In thermodynamics we deal with two basic forms of energy: heat and work. Work (w) is defined to be all forms of energy other than heat.

The law of conservation of energy tells us that energy can not be created or destroyed in a process: it can only be transferred between the system and the surroundings.

DH_system = -DH_surroundings

The first law of thermodynamics is just the mathematical formulation of the above definition of heat and work and the law of conservation of energy. In any process, the total change in energy of the system, DE, is equal to the heat absorbed from the surroundings + the work done on the system by the surroundings

DE = q + w

Another, simpler way of stating the First Law of Thermodynamics is by Clausius: "The energy of the universe is a constant" This is obvious when you think about it: if energy is only transferred between system and surroundings and both exist in the universe, then the total energy of the universe doesn't change during any process.

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