Question

How can one define the internal energy of a system?

Answer 1

Definition of Internal Energy:

1. The sum of all the energies contained in a system, such as mechanical, chemical, potential, kinetic, and so on, is its internal energy. Internal energy is denoted by the letter U, and joules are the unit of measurement (J).

Internal Energy:

1. The total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational motion and electric energy of atoms within molecules constitutes the internal energy of a system with some specific boundaries.
2. The internal energy is a state function that is it only depends upon initial and final points.
3. The internal energy is extensive quantity.
4. For a closed system, the internal energy is essentially defined by $\Delta U=q+W$
5. $\Delta U$ is the change in internal energy, $q$ is the heat and $W$ is the mechanical work done.