Thermo! Intensive properties and const. vol. compression

sidneykidney

Hello. I need help with some thermo basics.

I can't understand the concept of constant volume compression/expansion. Please help me understand how it occurs.

Also, I am not clear on how pressure is an intensive property. To my understanding it IS a quantity dependent on system size (eg: compression) or mass in the system (more mass = more force). Please help me with where I might be getting it wrong. Similarly with temperature.

haruspex

Compression here means, literally, increasing pressure. If you heat a gas at constant volume the pressure increases.
The use of the word "expansion" in that context does strike me as wrong, though. Decompression would be better.

If I have a container of gas at uniform pressure P, the pressure is the same within each region. If I partition it into separate regions, each will inherit the same pressure.
Suppose it's a rectangular box. The total force on a wall of area A will be A.P. So yes, scaling up means more force, but not more pressure.

sidneykidney

Thanks a lot. My confusion stemmed from my understanding of the word compression as a reduction in volume.

One final question: Are the definitions of intensive and extensive properties valid only for a fixed system state?

As in, I understood what you are saying about the container and the uniform pressure within, but I was confused thinking about the changes to P and T when the size is changed by say, compression, using a piston. But then, that is work being performed on the system, changing its state.

Thanks again. :)

haruspex

Not sure what you mean by this next question. If the system state changes then the properties can change.
See if http://en.wikipedia.org/wiki/Intensi...ive_properties helps.

Andrew Mason

I don't think you will find this definition of "compression" used that way in thermodynamics. Compression refers to a reduction in volume by the application of external pressure.

Compression of a quantity of gas (volume reduction) occurs if the external pressure exceeds the internal pressure of the gas. This can occur either because the temperature of the gas decreases or the external pressure applied to the gas increases. Expansion is the opposite. Expansion occurs if the internal pressure exceeds the external pressure applied to the gas. This can occur because the temperature of the gas increases or the external pressure decreases. If the volume is fixed, changes in pressure or temperature do not result in expansion or compression.
You have to be careful in defining what the terms mean. Intensive refers to a property of a system that does not change if you duplicate the system and place the two systems together: eg. temperature, density, energy density. Extensive refers to a property that does change if you duplicate the system and place the two together eg. volume, mass, energy. If you stick to this definition, you can see that pressure will not change if the system is duplicated and the two are placed together.

AM

haruspex

But "constant volume compression" was stated, so it can't be a reduction in volume.

Andrew Mason

I didn't read it that way but I see what you mean. I thought "constant volume compression/expansion" was a reference to three types of processes: ie. constant volume (and) compression or (/) expansion not two types: "constant volume compression" and "constant volume expansion". The latter makes no sense. Perhaps sidneykidney can explain what he meant.

AM

haruspex

I see the same usage at http://www.engineeringtoolbox.com/co...es-d_605.html:
"If a compression or expansion takes place under constant volume conditions".
Compression at constant volume I can justify, but expansion?

Andrew Mason

I think the author meant to say "If a compression or expansion takes place under constant entropy conditions". If a process takes place under constant volume conditions, it is isochoric and not isentropic. It cannot be isentropic because there is necessarily heat flow if pressure/temperature changes under constant volume conditions.

The author is clearly describing a reversible adiabatic expansion or compression. There is no change in entropy because Q = 0.

AM