Internal energy (U)

Every substance is associated with a definite amount of energy. This energy stored in a substance is its internal energy, denoted by $U$. The internal energy of a system is made up of the kinetic and potential energies of the individual particles of the system.

where $U_1$ and $U_2$ are the internal energies of the initial and final states, respectively. $U$ is a state function and an extensive property.

A transfer of energy (as heat or work) from the system would change its internal energy. To know $\Delta U$, the energy supplied to or removed from the system needs to be monitored:

i. The energy transferred to the system by heating it or performing work on it is added to the system.

ii. The energy transferred from the system by cooling or by performing work on the surroundings is removed from the system.

The following examples illustrate how to determine $\Delta U$:

i. 30 kJ of heat is supplied to the system. It would be added to the internal energy of the system, and $\Delta U = +30$ kJ. …