Midterm 1, question 4

[phiryn]

For the question:

A mechanical device and its battery together do 500kJ of work. the battery also releases 250 kJ of energy as heat and the mechanical device releases 50kJ as heat due to friction. What is the change in internal energy of the system with the system regarded as the battery and mechanical device together.

Why isn't internal energy, 200 kJ?

For q, since it was both releasing heat I assumed: -250 + -50 = -300, why is this wrong? Isn't heat released mean q = (-)

[Dhruv Suresh 2J]

Hi, I think that you are right about q being -300 kJ. However, I believe that w (work) would equal to -500 kJ as the system is doing the work; thus, the overall change in internal energy would be: -300 kJ + (-500 kJ) = -800 kJ.

[106036120]

Hi, you're right about q but the question also says that the system did work.

The formula, U = q + w, tells you the change in internal energy, which is what the question is asking. You plug in your q and the amount of work the system did (-500 kJ) and you get that the change in internal energy is -800kj.

[phiryn]

Hi, I think that you are right about q being -300 kJ. However, I believe that w (work) would equal to -500 kJ as the system is doing the work; thus, the overall change in internal energy would be: -300 kJ + (-500 kJ) = -800 kJ.

So if work is being done TO the system, it would be +500 kJ?