Specific heat capacity: the energy needed to raise the temperature of a substance, the equation linking change in thermal energy to mass, specific heat capacity and temperature change, and the required practical.

What this dot point is asking

AQA wants you to define specific heat capacity, use the equation linking change in thermal energy to mass, specific heat capacity and temperature change, and describe the required practical for measuring the specific heat capacity of a material.

What specific heat capacity means

A material with a high specific heat capacity, such as water ($c \approx 4200\,J/kg\,^{\circ}C$), needs a large amount of energy to warm up and stores a large amount of energy. This is why water is used in central-heating systems and as a coolant. The same property explains why coastal areas have milder climates than places far inland: the sea warms up and cools down slowly because of water's high specific heat capacity, so it moderates the temperature of nearby land. By contrast, metals such as copper and aluminium have low specific heat capacities (around $385$ and $900\,J/kg\,^{\circ}C$), so they heat up quickly for a given energy input, which is useful in cooking pans where you want a fast temperature response.

It is worth being clear about what specific heat capacity is not. It is not the same as the total heat capacity of an object, which depends on how much of the material there is. A large swimming pool and a cup both contain water with the same specific heat capacity, but the pool needs far more energy to warm by one degree because it has much more mass. The specific heat capacity is a property of the material itself, independent of the amount.

The equation

Required practical: measuring specific heat capacity

You heat a known mass of the material (a metal block or a beaker of water) with an electric immersion heater. A joulemeter (or voltmeter and ammeter with a timer) measures the energy supplied, and a thermometer records the temperature rise.

In the standard version of the practical, a cylindrical metal block has two holes drilled in it, one for the heater and one for the thermometer (a little oil or water in the thermometer hole improves thermal contact). You record the temperature, switch on the heater for a measured time while recording the energy supplied, and record the final temperature. The energy supplied electrically can be found from $E = Pt$ if you know the power, or read directly from a joulemeter. Plotting temperature against energy supplied gives a straight line whose gradient can be used to find the specific heat capacity, which often gives a more reliable result than a single pair of readings.

Try this

Q1. Define specific heat capacity. [2 marks]

• Cue. The energy needed to raise the temperature of $1\,kg$ of a substance by $1\,^{\circ}C$.

Q2. Calculate the energy needed to raise the temperature of a $2\,kg$ aluminium block ($c = 900\,J/kg\,^{\circ}C$) by $15\,^{\circ}C$. [3 marks]

• Cue. $\Delta E = mc\Delta\theta = 2 \times 900 \times 15 = 27000\,J$.