# Specific thermal capacity of aluminium

##### 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.

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

a Weigh 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.

b 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

1 The transfer of energy from the power supply to thermal energy in the body
= 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

2 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.

3 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.

Thermal energy losses are 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 losses. 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-checked in December 2006