Introduction to SAT II Physics - FreeExamPapers

is concerned, but we have included it for the sake of completeness.
Heat Engines
A heat engine is a machine that converts heat into work. Heat engines are important not only
because they come up on SAT II Physics, but also because a large number of the machines we use
—most notably our cars—employ heat engines.
A heat engine operates by taking heat from a hot place, converting some of that heat into work,
and dumping the rest in a cooler heat reservoir. For example, the engine of a car generates heat by
combusting gasoline. Some of that heat drives pistons that make the car do work on the road, and
some of that heat is dumped out the exhaust pipe.
Assume that a heat engine starts with a certain internal energy U, intakes heat
from a heat
source at temperature , does work , and exhausts heat into a the cooler heat
reservoir with temperature
. With a typical heat engine, we only want to use the heat intake,
not the internal energy of the engine, to do work, so
tells us:
. The First Law of Thermodynamics
To determine how effectively an engine turns heat into work, we define the efficiency, e, as the
ratio of work done to heat input:
Because the engine is doing work, we know that > 0, so we can conclude that > .
Both and are positive, so the efficiency is always between 0 and 1:
Efficiency is usually expressed as a percentage rather than in decimal form. That the efficiency of
a heat engine can never be 100% is a consequence of the Second Law of Thermodynamics. If
there were a 100% efficient machine, it would be possible to create perpetual motion: a machine
could do work upon itself without ever slowing down.
EXAMPLE
80 J of heat are injected into a heat engine, causing it to do work. The engine then exhausts 20 J of
heat into a cool reservoir. What is the efficiency of the engine?
196