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Amrit Pal , asked a question
Subject: Physics , asked on 26/10/12

What is heat engine? Explain the principle of working of heat engine ? Define its efficiency ?


Nitish Kumar Gupta , Meritnation Expert added an answer, on 29/10/12

Heat engine is a device which converts heat energy into mechanical work by exploiting the temperature difference between a hot source and a cold sink. Heat is transferred from the source to the sink, through the working substance of the engine. During this transfer, some of the heat energy is converted into work. The working substance may be a gas or a liquid.

Heat Engine Principle

For the typical heat engine, as shown, the working agent is assumed to complete a cyclic process. There is no net gain or loss of energy within the boundary (Δ E = 0). This is in accordance with the First Law of Thermodynamics or the law of conservation of energy.

Qi + Wi = Qo + Wo

Also the net work done by the engine is equal to (the work out - the work expended ).

W = Wo - Wi.....= Qi - Qo

The efficiency of the heat engine is the ratio of the useful work output and the energy input i.e.


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Diksha Pandey , added an answer, on 27/10/12
10 helpful votes in Physics

it is a device that converts heat energy continuosly into mechanical energy. it consists of 3 parts- source,sink and working substance.....the working substance absorbs heat from the source,sourceon losing heat gets cooled then the working substance does some mechanical (PV) work and rejects the left out heat to the sink and as such temp.of sink rises.no heat energy can convert all heat energy into mechanical energy without losing heat..as we also know that it is not possible 4 a self acting machine unaided by any external agency to transfer heat from a body to another body at higher temp.- Claussius statement.

efficiency of a heat engine is defined as the ratio of net work done by heat engine in one cycle to the amount of heat absobed from the source at higher temp.working substance absorbs this hear from the source 

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