The first law of thermodynamics (also called conservation of energy) expresses the principal of the conservation of energy. This means that the total amount of energy in the universe is constant and that it can neither be created or destroyed. This law states that for every gain in some type of energy will result in the loss in some other form.
In general, the conservation of energy can be stated as follows:
\(\triangle U = Q \;-\; W \;\)
\(\triangle U\) = internal energy
\(Q\) = heat added to the system
\(W\) = work done by the system