• The first law of thermodynamics is a restatement of the law of conservation of energy. 
• It states that energy cannot be created nor destroyed in an isolated system; energy can only be transferred or changed from one form to another.

When heat energy is supplied to a gas, two things may occur:

1. The internal energy of the gas may change
2. The gas may do some external work by expanding

According to the first law of Thermodynamics:

δQ = δW + ΔU

When a process is executed by a system, the change in stored energy of the system is numerically equal to the net heat interaction minus the net work interaction during the process:

ΔU = δQ – δW 

where,

U is the internal energy which is introduced by this law.

For the cyclic process: ΔU = 0

The first law of thermodynamics states that for a cyclic process, the cyclic integral of heat is equal to the cyclic integral of work.