Is the Gas Heated and Cooled Equally in Phase C->D on the Pressure/Volume Graph?

[fawk3s]

[PLAIN]http://img686.imageshack.us/img686/7122/gasgraph.png

It is said that during the phase C->D, the gas is heated. I can see at some points, the pressure*volume is higher than the initial one, which is 10*2=20, but its also lower at some points. So as it starts off as 10*2, and ends as 2*10, I would say that it is both heated and cooled equally. Where am I wrong?

Thanks in advance !

 

[fawk3s]

Or is the statement wrong?

 

[ehild]

Being heated means that heat is transferred to the gas or other object, but this does not necessarily mean that the temperature increases. When you make boiled egg, the water is heated but the temperaure of the water remains 100 °C.
The process from B to C looks an isotherm expansion. The gas takes on heat but does work, equal to this heat and the temperature does not change during the process.

ehild

 

[fawk3s]

B to C actually looks more of an isobaric to me. Since pressure is constant and volume increases, that means the temperature must increase aswell.
And I still can't see how C to D must be heated. The pressure and volume multiplication is the same in both C and D, so I can't really see why the gas would need any outside work done, as in heating it?

Thanks in advance,
fawk3s

 

[ehild]

And I still can't see how C to D must be heated. The pressure and volume multiplication is the same in both C and D, so I can't really see why the gas would need any outside work done, as in heating it?


fawk3s

During the process from C to D (it was a mistake writing BC in my previous post) the gas does not need work from the outside world, but does work on it.
PV=constant is characteristic to an isothermal process in case of ideal gases. During such process, the heat gained is equal to the work done by the gas, and the internal energy does not change.

ehild

 

[fawk3s]

Let me see if I got this straight. In the process from C to D, heat is needed to do work on the outside world. Right?
But a follow up question. If the process was from D to C, the heat or the energy would need to be taken away from the gas in order to do the opposite work, as in cooled?

 

[ehild]

If some outer force does work on the gas, the gas most be cooled in order to keep its temperature constant.

ehild

 

[fawk3s]

Think I get it. Thanks ehild !