. In a certain process a small engine used heat from gasoline at a rate of 50 W

Question

.

In a certain process a small engine used heat from gasoline at a rate of 50 W for 20 minutes. During the same process there was a total of 15 kJ of work output over the 20 minute period. Based on the sign convention used in our class (energy as work entering system is negative, heat as work entering system is positive), answer the following four questions.

In this process, the amount of heat transfer, Q, is

A) 1000 J
B) 1000 kJ
C) -60 kJ
D) 60 kJ
E) 60 J

25.

In this process the engine power output, is

A) 50 kJ
B) 15 kJ
C) 15 J
D) 12.5 kW
E) 12.5 W

26.

In this process, the engine efficiency is,

A) 10%
B) 25%
C) 50%
D) 100%
E) 4.0

Explanation / Answer

24)

heat is enetering system so positive
Heat = power * time
      = 50 *20*60
      =60000 J
       = 60 KJ
Ans: E

25)
Power = work / time
             = 15000/(20*60)
             = 12.5 w

Ans: E

26)
efficiency= output power * 100/input power
               =12.5*100/50
              = 25 %

Ans: B

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