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

100%

12.5%

75%

33%

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%