# Thermodynamic Signs

1. Apr 19, 2008

### yuvlevental

1. The problem statement, all variables and given/known data
A thermodynamic system changes isochorically from state A to state B where the temperature is greater than state A. The system then expands isobarically to state C where the temperature is greater than state B. From state C the system returns to its original state A linearly. Select Zero, Positive, or Negative for all selections.
W C → A
Q A → B
W B → C
∆U C → A
Q B → C
∆U A → B

2. Relevant equations
∆U = Q + W
W = WORK ON GAS
Q = HEAT ADDED TO GAS
W = -P∆V

3. The attempt at a solution
W C → A +
Q A → B +
W B → C -
∆U C → A 0
Q B → C 0
∆U A → B +

WHAT AM I DOING WRONG?

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Last edited: Apr 19, 2008
2. Apr 19, 2008

### Mapes

Determine ∆U for B → C, then find which one of your results is inconsistent regarding the B → C process.

3. Apr 19, 2008

### yuvlevental

It's W = -P∆V. The volume is increasing because the temperature is increasing. So the work is negative. So I can't find anything inconsistent regarding the B → C process...

4. Apr 19, 2008

### yuvlevental

anyone??

5. Apr 19, 2008

### yuvlevental

really, i need tp know the answer

6. Apr 19, 2008

### Andrew Mason

Make sure that you are using the right convention for W. The convention is to use W done by the gas as positive and W done on the gas as negative. The first law is:

$$\Delta Q = \Delta U + W$$ or

$$\Delta U = \Delta Q - W$$

Also, we can't tell what you are doing wrong without knowing your reasoning.

ie. What is your reason for saying that:

1. the work done by the gas from C to A is positive?

2. the heat flow into the gas from A to B is positive?

3. the work done by the gas from B to C is negative?

4. the change in internal energy of the gas from C to A is 0?

5. the heat flow into the gas from B to C is 0?

6. the change in internal energy of the gas from A to B is positive?

AM