Download Ch 14.3 PPT - Using Heat

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Transcript
Heat and Temperature
Section 3
Using Heat
Chapter 14.3
Heat and Temperature
Section 3
Laws of Thermodynamics
〉What happens to heat energy when it is
transferred?
〉The first law of thermodynamics - total energy
used in any process is conserved, whether that
energy is transferred as a result of work, heat, or
both.
〉The second law of thermodynamics - energy
transferred as heat always moves from an object
at a higher temperature to an object at a lower
temperature.
Heat and Temperature
Section 3
Laws of Thermodynamics
• Work can increase average kinetic energy by – mechanical processes: processes in which energy is
transferred by work
• The disorder of a system tends to increase.
– Over time, in any given system left to itself, the entropy
of that system will tend to increase.
• entropy: a measure of the randomness or disorder of
a system
• Usable energy decreases in all energy transfers.
– As entropy increases – usable energy decreases
• Conduction and radiation may cause energy to be
transferred to the surrounding.
Heat and Temperature
Section 3
Heat Engines
〉Heat Engine: a machine that transforms heat into
mechanical energy, or work
〉 2 main types:
〉 Internal-combustion engines
〉 External-combustion engines
〉What do heat engines do?
〉 In a heat engine, potential chemical energy and internal
kinetic energy are converted to mechanical energy by
the process of combustion.
Heat and Temperature
Section 3
Heat Engines, continued
• Internal-combustion engines burn fuel inside the engine.
– An automobile engine is a four-stroke engine,
because four strokes take place for each cycle of the
piston.
– The four strokes are:
• intake
• compression
• power
• exhaust
• Internal-combustion engines always generate heat.
– Friction and other forces cause much of the energy to
be lost to the atmosphere as heat.
Heat and Temperature
Section 3
Heat Engines, continued
• Automobile engines (Internal-combustion) use carburetors
or fuel injectors.
– A carburetor is the part of the engine in which liquid
gasoline becomes vaporized.
– Some engines have fuel injectors instead of
carburetors.
Heat and Temperature
Section 3
Internal Combustion Engine
Heat and Temperature
Section 3
External-Combustion Engines
• External-Combustion Engine
– an (internal) working fluid is heated through the
engine wall or a heat exchanger by combustion in an
external source.
– The fluid (by expanding and acting on the mechanism
of the engine) produces motion and usable work.
– The fluid is then cooled, compressed and reused
(closed cycle), or (less commonly) dumped, and cool
fluid pulled in (open cycle air engine).
• Example: Steam Engine
Heat and Temperature
Section 3
External-Combustion Engine