Defining internal energy
- Internal energy, U is determined by the state of the system
- Temperature
- The random motion of molecules
- The state of matter: gases have the highest internal energy, solids have the lowest
- Intermolecular interactions between the particles
- The molecules of all substances have both kinetic and potential energies
- Kinetic energy is due to the speed of the molecules
- Potential energy is due to the intermolecular forces between the molecules and varies with the separation of particles
- The internal energy of a substance is defined as:
The sum of the random distribution of kinetic and potential energies within a system of molecules
Internal energy of water molecules
All molecules in a substance possess both kinetic and potential energies
- The internal energy of a system can increase by:
- Doing work on it
- Adding thermal energy to it (heating it)
- The internal energy of a system can decrease by:
- Losing thermal energy to its surroundings
- The system doing work on its surroundings (e.g. a gas pushing a piston)
Examiner Tip
When an exam question asks you to define “internal energy”, you can lose a mark for not mentioning the “random motion” of the particles or the “random distribution” of the energies, so make sure you include one of these in your definition!