# Q for Isobaric Processes

1. Nov 20, 2013

### songsteel

Just wanted to ask why is it that for isobaric processes, when we wanted to find Q, sometimes we use Q = nCpΔT and sometimes we use Q = nCvΔT + p(Vf - Vi)?

How do we determine which should be used and when?

2. Nov 20, 2013

### Tanya Sharma

They are not different things .Both of the above expressions are correct and can be used simultaneously , or separately depending on the situation.

For an isobaric process ΔQ = nCPΔT.

From First law of Thermodynamics ΔQ = ΔU+ΔW .

Now ΔU=nCVΔT applies to all kinds of processes involving an ideal gas.

So ,putting values of ΔQ and ΔU ,we have nCPΔT = nCVΔT + pΔV

To elaborate it further-

Case 1) What is the heat required to raise the temperature of 'n' moles of an ideal monoatomic gas by 'ΔT' .

You can simply use Q = nCPΔT

Case 2) How much heat is supplied to 'n' moles of an ideal monoatomic gas in a chamber fitted with a light piston , when the temperature changes by 'ΔT' and volume changes by ΔV ? Consider the process to be isobaric

Here ,use ΔQ = nCVΔT + pΔV

Case 3) What is the change in the volume of 'n' moles of an ideal monoatomic gas in a chamber fitted with a light piston , when the temperature changes by 'ΔT' ? Consider the process to be isobaric .

Now use nCPΔT = nCVΔT + pΔV

Edit:Removed the word - "insulated" and removed a typo

Last edited: Nov 20, 2013
3. Nov 20, 2013

### Staff: Mentor

Hi Tanya. You need to "lose" the word insulated from cases 2 and 3. If Q is not equal to zero, the chamber is not insulated. In all three cases, you need to lose the word monoatomic, and in case 1 you need to add the words at "at constant volume". Also, in item 2 there is a typo: replace ΔU with ΔV.

Chet

4. Nov 20, 2013

### Tanya Sharma

Hi Chet...

Yes you are right .I have edited my post.

Why? These are just examples i have taken using monoatomic ideal gas.

Why ? The heat supplied at constant pressure is nCPΔT

Last edited: Nov 20, 2013
5. Nov 20, 2013

### Staff: Mentor

It might give the impression that the relationship applies only to a monoatomic ideal gas.

Oh, sorry. I misread the Cp as Cv. I think it would be worthwhile adding the words "at constant pressure" to Case 1.

Chet

6. Nov 20, 2013

### Tanya Sharma

Well..these are just random questions/cases I formed .But...you may be correct.

Right.

Thanks for your valuable suggestions .I very much like reading your posts .They are full of insight.

Last edited: Nov 20, 2013