Conservation of Energy

Energy can neither be created nor destroyed.  It may be transferred, transported or transformed, but the total amount of energy in the universe, or in a totally isolated system, never changes.  Hence, if system 1 interacts with system 2 and loses energy in the amount ΔE in the process, then system 2 must have an increase of energy in the amount ΔE.

Total energy change = Energy change of system 1 + Energy change of system 2 =
-ΔE + ΔE = 0.