Specific heat capacity required practical

Aim: To find the specific heat capacity of a material, such as an aluminium block or water, by measuring the energy supplied to it and its rise in temperature.

Variables

• Independent: Energy transferred to the block (which increases with heating time)
• Dependent: Temperature of the block
• Control: Mass of the block; Material of the block; Power of the heater (keep the supply settings the same); Amount of insulation around the block

Equipment

• 1 kg metal block with holes for a heater and a thermometer
• Immersion heater (about 12 V)
• Low-voltage power supply
• Joulemeter, or an ammeter and a voltmeter
• Thermometer or temperature probe
• Stopwatch
• Insulating material to wrap the block
• Top-pan balance
• A few drops of water or oil for the thermometer hole

Method

1. Measure the mass of the block on a balance and record it in kg.
2. Wrap the block in insulation, leaving the two holes uncovered.
3. Put the heater in the large hole and the thermometer in the small hole, adding a drop of water to the thermometer hole for good thermal contact.
4. Connect the heater to the power supply with a joulemeter (or an ammeter in series and a voltmeter in parallel).
5. Record the starting temperature of the block.
6. Switch on the supply and start the stopwatch at the same moment.
7. Record the temperature and the joulemeter reading (or current and potential difference) every minute for about 10 minutes.
8. Switch off and work out the energy supplied at each time, using E = P t with P = V I if you have no joulemeter.
9. Plot temperature against energy supplied and find the gradient of the straight part of the line.

Safety

• The heater gets very hot: do not touch it while it is on or straight after use, and let it cool before putting it away.
• Stand the block on a heatproof mat.
• Keep water away from the power supply and the electrical connections.

Results and calculations

Record time, energy supplied and temperature in a table. Plot temperature (y-axis) against energy supplied (x-axis); the gradient equals 1 ÷ (m c), so c = 1 ÷ (gradient × m). You can also use ΔE = m c Δθ directly, with E = P t and P = V I, rearranged to c = ΔE ÷ (m Δθ).

Common mistakes and improvements

• Energy is lost to the surroundings, so the temperature rise is smaller than it should be and c comes out too high: insulate the block well, including a lid on top.
• The thermometer does not touch the block properly: add a drop of water or oil in the hole so readings keep up with the block's real temperature.
• The temperature keeps rising after the heater is switched off because the heater is still hot: use only the straight-line part of the graph, not the first minute or the end.
• Reading the thermometer at an angle causes parallax error: read it at eye level, or use a digital probe for better resolution.

Exam tips

• Be ready to explain why the calculated value is higher than the true value: some of the energy supplied heats the surroundings instead of the block.
• Practise rearranging ΔE = m c Δθ and give the unit of c as J/kg °C.
• If asked how to improve the method, give a specific action, such as adding more insulation or using a lid, and say why it helps.
• Know that the gradient of a temperature–energy graph is 1 ÷ (m c), and that you need to convert mass in grams to kilograms.
• Draw the heater circuit correctly: ammeter in series with the heater and voltmeter in parallel across it.