Air contained in a cylinder comprises the system. A cycle is completed as follows: Piston does 85 KJ of work on air during compression stroke while 42 KJ of heat is rejected to the surroundings. During expansion stroke, air does 120 KJ of work on the piston. Determine the heat transfer.
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This question requires the application of the first law of thermodynamics.
From the first law of thermodynamics :
Q = W + ΔE
Where :
Q = the total energy.
W = Work done
ΔE = Change in energy
From the formula :
ΔE = Q - W
In the first process:
Q = -42( it is negative since heat is lost)
W = -85(work done on air)
ΔE = -42 - (-85) = 43 KJ
In the second process :
Q = W + ΔE
W = 120 (work air does on piston)
ΔE = -43 kJ(It is negative since end conditions are reversed.
Q = 120 - 43 = 77
= 77 KJ
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