A heat engine with an efficiency of 20% does 400·J of work in each cycle. (a) Ho

Question

A heat engine with an efficiency of 20% does 400·J of work in each cycle. 

(a) How much heat must be supplied from the high-temperature source in each cycle?
 
(b)Suppose you change the engine by increasing the temperature of the high-temperature source, but continue to exhaust the waste heat at the same temperature as before. How much heat would need to be supplied to the engine to once again do 400·J of work in each cycle? Choose one answer only.

a) more heat
b) same amount of heat    
c) less heat
d) could be more, same, or less heat...depending on the heat engine

Explanation / Answer

a)

efficiency, n = W/Q

So, 0.2 = 400/Q

So, Q = 400*5 = 2000 J

b)

From the relation,

n = 1 - Tl/Th

As Th increases, n increases

So, for same work done, Q will decrease so that n increase

So, answer (c) less heat

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