What are some common misconceptions in thermodynamics?

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Homework Help Overview

The discussion revolves around common misconceptions in thermodynamics, focusing on problems related to ideal gases, energy transfer, and work done by gases during expansion and compression. Participants are exploring various scenarios involving heat transfer, work calculations, and the behavior of gases under different conditions.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants attempt to apply formulas related to heat transfer and work done by gases, such as Q=mct and W=PΔV. There are questions about the correct application of these formulas, particularly regarding the constants and units involved. Some participants express confusion about how to relate work done to changes in internal energy and temperature.

Discussion Status

Several participants have provided insights and partial guidance on the problems, suggesting formulas and discussing the implications of changing conditions. However, there is no explicit consensus on the correct approaches, and multiple interpretations of the problems are being explored.

Contextual Notes

Participants are facing challenges due to missing information, such as specific heat capacities and the need for unit conversions. There are also concerns about whether certain assumptions, like constant pressure or the necessity of using multiple layers in heat transfer calculations, are valid.

Kawrae
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Sorry for some many questions here. I'm stuck on all of these problems and it's starting to really frustrate me. I'm not sure what more to do.

1. A sample of Helium behaves as an ideal gas as energy is added by heat at constant pressure from 273 K to 403 K. If the gas does 20.0 J of work, what is the mass of helium present?

>> At first I was going to use Q=mct, but I do not know c of helium and it's not in my physics book. Then I tried to use Q=mL which I think should work, but I don't know how to convert W to Q.

2. A gas is compressed from 10.00 L to 3.00 L at a constant pressure of 0.800 atm. In the process, 330 J of energy leaves the gas by heat. What is the work done by the gas? What is the change in its internal energy?

>> I know trial and error isn't the way to approach physics, but I'm lost. I tried using W=Pln(Vf/Vi) and got an answer of .963 J. I don't understand why this is wrong...

3. A thermal window with an area of 6.00m^2 is constructed of two layers of glass, each 4.00 mm thick and separated from each other by an air space of 4.00 mm. If the inside is at 20.0 C and the outside is at -34.0 C, what is the rate of energy transfer by conduction through the window?

>> I know I need to use the formula H=kA(change in T/L). But... do I have to do this using only the air in the middle of the two panes of glass? Or do I have to do the formula three times: one for the air, and two each piece of glass? And if so, is the change in temperature the same?

4. Gas in a container is at a pressure of 1.40 atm and a volume of 6.00 m^3. What is the work done by the gas if it expands at a constant pressure to twice its initial volume.

>> I think I need to use W=Pln(Vf/Vi). When I did this, though, I got .970... which isn't the answer according to webassign. I honestly don't know why this doesn't work. Does P have to be in a different unit?
 
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Kawrae said:
Sorry for some many questions here. I'm stuck on all of these problems and it's starting to really frustrate me. I'm not sure what more to do.

1. A sample of Helium behaves as an ideal gas as energy is added by heat at constant pressure from 273 K to 403 K. If the gas does 20.0 J of work, what is the mass of helium present?

>> At first I was going to use Q=mct, but I do not know c of helium and it's not in my physics book. Then I tried to use Q=mL which I think should work, but I don't know how to convert W to Q.

2. A gas is compressed from 10.00 L to 3.00 L at a constant pressure of 0.800 atm. In the process, 330 J of energy leaves the gas by heat. What is the work done by the gas? What is the change in its internal energy?

>> I know trial and error isn't the way to approach physics, but I'm lost. I tried using W=Pln(Vf/Vi) and got an answer of .963 J. I don't understand why this is wrong...

3. A thermal window with an area of 6.00m^2 is constructed of two layers of glass, each 4.00 mm thick and separated from each other by an air space of 4.00 mm. If the inside is at 20.0 C and the outside is at -34.0 C, what is the rate of energy transfer by conduction through the window?

>> I know I need to use the formula H=kA(change in T/L). But... do I have to do this using only the air in the middle of the two panes of glass? Or do I have to do the formula three times: one for the air, and two each piece of glass? And if so, is the change in temperature the same?

4. Gas in a container is at a pressure of 1.40 atm and a volume of 6.00 m^3. What is the work done by the gas if it expands at a constant pressure to twice its initial volume.

>> I think I need to use W=Pln(Vf/Vi). When I did this, though, I got .970... which isn't the answer according to webassign. I honestly don't know why this doesn't work. Does P have to be in a different unit?

For 1... you know that P(deltaV) = work with constant pressure...

you also know that PV = nRT...

so PV1 - PV2 = nRT1 - nRT2 = P(deltaV) = nR(delta T) = work

you can figure out the number of moles... which you can use to determine the mass..
 
1. The number c you looking for is 5.193 J/(grams*K)
Q=mc(delta T)
where delta T = 403 - 273

2. PV = nRT
If you change Volume, and Pressure stays the same - something's got to give. So T must rise.

3. http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html
This should help you understand difference between convection and conduction
 
Last edited by a moderator:
cronxeh said:
1. The number c you looking for is 5.193 J/(grams*K)
Q=mc(delta T)
where delta T = 403 - 273

2. PV = nRT
If you change Volume, and Pressure stays the same - something's got to give. So T must rise.

3. http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html
This should help you understand difference between convection and conduction


For number 1... even if you use that equation... how are you going to solve it?
that gives you the energy change ... what good does that do? If a problem doesn't give you the value... i don't think it is needed to solve the problem, since enough information is available...
 
Last edited by a moderator:
Kawrae said:
Sorry for some many questions here. I'm stuck on all of

2. A gas is compressed from 10.00 L to 3.00 L at a constant pressure of 0.800 atm. In the process, 330 J of energy leaves the gas by heat. What is the work done by the gas? What is the change in its internal energy?

>> I know trial and error isn't the way to approach physics, but I'm lost. I tried using W=Pln(Vf/Vi) and got an answer of .963 J. I don't understand why this is wrong...

For 2... P(deltaV) = work done on the system which is equal to the total energy...

q(heat) + Work = total energy... <--- solve for Work...
 
cronxeh said:
2. PV = nRT
If you change Volume, and Pressure stays the same - something's got to give. So T must rise.

I understand why the temperature must rise... but how does that help me find the work done by the gas? :confused:
 
If anyone helping me on this has AIM, can you IM me at ecksCavator? I'm so lost...
 
Kawrae said:
Sorry for some many questions here. I'm stuck on all of these problems and it's starting to really frustrate me. I'm not sure what more to do.



3. A thermal window with an area of 6.00m^2 is constructed of two layers of glass, each 4.00 mm thick and separated from each other by an air space of 4.00 mm. If the inside is at 20.0 C and the outside is at -34.0 C, what is the rate of energy transfer by conduction through the window?

>> I know I need to use the formula H=kA(change in T/L). But... do I have to do this using only the air in the middle of the two panes of glass? Or do I have to do the formula three times: one for the air, and two each piece of glass? And if so, is the change in temperature the same?

4. Gas in a container is at a pressure of 1.40 atm and a volume of 6.00 m^3. What is the work done by the gas if it expands at a constant pressure to twice its initial volume.

>> I think I need to use W=Pln(Vf/Vi). When I did this, though, I got .970... which isn't the answer according to webassign. I honestly don't know why this doesn't work. Does P have to be in a different unit?

P should be in Pascals... for P(deltaV) to work... However though I'm not sure about W = Pln(Vf/Vi), but you can try it out...

I haven't done thermodynamics for energy transfer... so i can't help you on 3...
 

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