Internal energy: Energy is stored inside a system by the particles (atoms and molecules) that make up the system. This is called internal energy.

The idea

Every system is made of particles, and those particles store energy. The energy stored by the particles of a system is called its internal energy. The particles have kinetic energy because they move or vibrate, and potential energy because of their positions and the forces between them. Heating a system transfers energy to its particles, increasing their kinetic or potential energy, so internal energy rises. For example, heating 1 kg of water from 20 °C to 80 °C increases its internal energy; the particles vibrate and move faster. Cooling reduces internal energy. Internal energy is not the same as temperature: temperature measures the average kinetic energy of the particles, while internal energy is the total energy stored by all the particles. A larger mass at the same temperature has more internal energy because it contains more particles.

What earns the marks

• Internal energy is energy stored by the particles that make up a system.
• Particles have kinetic energy due to movement or vibration and potential energy due to their positions and the forces between them.
• Heating a system transfers energy to its particles, increasing internal energy; cooling decreases it.
• Internal energy is a total energy store, whereas temperature relates to the average kinetic energy of the particles.
• For the same material and temperature, a larger mass has more internal energy because it contains more particles.

Common mistakes and how to put them right

• Saying internal energy is the same as temperature.

  Put it right: Temperature is a measure of average kinetic energy of particles, while internal energy is the total kinetic plus potential energy of all particles.

• Thinking internal energy only includes kinetic energy.

  Put it right: It also includes potential energy from particle positions and intermolecular forces.

• Believing a substance at a lower temperature always has less internal energy than one at a higher temperature.

  Put it right: Mass matters too, so a large cool object can store more internal energy than a small hot object.

Exam tips

• Define internal energy in terms of the particles of the system, not just 'heat inside'.
• When comparing internal energies, state both temperature and mass because both affect the total.
• Use the idea of energy transfer by heating to explain changes in internal energy, and link particle motion to kinetic energy.