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Workshop 7. Heat, Work, and Efficiency
A machine's energy output cannot be greater than its
input. In this session, look at the energy that goes into useful
work, examine how some always ends up as heat, and see why systems
are never 100% efficient.
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Program Questions
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Answers
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1. How can we get work from heat?
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We get work from heat by exploiting
a temperature difference between two parts of a system, usually,
by expanding and contracting a gas.
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2. Why do hot things cool down and cold
things warm up?
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Heat is the disordered motion of molecules.
Hot places — ones where the molecules are moving more
vigorously — will scatter their kinetic energy by the
randomly bumping and jostling slower moving, or cooler, molecules.
This makes the hot places cool down. When the cooler molecules
start moving faster, they become warmer.
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3. What factors limit a system's efficiency
in getting work from heat?
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The key factor is the temperature difference
between the system's heat source and the heat sink. The greater
the temperature difference, the higher the possible efficiency.
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4. How are heat, work and friction related?
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Friction converts the ordered energy
of mechanical motion into the disordered — and less useful
— energy of heat.
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 Further
Explanations
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