- Every particle in a body has potential energy, due to their state and position, and, kinetic energy, due to their motion. Collectively, the sum of these energies is called the internal energy of the body.
- Potential energy of particles in a body tends to be small and does not change very much, a change in internal energy is generally due to a change in kinetic energy.
- The temperature of an object gives a measure of how hot or cold it is, but it is not a measure of how much internal energy the object contains.
Example: Consider a box resting on a horizontal surface. It contains internal energy due to the molecules that make up the box having potential energy and kinetic energy. The potential energy and kinetic energy referred to are both internal. You can visualise internal potential energy as the energy to assemble the box and internal kinetic energy as the energy the molecules possess (the molecules in the box are vibrating constantly due to thermal energy).
When the box is pushed along the horizontal surface, the box acquires external kinetic energy. This external kinetic energy has nothing to do with the internal kinetic energy. In Work, energy and power, the kinetic energy that is referred to is the external kinetic energy.