Peoples Physics Book Version-3

19.1. The Big Ideas http://www.ck12.org

• In a practical heat engine, the change in internal energy must be zero over a complete cycle. Therefore, over
  a complete cycleW=∆Q.


• The work done by a gas during a portion of a cycle =P∆V, note∆Vcan be positive or negative.


• The efficiency of any heat engine :η=W/Qin


• An ideal engine, the most efficienttheoreticallypossible, is called a Carnot Engine. Its efficiency is given by
  the following formula, where the temperatures are, respectively, the temperature of the exhaust environment
  and the temperature of the heat input, in Kelvins. In a Carnot engine heat is input and exhausted in isothermal
  cycles, and the efficiency isη= 1 −TcoldT
  hot.T heStirlingengineisarealli f eheatenginethathasacyclesimilartothetheoreticalCarnotcycle.T heStirlingengineisverye f f icient(es peciallywhencom paredtoagasolineengine)andcouldbecomeanim portant playerintoday′sworldwheregreenenergyande f f iciencywillreignsu preme.

• MEDIA

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Key Equations

Q=mc∆T; the heat gained or lost is equal to the mass of the object multiplied by its specific heat multiplied by the

change of its temperature.

Q=mL; the heat lost or gained by a substance due to a change in phase is equal to the mass of the substance

multiplied by the latent heat of vaporization/fusion (Lrefers to the latent heat)

1 cal = 4.184 Joules ; your food calorie is actually a kilocalorie (Cal) and equal to 4184 J.

Qin=Qout+W+∆U

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

(

1

2

mv^2

)

avg

=

3

2

kT

The average kinetic energy of atoms (each of massmand average speedv) in a gas is related to the temperatureT

of the gas, measured in Kelvin. The Boltzmann constantkis a constant of nature, equal to 1. 38 × 10 −^23 J/K.

P=

F

A

The pressure on an object is equal to the force pushing on the object divided by the area over which the force is

exerted. Unit for pressure are N/m^2 (called Pascals)

PV=NkT