# I Heat and internal energy

1. Sep 16, 2016

A gas in a cylinder with constant pressure, the gas cooled down and its internal energy decreased as well as its volume. The heat Q will be flowing into the gas or out of the gas?

My try for the solution: As the volume decrease the work done by the gas will be negative.
The gas cooled down, so its internal energy will be negative too, and use the first law equation, the heat will be negative and will flowing out of the gas, but what make me stop and think is:
If the internal energy and the volume decreased and the heat is flowing out of the gas, how the pressure is still constant?

2. Sep 16, 2016

### ehild

Assume it is an ideal gas: PV=nRT. The pressure remains constant if the volume and the pressure temperature are proportional during the cooling process.

Last edited: Sep 17, 2016
3. Sep 16, 2016

Ok, that means heat will be flowing out of the system.
What would be If we consider it as a real gas?

4. Sep 17, 2016

### ehild

The real gases also obey some state equation, relation among pressure, volume and temperature. With proper heat transfer, the temperature and volume can be adjusted in the way that ensures constant pressure.

5. Sep 17, 2016

### lychette

I am having trouble trying to understand what you mean by this statement !
First of all I understand that a gas under constant pressure obeys Charles' law...there are only 2 variables, V and T...Charles' law.
You suggest that if volume and pressure are poroportional (!!) then the pressure remains constant, I do not get this.

6. Sep 17, 2016

### ehild

Sorry, I meant volume and temperature being proportional: V/T = const.

7. Sep 17, 2016

### lychette

i.e Charles' law

8. Sep 17, 2016

Yes.