SAT Subject Physics Practice Question 678: Answer and Explanation
14. A heat engine receives 150 Joules of heat energy and performs 70 Joules of work. If heat cannot be removed from the heat engine in any other way, by what value does the internal energy of the system change?
A. 0 J
B. 40 J
C. 80 J
D. 120 J
E. 150 J
Correct Answer: C
The correct answer is (C). The first law of thermodynamics is the answer here. ΔQ = ΔU + ΔW. The work done by the system is 70 Joules, but 150 Joules of heat was added. Rearrange the first law equation and subtract the work from the input heat to find the increase in internal energy. ΔQ - ΔW = ΔU leads us to 150J - 70J = 80J.