- #1

jaumzaum

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- Homework Statement
- See below

- Relevant Equations
- See below

Hello guys. I am a Physics student and I think one of the questions I had to solve in an admission exam from a certain university is incoherent. Can you help me to understand if it is in fact incoherent or not, and if it needs annulment?

The question is:

I am concerned only with letter A.

Why I think it's wrong?

The question says the cylinder is adiabatic. I will also assume that the process ais reversible and that the transformation inside the cylinder is isobaric. If P is the pressure inside the cylinder:

$$W = P \Delta V = n R \Delta T = 3 \cdot 8,31 \cdot 10=249,3 J$$

$$Q = n C_P \Delta T = 3 \cdot 5 \cdot 4,18 \cdot 10 = 627 J$$

$$\Delta U = Q-W \approx 378 J$$

$$\gamma = Q/\Delta U \approx 5/3 $$

Typical of any monoatomic gas.

However, the work is also:

$$W=P \Delta V = (P_{atm } + mg/A) \Delta V = P_{atm} \Delta V + mgH = P_{atm} \Delta V + 400 = 249,3 J$$

$$P_{atm} \Delta V = -151,7 J$$

So this makes the question impossible, as we cannot have negative atmosferic pressure. The question didn't give the value of R either, nor the value of Pa. If we consider the process is not reversible, is it still possible that this would still be valid? I am not seeing any way this could be true.Can you guys help me? Thank you very much!

The question is:

*Three moles of a certain ideal gas, whose molar heat at constant pressure is 5.00 cal / mol.K, is inside the cylinder in the figure below. The gas receives heat from a thermal source (not shown in the figure) in such a way that its temperature increases by 10.0 ° C. When absorbing heat, it is verified that the piston, adiabatic and of negligible mass, rises 2.00 meters. On the piston we have block 1 of mass m1 = 20.0 kg. Consider: g= 10 m/ s ^2 and 1.00 cal = 4.18 J.*

a) Calculate the change in the internal energy (in joules) of the gas. (4 points)

b) At the end of the gas expansion, block 1 at rest on the horizontal surface AB, with negligible friction, is reached by block 2 with mass m2 = 10.0 kg and speed equal to 5.00 m / s. Calculate the recoil speed of block 2, knowing that the restitution coefficient is 0.800. (7 points)a) Calculate the change in the internal energy (in joules) of the gas. (4 points)

b) At the end of the gas expansion, block 1 at rest on the horizontal surface AB, with negligible friction, is reached by block 2 with mass m2 = 10.0 kg and speed equal to 5.00 m / s. Calculate the recoil speed of block 2, knowing that the restitution coefficient is 0.800. (7 points)

I am concerned only with letter A.

Why I think it's wrong?

The question says the cylinder is adiabatic. I will also assume that the process ais reversible and that the transformation inside the cylinder is isobaric. If P is the pressure inside the cylinder:

$$W = P \Delta V = n R \Delta T = 3 \cdot 8,31 \cdot 10=249,3 J$$

$$Q = n C_P \Delta T = 3 \cdot 5 \cdot 4,18 \cdot 10 = 627 J$$

$$\Delta U = Q-W \approx 378 J$$

$$\gamma = Q/\Delta U \approx 5/3 $$

Typical of any monoatomic gas.

However, the work is also:

$$W=P \Delta V = (P_{atm } + mg/A) \Delta V = P_{atm} \Delta V + mgH = P_{atm} \Delta V + 400 = 249,3 J$$

$$P_{atm} \Delta V = -151,7 J$$

So this makes the question impossible, as we cannot have negative atmosferic pressure. The question didn't give the value of R either, nor the value of Pa. If we consider the process is not reversible, is it still possible that this would still be valid? I am not seeing any way this could be true.Can you guys help me? Thank you very much!

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