# Conversion of heat into work

I am trying to learn Second law of thermodynamics, but could not intuitively understand why

heat can not be completely converted into work in a cyclic process but can be completely converted into work in a non-cyclic process.

If someone can explain it with some example having rotational motion of turbine than linear motion of piston it would be really helpful (because some people used the concept that you need to do some work to bring the piston back to its original position, which should not be the case in turbine motion).

Also, if you are using the concept of entropy, please define it first because I have learned two meanings of entropy:

1. entropy from the classical thermodynamics point of view where it is just $$\frac{Q}{T} ;$$

2. entropy from statistical thermodynamics point of view where it is the probability of a microscopic state of system it could be in.

• I am not sure what you mean by "...some example having rotational motion of turbine than linear motion of piston...". As far as I am aware the only intuitive way to understand why heat cannot be converted completely into work in a cycle is to show that if it did then we could obtain unphysical phenomena such heat flowing spontaneously from cold body to hot body. – Deep Dec 23 '16 at 5:10
• Here i mean if you are explaining with some example then the one in which heat is converted into work through rotational motion of turbine will be better then the one in which heat is converted into work by linear motion of piston. – Divyansh raka Dec 23 '16 at 15:29
• You mean the reciprocating linear motion of piston? – Narasimham Jun 17 '17 at 23:17
• Yes exactly.@Narasimham – Divyansh raka Jun 19 '17 at 9:02
• Can anyone explain how heat can be completely converted to work in a non-cyclic process? – David White Aug 17 '18 at 19:31