Internal energy is the sum of all the energies of a molecule in the frame of reference of the system.
According to \(1^{\text {st }}\) law,
\(\Delta Q=\Delta U+\Delta W\)
where \(\Delta U\) is internal energy, \(\Delta W\) is the work done and \(\Delta Q\) is the heat transferred.
\(\Delta U=\Delta Q-\Delta W\)