The Physics Question Bank

Specific heat capacity questions appear on almost every paper, both as calculations and as questions about the required practical. Here’s everything you need.

What is specific heat capacity?

The specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1 kg of the substance by 1 °C. Water has a high specific heat capacity of 4200 J/kg°C, which is why it takes so long to boil a full kettle.

The equation

ΔE = m × c × Δθ

• ΔE = change in thermal energy (J)
• m = mass (kg)
• c = specific heat capacity (J/kg°C)
• Δθ = temperature change (°C)

Worked example

How much energy is needed to heat 2 kg of water from 20 °C to 70 °C? (c = 4200 J/kg°C)

1. Temperature change: 70 − 20 = 50 °C
2. Substitute: ΔE = 2 × 4200 × 50
3. Answer: ΔE = 420,000 J (420 kJ)

A common mistake is to use the final temperature (70 °C) instead of the change in temperature. Always subtract first!

The required practical

1. Measure the mass of a metal block (usually 1 kg).
2. Wrap the block in insulation.
3. Put an electric heater in one hole and a thermometer in the other. A few drops of water or oil in the thermometer hole improves thermal contact.
4. Connect the heater to a joulemeter (or use an ammeter and voltmeter) and record the starting temperature.
5. Switch on and record the temperature and energy supplied every minute for about 10 minutes.
6. Plot a graph of temperature against energy. Use the gradient to calculate c.

Why is the result often too high?

Some of the energy supplied is lost to the surroundings instead of heating the block. So the temperature rise is smaller than it should be for the energy measured, which makes the calculated specific heat capacity higher than the true value. Insulating the block reduces this energy loss.

Top tips

• Convert mass to kg (e.g. 500 g = 0.5 kg).
• Use the temperature change, not a single temperature.
• Be ready to rearrange: c = ΔE ÷ (m × Δθ).