# Heat and internal energy

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?

ehild
Homework Helper
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?
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.

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

ehild
Homework Helper
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.

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

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.

ehild
Homework Helper
You suggest that if volume and pressure are poroportional (!!) then the pressure remains constant, I do not get this.
Sorry, I meant volume and temperature being proportional: V/T = const.

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

i.e Charles' law

ehild
Homework Helper
i.e Charles' law
Yes.