Syllabus coverage: 2.2.2, experiment for a solid and a liquid; practical skills

6.1 Solid block method

Measure the mass m of a metal block. Insert an electric heater and thermometer into close-fitting holes, using a small amount of thermal paste or oil if appropriate to improve contact. Wrap the block in insulation. Record the initial temperature, switch on the heater and measure the potential difference V, current I and heating time t. Record the final temperature and calculate Δθ.

The electrical energy supplied is E = VIt. Assuming this energy heats the block, c = VIt/(mΔθ). The potential difference is measured across the heater and current through it. If V or I varies, record several values or use a data logger and estimate the average power.

6.2 Liquid method

Find the mass of liquid by subtracting the mass of the empty container from the mass of container plus liquid. Place an immersion heater and thermometer in the liquid. Use a lid and insulation. Stir gently throughout heating so the liquid temperature is as uniform as possible. Record V, I, t and the temperature rise.

Calculate c using the mass of the liquid. A common error is to use the total mass including the container. The heater must be fully immersed but must not touch the container. The container also absorbs energy, so in a more accurate method its heat capacity is included or a calibration correction is made.

Figure 13. Original KG2UNI diagram.

6.3 Reducing heat transfer

Insulation reduces conduction and convection from the sides. A lid reduces convection and evaporation from the top. A reflective outer surface can reduce infrared emission. A larger temperature rise gives a smaller percentage uncertainty in Δθ, but heating too far above room temperature increases the rate of heat loss.

A useful compromise is to heat steadily through a moderate range. Another technique is electrical compensation or cooling correction, but at this level it is enough to explain that heat loss makes the supplied electrical energy exceed the energy gained by the sample.

6.4 Graphical approach

Record temperature at regular time intervals. A temperature-time graph should rise during heating. If power is constant and losses were negligible, the gradient would be constant. In reality the graph may become less steep as the sample gets hotter because the temperature difference from the surroundings increases and losses grow.

The initial gradient can be used to reduce the influence of later heat loss. If a straight-line section is obtained, energy supplied over that interval is PΔt and the corresponding temperature rise is read from the graph.

Figure 14. Original KG2UNI diagram.

6.5 Evaluation language

Good evaluation names the problem, states its effect and proposes a linked improvement. For example: “Energy is transferred to the surroundings, so VIt is larger than the energy gained by the block and the calculated c is too large. Wrap the block in thicker insulation and use a lid where appropriate.”

Do not write “repeat for accuracy” without saying what repetition achieves. Repeating and averaging reduces random variation. It does not remove a systematic heat loss. Also avoid “use better equipment” unless the specific improvement is named, such as a temperature probe with finer resolution or a power supply that maintains constant voltage.

Worked examples

Practical focus

Examination guidance

• A method question needs measurements, apparatus arrangement and calculation.
• State where the ammeter and voltmeter are connected if a circuit diagram is required.
• Link every suggested improvement to a named source of error.

Check your understanding

1. Write the expression for electrical energy supplied to a heater.
2. Why is a lid used when heating a liquid?
3. Why should the liquid be stirred?
4. A heater operates at 10 V and 2.5 A for 300 s. Find the energy supplied.
5. What is the effect of heat loss on the calculated c if ignored?