Cyclic Process

What Is Cyclic Process?

The process in which the initial and final state is the same is known as a cyclic process. It is a sequence of processes which leaves the system in the same state in which it started.

When a system undergoes a cyclic process, its initial and final internal energies are equal. Hence, the internal energy change in any cyclic process is zero. Applying the first law of thermodynamics to a cyclic process, we get

\(\Delta E=Q+W\)

As \(\Delta E=0\),


Hence, the work done by the system in a cyclic transformation is equal to the heat absorbed by the system. The net work involved in a cyclic process is the area enclosed in a P-V diagram. If the cycle goes clockwise, the system does work. If the cycle goes anticlockwise, then the work is done on the system every cycle. An example of such a system is a refrigerator or air conditioner. The Carnot engine is the best example of a cyclic process.

What Is Heat Engine?

A heat engine is a device by which a system is made to undergo a cyclic process that results in the conversion of heat to work.

  • Heat engines consist of a working substance– the system. For example, steam in a steam engine is the working substance.
  • The working substance goes through a cycle of several processes. In some of the processes, it absorbs heat from a reservoir at a high temperature.
  • In some processes, the working substance releases some amount of heat to an external reservoir at a lower temperature.
  • The work done by the system in a cycle is transferred to the environment through some arrangements. For example, the working substance may be in a cylinder with a moving piston that transfers mechanical energy to the wheels of the vehicle.

The schematic representation of a heat engine is represented as follows:

The cycle is repeated again and again to get the useful work done for some purpose.

What Is Refrigerator?

A refrigerator is a reverse of the heat engine. Here, the working substance extracts some amount of heat from the cold reservoir at some temperature T1. An external work W is done on it and heat Q is released to the hot reservoir at a temperature T2. The schematic representation of a refrigerator is given below.