# Change in thermal energy of gas

## Homework Statement

A gas is compressed from 600cm3 to 200cm3 at a constant pressure of 400kPa . At the same time, 100J of heat energy is transferred out of the gas.

What is the change in thermal energy of the gas during this process?

## Homework Equations

Wgas=p(Vf-Vi)=p/\v

## The Attempt at a Solution

400k(400cm.3)=p/\v

I came up with a wrong answer of 160 J. Any help would be appreciated.

Thanks,
Jared

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kuruman
Homework Helper
Gold Member
You calculated the work done on the gas not the change in thermal energy. What equation gives you the change in thermal energy of a gas?

I found the correct equation to use is Eth=Q-Wgas.

Alright. Eth=100J-160J = 60 J is that answer required to be in a positive or negative 60J?

Thanks,
Jared

Last edited:
kuruman
Homework Helper
Gold Member
Using the first law is the correct thing to do. It says ΔE = Q - W. You need to pay attention to the symbols and their signs. Is Q positive or negative? What about W? Positive or negative and why?

ideasrule
Homework Helper
BTW, the sign on W is purely arbitrary. Thermodynamics has adopted the convention that W should be considered from the point of view of the gas: that is, the work done by the gas is W.

I am pretty certain the answer is -60J. Although, I don't feel confident that you can have a negative thermal energy change.

@ideasrule- I don't quite understand what you mean?

kuruman