Finding the change of internal energy

In summary, a student moves out of a dormitory and does 2.69 x 104 J of work. This results in a decrease in his internal energy of 5.52 x 104 J.
  • #1
Ester
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0
In moving out of a dormitory at the end of the semester, a student does 2.69 x 104 J of work. In the process, his internal energy decreases by 5.52 x 104 J. Determine each of the following quantities (including the algebraic sign): (a)W, (b) delta U, and (c)Q.

I get the following using formula: delta U = Q - W

(a) 2.69E4 J

(b) -5.52E4 J

(c) -28300 J

I know that part a and c are right, but part b isn't for some reason I do not understand. I think it should be right, but it marks it wrong. It's probably the easiest problem I've ever encountered, but for some reason I'm having trouble getting it. Does anyone know what the answer should be?
 
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  • #2
perhaps the change in potential energy is his initial potential energy minus the work he does. If I have 5 units of energy in me, and I mow the lawn using two, I have 3 units of energy left for the day...make sense?
 
  • #3
Ester said:
I know that part a and c are right, but part b isn't for some reason I do not understand.
I don't understand how b could be wrong either; after all, dU is given in the problem statement. It looks correct to me. (And if b is wrong, how could c be right, since c depends on a and b.)
 
  • #4
If a and c are right and the formula is right then b is right.

If b is not right then either a is wrong, and/or c is wrong, and/or the formula is wrong.
 
  • #5
for some reason, my other friends use the same formula and get the answers that are the same as mine, and they get marked right, but i get marked wrong. :(
 
  • #6
I would go talk to the professor who taught the class and gave the exam. Ask a friend of yours to be with you and take both sets of answers when you go to see the prof.

What is Q, by the way?
 

1. What is internal energy?

Internal energy is the total energy contained within a system, including both its kinetic and potential energies.

2. How is the change of internal energy calculated?

The change of internal energy is calculated by taking the difference between the final and initial internal energy values of a system.

3. What factors can cause a change in internal energy?

A change in internal energy can be caused by heat transfer, work done on or by the system, or a change in the system's composition or state.

4. Can the change of internal energy be negative?

Yes, the change of internal energy can be negative if the system loses energy through heat transfer or work done by the system. This indicates a decrease in the total energy of the system.

5. How is the change of internal energy related to the first law of thermodynamics?

The first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. In other words, the first law relates the change in internal energy to the energy interactions within a system.

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