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In thermodynamics, a system is a part of the universe which is of interest and the rest of the universe is known as surroundings. The first law of thermodynamics states thatthe total energy of the system including the surroundings remains constant, i.e. energy can neither be created nor destroyed but the energy can be transformed from one form to another.

Every system has a certain amount of energy which is denoted by U. Some of this energy can be used to perform work when a system undergoes a change.The change in the energy of the system is due to the difference between the heat (q) which is absorbed by the system from its surrounding and the work (w) done by the system on the surrounding.

According to the first law of thermodynamics, the relationship between energy (U) work (w) and heat (q) is given by:

Here, Greek letter ∆(delta) represents change.