Wednesday, January 7, 2015

Heat Engine and Second Law of Thermodynamics

Heat engine

Heat engine is a device which converts supplied heat into work. It is not practically possible to convert all the heat supplied into work without any wastage. It is simply because it is a disordered format of energy.

A heating and has three major parts. One is the source which supplies the heat energy, second one is the working system which will do some work with the supplied heat energy and the third one is a sink which will absorb the wasted heat energy.

The temperature of the source will be obviously always more than that of the sink. The heat energy supplied by the source is first of all used to do some external work. The entire heat energy cannot do the work and some of the heat energy is rejected to the Sink.

The efficiency of a heating and is defined as the ratio of the work done to the supplied heat energy. We can also express it in terms of the temperature of the source and sink as shown below.

T1 is the absolute temperature of the source and T2 is the absolute temperature of the sink.

Efficiency of a heating mean is always less than hundred percent.

Carnot’s heat engine

The working substance of this ideal engine is taken through a reversible cycle consisting of the four steps. The first step is isothermal expansion, second step is a adiabatic expansion. The third step is isothermal compression and the fourth step is the adiabatic compression so that the system comes back to its original state.

In each of this case is the work done equation is expressed as shown below.


It is ideal heat engine working in the reverse direction. A small portion of the heat energy is passed from the sink towards the working substance. From external direction some work is also done on the working substance and the the total generated heat is passed towards the source.

Problem and solution

In a heat engine if the source and sink are at temperatures 100 and 27 in centigrade and 27°C what is its efficiency ?

Problem and solution

This problem is also facing on the efficiency of the system.

Second law of thermodynamics

The first law of thermodynamics use the concept of conservation of energy and it won’t gives the direction of the flow of heat. It doesn't explain the direction which heated transfer takes place.

The second law views the direction of the flow of the heat. As per this law, heat always flows from a body of higher temperature to lower temperature. If the heat has to flow from a body of lower temperature to your body of higher temperature they shall be some support of an external agency. Without that external support it is impossible for the heat energy to transfer from a body of lower temperature to your body of higher temperature.

It can also be expressed in some another way. It is impossible for any self acting machine without any external support to transfer the heat from lower temperature to higher temperature.

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