Internal Energy and the Kinetic Theory of Matter

Every substance is made of enormous numbers of tiny particles: atoms, molecules, or ions. The kinetic theory of matter is the model that explains the large-scale behavior of materials in terms of the continuous motion of these particles. According to this model, all particles are in constant random motion, and the nature of that motion, together with the forces between particles, determines whether a substance is a solid, a liquid, or a gas. The kinetic theory transforms abstract thermal quantities into a concrete mechanical picture of jostling, vibrating, and colliding particles.

The total energy contained within a substance due to the motion and arrangement of its particles is called its internal energy. Internal energy has two distinct contributions. The first is the kinetic energy of t