

#### Teaching notes

1 Energy carried by the electric current to the water.
The extra thermal store of 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.

2 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 losses increase because of the higher temperature difference between the container and its surroundings.

3 For experiment c the temperature rise of the water is noted and its mass measured. The energy transferred to the water is again calculated from (mass of water) x (temperature rise). Ignoring energy losses to the surroundings, this is the energy transferred from the Bunsen burner. A similar calculation in d gives the energy transferred from 1 cm3 of alcohol.

4 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.

5 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-checked in March 2006