Define heat engine .

Explanation

Heat Engine: In the modern technological world, the role of automobile engines plays a vital role in for transportation. In motor bikes and cars there are engines which take in petrol or diesel as input, and do work by rotating wheels. Most of these automobile engines have efficiency not greater than `40%`. The second law of thermodynamics puts a fundamental restriction on efficiency of engines. Therefore understanding heat engines is very important.
Reservoir: It is defined as a thermodynamic system which has very large heat capacity. By taking in heat from reservoir or giving heat to reservoir, the reservoir.s temperature does not change.
Example: Pouring a tumbler of hot water in to lake will not increase the temperature of the lake. Here the lake can be treated as a reservoir.
When a hot cup of coffee attains equilibrium with the open atmosphere, the temperature of the atmosphere will not appreciably change. The atmosphere can be taken as a reservoir.
We can define heat engine as follows: Heat engine is a device which takes heat as input and converts this heat in to work by undergoing a cyclic process.
A heat engine has three parts: (a) Hot reservoir (b) Working substance (c) Cold reservoir
A Schematic diagram for heat engine is given below:
1. Hot reservoir (or) Source: It supplies heat to the engine. It is always maintained at a high temperature `T_(H)`
2. Working substance: It is a substance like gas or water, which converts the heat supplied into work.
3. Cold reservoir (or) Sink: The heat engine ejects some amount of heat `(Q_(L))` in to cold reservoir after it doing work, It is always maintained at a low temperature `T_(L)`.
The heat engine works in a cyclic process. After a cyclic process it returns to the same state. Since the heat engine returns to the same state after it ejects heat, the change in the internal energy of the heat engine is zero.
The efficiency of the heat engine is defined as the ratio of the work done (out put) to the heat absorbed (input) in one cyclic process.
Let the working substance absorb heat `Q_(H)`, units from the source and reject `Q_(L)`, units to the sink after doing work W units.
We can write, `"Input heat" =" Work done" +" ejected heat"`
`Q_(H) = W+Q_(L)`
`W = Q_(H) -Q_(L)`
Then the efficiency of heat engine
`eta=("Qoutpu")/("Input")=(W)/(Q_(H))=(Q_(H)-Q_(L))/(Q_(H))=1-(Q_(L))/(Q_(H))`
Note here that `Q_(H),Q_(L)` and W all are taken as positive, a sign convention followed in this expression.
Since `Q_(L) lt Q_(H)`, the efficiency `(eta)` always less than 1. This implies that heat absorbed is not completely converted into work. The second law of thermodynamics placed fundamental restrictions on converting heat completely into work.