Howdy all, I have a final coming up, and I'm having a very difficult time grasping a few concepts about the ideal gas laws, specifically a problem with isobaric compression.(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

Alright, the intro to the problem is:

A quantity of ideal gas is slowly compressed to 1/3 of its original volume. In this compression, the work done on the gas has a magnitude of 800J. For the gas, Cp= 7R/2

It then breaks up into several problems based on a situation

a.) If the problem is isothermal, what is the heat flow (Q) for the gas? Does the heat flow into or out of the gas?

b.) If the problem is isobaric, what is the change in internal energy of the gas? Does the internal energy increase or decrease?

2. Relevant equations

P1V1=P2V2

T1V1=T2V2

W=p(dV) (for an isobaric problem only?)

Cp= Cv + R

dU= Q - W

dU(isothermic) = 0

W = nRTln(V2/V1)

pV= nRT

3. The attempt at a solution

a.) Ok, because isothermic reactions have a dU of 0, this one was pretty simple

dU = Q - W

(0) = (Q) - (-800J)

Q = -800J, I know that heat flows out of the gas, but I don't know why?

-----------------------------------------------------------

b.) This one is the one I'm so confused on, and I've gone through every equation and it feels like I need 1 more variable.

Ok, so for isobaric, P remains constant, so dP = 0

Here's what I've worked through.

Q = dU + W

-----------------

Q = n(Cp)(dT)

Q = n(7R/2)(dT)

Q = n(29.1)(dT)

------------------

T1V1 = T2V2

T1V1 = T2(1/3 V1)

((T1V1)/T2) = 1/3 V1 or ((T1V1)/(1/3V1) = T2

-------------------------

p(dV) = W = 800J

p(V2-V1) = W = 800J

p(1/3V1 - V1) = W = 800J

-------------------------------

From the answer I have, I know that dU is supposed to be -2000 J.....problem is, I don't know how to get it. It seems like I need just 1 more variable to figure out the entire thing!!

Any advice?

Thanks,

Garrett

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Isobaric Compression

**Physics Forums | Science Articles, Homework Help, Discussion**