Internal Energy
- The internal energy of a substance includes both the kinetic and potential energy of the particles it contains
- The molecules of all substances contain both kinetic and potential energies
- Kinetic energy is due to the speed of the molecules
- Potential energy is due to the separation between the molecules
- The amount of kinetic and potential energy a substance contains depends on its phase of matter (solid, liquid or gas)
- This is known as the internal energy
- The internal energy of a substance is defined as:
The sum of the randomly distributed kinetic and potential energies of the particles in a body
- The symbol for internal energy is U, with units of joules (J)
- Particles are randomly distributed, meaning they all have different speeds and separations
- The internal energy of a system is determined by:
- Temperature (higher temperature, higher kinetic energy and vice versa)
- The random motion of molecules
- The phase of matter: gases have the highest internal energy, solids have the lowest
- Intermolecular interactions between the particles
- 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
- Changing state from a gas to a liquid or liquid to a solid
Exam 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!