Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 19 - Heat Engines and Refrigerators - Exercises and Problems - Page 551: 45

Answer

No.

Work Step by Step

To find if the salesman's claims are right or wrong, we need to find the maximum coefficient of performance of a refrigerator between two reservoirs of $T_H=(22+273)\;\rm K$ and $T_C=(2+273)\;\rm K$. This coefficient of performance is called the Carnot coefficient which is given by $$K_{\rm Carnot}=\dfrac{T_C}{T_H-T_C}$$ Plugging the known; $$K_{\rm Carnot}=\dfrac{(2+273)}{(22+273)-(2+273)}=\bf 13.75$$ Now this is the maximum performance we can get from this refrigerator, so let's find the real coefficient of performance of it to compare the results. $$K=\dfrac{Q_C}{W_{in}}\tag 1$$ We know that DreamFridge uses 100 W of power to remove 100 kJ of heat per minute. This means that the work is given by $$W_{in}=Pt$$ where $P$ is the power and $t$ is the time. Plugging into (1); $$K=\dfrac{Q_C}{Pt}$$ Plugging the known; $$K=\dfrac{100\times 10^3}{100\times 60}=\bf16.67 $$ And since the real performance according to the claims of the salesman is greater than the Carnot performance, then he lies and you should not buy this one.
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