Specific heat capacity required practical

In the specific heat capacity practical, you heat a metal block with an electric heater, record its temperature every minute, and work out how much energy the heater has supplied. Plotting temperature against energy supplied gives a straight line whose gradient is 1 ÷ (mc), so the specific heat capacity is c = 1 ÷ (gradient × mass). In AQA's GCSE Physics it is required practical 1, and in Combined Science Trilogy it is required practical 14.

Aim

AQA's specification describes it as an investigation to determine the specific heat capacity of one or more materials, linking the decrease of one energy store (or work done) to the increase in temperature and the increase in thermal energy stored. In practice: find c for copper, aluminium and iron blocks. The theory, ΔE = mcΔθ, is covered in our guide to specific heat capacity.

Equipment

AQA's handbook suggests, for each group:

• 1 kg blocks of copper, iron and aluminium, each with two holes, one for the heater and one for the thermometer;
• a thermometer, and a pipette to put a little water in the thermometer hole;
• a 30 W heater and a 12 V power supply;
• insulation to wrap around the block;
• an ammeter, a voltmeter and connecting leads;
• a stopwatch or stopclock;
• a balance that can measure more than 1 kg;
• a heatproof mat.

Method

These steps follow the student worksheet in AQA's handbook.

1. Measure and record the mass of the block in kg.
2. Wrap the insulation around the block.
3. Put the heater in the larger hole.
4. Connect the ammeter, power supply and heater in series, and the voltmeter across the heater.
5. Use the pipette to put a small amount of water in the other hole, then put the thermometer in it. The water gives better thermal contact than air.
6. Set the power supply to 12 V and switch on.
7. Record the ammeter and voltmeter readings. They should not change during the experiment.
8. Measure the starting temperature and start the stopclock.
9. Record the temperature every minute for 10 minutes.
10. Repeat with the aluminium and iron blocks.

Variables

Results table

Fill in the last column with work done = power × time, where power = potential difference × current.

How to analyse the results

Work out the heater's power from P = VI, then the work done (energy supplied) at each time from E = Pt. Plot temperature on the y-axis against work done on the x-axis.

Here x is the energy supplied in kJ and y is the temperature in °C. AQA's handbook warns that the start of the graph will not be straight, because the block takes a while to warm up evenly. Draw the line of best fit through the straight part only, and find its gradient.

Rearranging ΔE = mcΔθ gives Δθ = ΔE ÷ (mc), so:

AQA's worksheet does it in two steps: the heat capacity of the block is 1 ÷ gradient, the energy needed to warm the whole block by 1 °C, and the specific heat capacity is that divided by the mass. Type in the readings from the straight part, and the Practical Data Lab gives the gradient of the best-fit line with its uncertainty.

Worked example: analysing a set of results

A student heats a 1.0 kg aluminium block. The ammeter reads 2.5 A and the voltmeter 12 V. On the straight part of the graph, the temperature is 26.0 °C at 6000 J and 38.0 °C at 18 000 J.

Power: P = 12 × 2.5 = 30 W, so the heater supplies 30 J every second and 18 000 J in 10 minutes.

AQA's handbook gives 913 J/kg °C for aluminium, so this result is about 9.5% too high. That is what you expect: some energy escapes to the surroundings, so the block warms less than it should, the gradient is smaller and c comes out bigger.

The Specific Heat Capacity Calculator works out c, ΔE, m or Δθ from the others, so you can check each step of your analysis.

Try it

Specific Heat Capacity Calculator

Work out heating energy and specific and latent heat

Open the calculatorGo to its page

These values are from AQA's handbook. It notes that 10 minutes with a 30 W heater supplies 10 × 60 × 30 = 18 000 J, enough to warm 1 kg of copper from room temperature to about 70 °C, iron to about 55 °C and aluminium to about 40 °C.

Here is a quick check.

Risks

Errors and improvements

• Energy lost to the surroundings. This is the biggest source of error, and makes c too high. Improve it by wrapping the block well in insulation and covering the top.
• Poor thermal contact. An air gap around the thermometer makes its reading lag behind the block. The drop of water helps.
• Not all the heater's energy goes into the block. Some warms the heater itself and the air. Make sure the heater fits snugly and is fully inside the hole.
• The temperature keeps rising after switching off. Energy stored in the heater still flows into the block, so take readings over time rather than relying on one final value.
• Reading the thermometer. A digital thermometer or temperature probe with a data logger gives better resolution and removes reading errors.
• Using the curved start. Use the gradient of the straight part, not just the first and last readings.

Common mistakes

• Mass in grams. A 1 kg block is 1 kg, but a 500 g block is 0.5 kg.
• Time in minutes. Work done = power × time needs time in seconds: 3 minutes is 180 s.
• Inverting the gradient wrongly. With temperature on the y-axis, c = 1 ÷ (gradient × m), not gradient ÷ m.
• Saying heat loss makes c too low. It makes the temperature rise too small, which makes c too high.

Practise physics in a short timed paper

GCSE Physics Higher and A-level Physics: Year 2 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so thermal physics and required practicals turn up regularly but not in every paper.

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