Matter: Internal Energy

Understanding Internal Energy

Internal energy is a measure of the total kinetic and potential energy of all the particles in a system.
Potential energy in this context relates to the energy stored in the particles due to their positions or the intermolecular forces acting between them.
The kinetic energy of the particles depends on their motion. More specifically, it is related to the speed at which particles move and vibrate.
The internal energy of a system can change when energy is transferred to or from the system, causing a change of state.

If a substance is heated, energy is transferred to the particles, increasing their kinetic energy. This increases the internal energy of the substance, and may lead to a change of state such as melting or evaporation.
If a substance is cooled, energy is transferred away from the system, reducing the kinetic energy of the particles. This decreases the internal energy of the system, and may lead to a change of state such as freezing or condensation.
During a change of state, the temperature remains constant. This is because all the energy transferred is used to change the potential energy of the particles, as they rearrange to change state.

There is a direct relationship between temperature and the kinetic energy of particles. As the temperature increases, the kinetic energy of the particles increases, and they move faster. Conversely, as the temperature decreases, the particles lose kinetic energy and move slower.
However, temperature doesn’t give an absolute measure of the internal energy. This is because internal energy also takes into account the potential energy, which depends on the physical state of the material and the forces between the particles, not just their kinetic energy.

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed. This is crucial in understanding changes of state and the concept of internal energy.
For instance, during fusion or phase transitions, energy is not lost; rather, it is transferred to breaking intermolecular forces, leading to a change in the state.