Transcript Heat Engines

Thermodynamics
Mechanical Equivalent of Heat
Heat produced by other forms of energy
 Internal Energy: total available potential &
kinetic energy of particles
 Adding heat increases internal energy
 Joule’s experiment with the paddles
stirring the water proved heat is form of
energy

Expansion and Work
Many thermodynamic processes involve
expansion or compression of gases
 Expanding gases exert pressure
(force/area) and can do work (car engine)
 Work done equals pressure times volume
change for constant pressure
 For expansion with pressure change, work
equals area under curve of pressure vs.
volume graph

First Law of Thermodynamics
Heat energy supplied to closed system
equals work done by system plus change
in internal energy of system.
 Q = DU + W
 1st Law is actually conservation of energy
restated to include heat energy
 If no work is done by system, heat added
equals change in internal energy

Adiabatic Processes
A process where no heat is added or
allowed to escape
 Process must happen very quickly or be
well insulated
 Adiabatic compression of gas causes
temp. increase
 Adiabatic expansion of gas causes cooling

Isothermal Processes
No temperature change occurs
 Isothermal expansion requires heat input
from surroundings
 Isothermal compression requires heat
emission to surroundings
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Specific Heats of Gases
Gases have two specific heats, one for
constant volume (cv ) , and one for
constant pressure (cp )
 (cv ) < (cp ) because work must be done
against pressure to change volume
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Second Law of
Thermodynamics
It is impossible to convert all heat energy
into useful work
 Device that uses heat to do work can
never be 100% efficient
 Some heat will remain and must be
expelled into low temp. sink
 Heat will never flow from a cold object to a
hot object by itself without work input
 Absolute zero is unattainable

Heat Engines
Any device that turns heat into mechanical
energy
 Anything that burns fuel or uses steam to
move or do work
 Ideal heat engine cycle:

takes heat from high temp. source,
 does work using part of the energy,
 expels remaining heat energy into low temp.
heat sink.
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Ideal Heat Engine Diagram
Efficiency of Heat Engines
Efficiency = work done/heat input
 Work done = heat energy used
 Sadi Carnot (1796 – 1832) showed max.
efficiency for any heat engine = temp.
difference between hot source and cold
sink divided by temp. of source
 eideal = (Thot - Tcold )/ Thot
 For max. efficiency, industry uses high
temp. source, large body of water for
sink.
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Types of Heat Engines
Steam Engines: first heat engines; Watt,
Fulton, Newcomen; external combustion
 Steam Turbine: uses high pressure steam
to turn wheel with many cupped fan-like
blades
 Gasoline Engines: internal combustion
engine; heat from expanding combustion
gases drive piston
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Types of Heat Engines
Diesel Engines: heat from compression
ignites fuel; efficient, powerful, but heavy
 Gas Turbines: air compressed by turbine
forced through combustion chamber; used
on airplanes
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Types of Heat Engines
Jet Engines: expanding combustion gases
forced out rear of engine; action-reaction &
conservation of momentum create forward
thrust
 Rockets: like jets, but carries own oxidizer
to work outside atmosphere. Thrust
depends on velocity of exhaust gases

Heat Pumps
Reverse cycle of heat engine; work from
electric motor causes heat to flow from
cool area to warm area
 Uses easily condensed vapor (freon);
motor condenses vapor in compressor;
When allowed to vaporize, it extracts its
heat of vaporization
 Basis for refrigerators, air conditioners

Heat Pump Diagram
Entropy
A measure of the disorder of a system
 Is related to the amount of energy that
cannot be converted into mechanical work
 Natural systems tend toward greater
disorder (greater entropy)
 Controls the direction of time

Entropy
Work must be done to decrease entropy of
system
 Heat is disordered energy, increased
entropy
 Change in entropy of system equals heat
added divided by absolute temperature
 DS = DQ/T (J/K)

Entropy
Entropy increased in melting, evaporation,
organic decay
 Total energy of universe is constant, but
usable energy decreases due to increased
entropy
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