Internal energy U of a system is the sum of all its microscopic kinetic energy, microscopic potential energy and other forms of microscopic energy it has. In another form, the change in the internal energy ΔU of a system can be expressed as the sum of the amount of heat transferred to (or from) the system Q and the work done to (or by) the system W. The values of Q and W would take a negative sign, in the case of heat removed from the system and work done by the system.
ΔU = Q + W
This explains the first law of thermodynamics as the energy in a system is conserved and can only be transferred but not created or destroyed.
Frequently Asked Questionsedit
What happens to the internal energy of an isolated system at absolute zero?edit
At absolute zero, the system would still retain its internal energy, mostly as potential energy. Though you would expect the kinetic energy to drop to zero at 0 K, there will be a little kinetic energy left in the system. This is known as the zero-point energy of the system.