This law, derived from the principle of conservation of energy, is expressed as E = Q - W, where E is the internal change in energy, Q is the heat the environment transfers into the system, and W is the work the system does. In other words, it means that: the internal energy in a system isolated from other systems remains constant the change in the internal energy of a system that is not isolated is equal to the energy that crosses its boundaries into or out of other systems It can be further extrapolated to mean that despite all the processes going on in the universe, the total energy contained in the universe remains constant.