Internal Energy and Specific Latent Heat — AQA GCSE Physics
Internal energy is the total energy of the particles in a system. Changing state requires energy without changing temperature.
Internal energy
The internal energy of a system is the total kinetic and potential energy of all the particles (atoms and molecules) in it.
Heating a system increases the internal energy, which either:
- raises the temperature, or
- changes the state (e.g. melting or boiling).
Specific latent heat
The specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature:
$$E = m \times L$$
- E = energy (J)
- m = mass (kg)
- L = specific latent heat (J/kg)
There are two types:
- Latent heat of fusion — for melting/freezing.
- Latent heat of vaporisation — for boiling/condensing.
Heating graphs
On a graph of temperature against time while heating, the flat (horizontal) sections show changes of state — the temperature stays constant because the energy is used to break the forces between particles, not to raise their kinetic energy.
Exam tips
- Internal energy = total kinetic + potential energy of the particles.
- During a change of state, temperature is constant (energy breaks bonds between particles).
- Learn E = mL for changes of state.
- Distinguish latent heat of fusion (melting) and vaporisation (boiling).