Peoples Physics Concepts

18.4. Heat Engine http://www.ck12.org

18.4 Heat Engine

• Give an overview of Heat Engines and Pressure Volume diagrams.


• Calculate ideal efficiencies of heat engines.

Students will learn about Heat Engines, Pressure Volume diagrams and how to calculate ideal efficiencies of heat

engines.

Key Equations

Qin=Qout+W+∆U

Uis the internal energy of the gas. (This is the first law of Thermodynamics and applies to all heat engines.)

ec= 1 −

Tc

Th

Efficiency of a Carnot (ideal) heat engine

whereTcandThare the temperatures of the hot and cold reservoirs, respectively.

adiabatic expansionis a process occurring without the exchange of heat with the environment

isothermal expansionis a process occurring without a change in temperature

isobaric expansionis a process occurring without a change in pressure

Guidance

Heat engines transform input heat into work in accordance with the laws of thermodynamics. For instance, as we

learned in the previous Concept, increasing the temperature of a gas at constant volume will increase its pressure.

This pressure can be transformed into a force that moves a piston.

The mechanics of various heat engines differ but their fundamentals are quite similar and involve the following

steps:

1. Heat is supplied to the engine from some source at a higher temperature(Th).


2. Some of this heat is transferred into mechanical energy through work done(W).


3. The rest of the input heat is transferred to some source at a lower temperature(Tc)until the system is in its
   original state.

A single cycle of such an engine can be illustrated as follows: