Internal Energy of a System – revision notes

On the basis of the kinetic theory of matter, we can discuss the concept of internal energy as the sum of the energies of individual components/constituents. This includes kinetic energy due to their random motion and their potential energy due to interactions amongst them.

Let us now discuss these.

Internal energy is the sum of kinetic energy and potential energy – how?

Internal energy consists of the Internal kinetic energy and Internal potential energy.

(a) Internal kinetic energy: According to kinetic theory, matter is made up of a large number of molecules. These molecules are in a state of constant rapid motion and hence possess kinetic energy. The total kinetic energy of the molecules constitutes the internal kinetic energy of the body.

(b) Internal potential energy: The energy arising due to the intermolecular forces is called the internal potential energy.

Example of internal energy: The internal energy of a metallic rod is made up of the kinetic energies of conduction electrons, potential energies of atoms of the metal, and the vibrational energies about their equilibrium positions.

How to increase the internal energy of a system?

The internal energy of the system may be increased by causing its molecules to move faster (gain in kinetic energy by adding thermal energy). It can also be increased by causing the molecules to move against inter-molecular forces, i.e., by doing work on it.

Internal energy symbol and formula

Internal energy is denoted by the letter U.

Formula: The internal energy of a system = Kinetic energy of molecules + Potential energy of molecules