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Hello everyone.
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This is problem 79 from chapter 18.
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It tells us that engine a has an efficiency of 66%.
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Engine b absorbs the same amount of heat from the hot reservoir and exhausts twice as much heat to the cold reservoir.
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Which engine has the greater efficiency? explain.
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So let's remember that efficiency is defined as the amount of work an engine does, divided by the heat absorbed at the hot reservoir.
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And also we can write this as qh minus qc over qh.
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This is qc.
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Okay, so if we're told that engine a has a certain efficiency and then engine b absorbs the same amount of heat, but it exhausts twice as much, that means that qc is twice as big in the case of engine b.
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Okay, so what is that going to do the efficiency? well, since there's a negative sign here, that means as you increase q sub c, then your efficiency goes down.
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It decreases.
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So the answer is that engine a has a greater efficiency.
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Engine b has a smaller efficiency, a less efficiency, because of this negative sign here because it exhausts more heat on q sub c.
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So that means the amount of work it's doing is actually less.
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So it has a smaller efficiency...