Heat engine

In thermodynamics, heat engine refers to a system capable of converting the heat input to the system into mechanical work.
eg.: Steam engine.
In thermodynamic analysis, while solving problems a typical heat engine is represented as follow.

Representation of heat engine.

Here the heat engine acts between source and sink.Source will be at high temperature

Thermodynamic system

System:

A system may be defined as a well defined region in space, which is considered for thermodynamic analysis.Thermodynamic system is classified into three categories.

1. Closed system
2. Open system
3. Isolated system

First law of thermodynamics

 It is nothing but the law of conservation of energy.

Closed system:

For a closed system, the law can be represented as

where Q is the heat input to the system considered
          W is the work obtained from the system
          dU is the change in internal energy of the working fluid of the system.

Let us consider a simple compression process in a piston cylinder arrangement.

 Before compression process, let the internal energy of the working fluid (say air) be U1.Due to compression internal energy of the working fluid increases considering the process as adiabatic (no heat exchange).Let it be U2.In this case, increase in the internal energy can be experienced as increase in pressure of the working fluid.
Then by first law

dU (U2-U1) = -W since Q=0 for an adiabatic process.

 -sign indicates that work is given to the system.

 

Zeroth law of thermodynamics

Statement:

                   "If a body A is in thermal equilibrium with a body B, and the body B is in thermal equilibrium with a body C, then the bodies A and C are said to be in thermal equilibrium."