Heat Engine - Practice Questions & MCQ

A heat engine is a device that converts heat into work continuously through a cyclic process.

The essential parts of a heat engine are

1. Source: It is a reservoir of heat at high temperature and infinite thermal capacity. Any amount of heat can be extracted from it.

2. Working substance: Steam, petrol, etc.

3. Sink: It is a reservoir of heat at low temperature and infinite thermal capacity. Any amount of heat can be given to the sink.

• Working of heat engine

As shown in the above figure, The working substance absorbs heat Q₁ from the source, does an amount of work W returns the remaining amount of heat (i.e Q₂) to the sink and comes back to its original state and there occurs no change in its internal energy.

To obtain work continuously, the same cycle is repeated over and over again.

•  The efficiency of the heat engine ()- It is defined as the ratio of useful work obtained from the engine to the heat supplied to it. The performance of the heat engine is expressed by means of “efficiency”.

$$
\text { I.e } \eta=\frac{\text { Work done }}{\text { Heat input }}=\frac{W}{Q_1}
$$

For a cyclic process $\Delta U=0$
so From the first law of thermodynamics, $\therefore \Delta Q=\Delta W \quad$ so $\quad W=Q_1-Q_2$

$$
\eta=\frac{Q_1-Q_2}{Q_1}=1-\frac{Q_2}{Q_1}
$$
 

Practically efficiency of an engine is always less than 1.