Question

In a heat engine, if 700 J of heat enters the system, and the piston does 400 J of work, what is the final internal (thermal) energy of the system if the initial energy is 1,200 J?

Answer 1

the answer would be 1,500 J or 1,700 J -which ever one is there-

Answer 2

We are given with:
Q = 700 J
W = 400

U1 = 1200 J

From the first law of thermodynamics
Q = ΔU + W
Q = U2 - U1 + W
U2 = Q - W + U1
U2 = 700 - 400 + 1200
U2 = 1500 J

We are given with
Q = 500 J
W = 300 J
U1 = 1500 J

U2 = 500 - 300 + 1500
U2 = 1700 J
the answer is 1) 1500 J, 2) 1700 J