Question

A steam engine absorbs 4 x 105 J and expels 3.5 x 105 J in each cycle. What is its efficiency?

Answer 1

Input heat, Qin = 4 x 10⁵ J
Output heat, Qout = 3.5 x 10⁵ J

From the first Law of thermodynamics, obtain useful work performed as
W = Qin  -  Qout
     = 0.5 x 10⁵ J

By definition, the efficiency is
η = W/Qin
   = 100*(0.5 x 10⁵/4 x 10⁵)
   = 12.5%

Answer: The efficiency is 12.5%