The specific heat capacity of a substance is the amount of energy required…

The idea

Specific heat capacity is an energy price per kilogram per degree. The word specific means per kilogram, and the unit J/kg °C carries the whole definition: joules, per kilogram, per degree Celsius. Water is unusually expensive at 4200 J/kg °C, while aluminium is about 900 and copper about 385. A high value has two consequences worth remembering: the substance needs a lot of energy to warm up, and it also releases a lot as it cools. That is why water is used in heating systems and engine coolants, and why the walls of a fusion reactor are chosen for a very high specific heat capacity, so that an enormous energy transfer produces only a small temperature rise and the walls stay below their melting point.

What earns the marks

• The energy needed to raise the temperature of one kilogram of the substance.
• Specifying a rise of one degree Celsius.
• Stating that a large energy transfer causes only a small temperature increase when the specific heat capacity is high.
• The consequence in context, for example that the reactor walls do not reach their melting point.

Common mistakes and how to put them right

• Omitting the one kilogram and defining it as the energy to raise the temperature of the substance by one degree.

• Writing the energy needed to raise the temperature to 1 °C rather than by 1 °C.

• Giving the definition of specific latent heat, which involves no temperature change at all.

• Assuming a high specific heat capacity means the material cannot store much energy.

Exam tips

• Learn the definition as three separate conditions: energy, one kilogram, one degree Celsius; each is worth checking before you move on.
• Quote the unit J/kg °C when asked for it; it reads the definition back to you if you forget the wording.
• In an application question, always finish with the effect on temperature, not just the value of c.