Change in Entropy and adiabatic process

[grscott_2000]

If a process is adiabatic and reversible, such as a gas in a tube that has been compressed, is it true to say that there is no change in entropy from the gas being uncompressed to being compressed?

If not, why not?

 

[Andrew Mason]

If a process is adiabatic and reversible, such as a gas in a tube that has been compressed, is it true to say that there is no change in entropy from the gas being uncompressed to being compressed?

If not, why not?

Entropy is defined as heat flow / temperature at equilibrium: dS = dQ/T. If the process is adiabatic and reversible, this means there is no heat flow and at all times the system is in equilibrium. So dQ=0. So that pretty much means that dS = 0 (ie. no change in entropy).

AM

 

[pixel01]

You can see that without taking into account the equation dS=dQ/T.
The entropy of the gas in a cylinder is increased when its temperature is increased and the entropy is decreased when its volume is reduced. When you compress the cylinder of gas without heat exchange, its volume is reduced while its tempereture raises, so entropy is unchanged. dS=0