# Heat Engine | Efficiency of Heat Engine | Physics Grade 11 Notes

### Thermodynamics

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#### Kinematics

Introduction to Kinematics

# Heat Engine | Efficiency of Heat Engine

Heat Engine
Heat engine is a device that continuously converts heat energy into mechanical energy.

1. Source: The source is the part of the heat engine which is a hot body at a constant temperature from which heat engine can draw heat
2. Sink: Sink is a hot body at a constant low temperature to which any amount of heat can be rejected.
3. Working Substance: Working substance is an ideal gas which is being supplied with heat performs mechanical work.
Efficiency of Heat Engine
It is defined as the ratio of the external work obtained to the heat energy absorbed by the working substance from the source. It is denoted by η.
η = external work obtained / heat energy absorbed from the source

In the above figure of the heat engine, the working substance absorbs Q1 amount of heat from the source at higher temperature T1, then it does W amount of work, and Q2 be the amount of heat rejected and flowed to the sink at a lower temperature T2. Then the amount of heat used to do work is given by
W = Q1 – Q2
Since the efficiency of the engine, η = W/Q1
η = (Q1 – Q2)/Q1
η = 1 - Q2/Q1
No engine converts all the heat absorbed from the source into work. SO, the efficiency of an engine is always less than one or 100%.

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