Q.What is Heat Engine? Write its principle and explain its construction and working.

A heat engine converts heat into work by cyclically absorbing heat Q1 from a hot source, converting part of it into work W, and rejecting the remaining heat Q2 to a cold sink; efficiency eta = W/Q1 = 1 - Q2/Q1.

What is a Heat Engine?

A heat engine is a device that converts heat energy, absorbed from a source at high temperature, into mechanical work, while operating continuously in a cycle (i.e., the working substance returns to its initial state after each cycle, so the process can repeat indefinitely).

Principle:

A heat engine works on the principle that heat CANNOT be converted completely into work in a cyclic process (this is essentially a statement of the second law of thermodynamics). Instead, only PART of the heat absorbed from a hot reservoir can be converted into useful work; the remaining part must necessarily be rejected to a cold reservoir.

Construction (essential parts):

Every heat engine has three essential parts:

1. Source (Hot Reservoir): A body at a high fixed temperature T1, from which the working substance absorbs heat Q1. It is assumed to have infinite heat capacity (so its temperature doesn't drop when it gives up heat).
2. Working Substance: A material (gas, steam, etc.) that undergoes a cyclic process, absorbing heat, doing work, and rejecting heat, then returning to its original state to repeat the cycle.
3. Sink (Cold Reservoir): A body at a low fixed temperature T2 (less than T1), which absorbs/receives the heat Q2 rejected by the working substance. It is also assumed to have infinite heat capacity.

Working:

In each cycle of operation:

1. The working substance absorbs heat Q1 from the source (at temperature T1).
2. Part of this heat is converted into mechanical work W by the working substance (e.g., by expanding and pushing a piston).
3. The remaining heat, Q2 = Q1 - W, is rejected by the working substance to the sink (at temperature T2).
4. The working substance returns to its initial state, ready to repeat the cycle. …