# Why is this ISOTHERMAL and not ISOBARIC?

1. Jun 17, 2013

### Farina

I've looked at multiple textbooks that all say this process is ISOTHERMAL (see attached image). I can see why it is indeed ISOTHERMAL, but couldn't also be termed ISOBARIC?

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2. Jun 17, 2013

### Haborix

Hello,

Quick question. Does the piston contain an ideal gas? If so, remember what an isotherm looks like on P vs. V diagram for an ideal gas.

I hope this helps. If not, then I'm sure someone else will come along and help you sort it out.

3. Jun 18, 2013

### Farina

It is an Ideal Gas - thank you. I definitely see how it's an isothermal process (due to the heat reservoir). I'm still not, however, seeing how it is also not an isobaric process.

4. Jun 18, 2013

### nasu

What makes the piston to move up?
You did not describe the actual mechanism illustrated here.

5. Jun 18, 2013

### Farina

According to the textbook, this is simply a Carnot process (see attached image). My point is this: if the piston is free to move, then it will move however it needs to move to maintain a constant pressure - at least this is how I'm used to hearing the description of an isobaric process.

So, again, isn't this BOTH an isobaric process and isothermal process?

This seems to be quite a brain teaser.

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6. Jun 18, 2013

### nasu

I think I understand your confusion.
You assume that the pressure of the gas is such that balances the weight of the piston.
If this were the case, the piston will not begin to move and the gas will not expand. And no heat will be absorbed, as the gas has the same temperature as the reservoir, in state A.

Imagine that the pressure in state A is larger than the weight of the piston divided by its area.
The gas pushes the piston and expands, lowering its pressure in the process. To keep its temperature constant during expansion it absorbs heat from the reservoir.

7. Jun 18, 2013

### Farina

Ah, yes - you're right, that's what I was missing. Thank you very much!!

8. Jun 20, 2013

### nasu

You are very welcome.