J - E expansion

[abvegto]

in the throtling process or porous plug experiment, the gas either cools (most) or is heated (hydrogen helium) when they pass from a region of high pressure to a region of low pressure.

 

i do not understand how a gas can be heated via this expansion?...becoz we know that

 

expansion => increase in volume => decrease in internal energy wch should => fall in temp always...but for hydrogen the temp. increses even after the decrease in its internal energy...how is this possible?

[Enthalpy]

Did we meet on another forum?

 

Same answer here: find where the internal energy has gone, since energy can't disappear. Expansion through a porous material is not an isentropic expansion.

[abvegto]

yes we have met before although i dunno where...

 

thats where iam having troubles...i cant figure out where this inernal energy has gone?....iam aware that this process is diff from adiabatic expansion

wch is an isentropic....where entropy increses due to increase in volume then simultaneously entropy decreases due to decrease in temp..therefore keeping the entropy const...

in adiabatic process

 

s(v ,T) = 0 = Ds = ds/dv _T Dv + ds/dT _v DT such that ds/dv = -ds / dT

 

in the throtlling process...if hydrogen is taken as the gas...after the expansion its temp. increases and its volume also increases thus s(v , T) is positive

i.e there is an increase in entropy => that the ability of hydrogen to do work has decreased => that internal energy of the hydrogen has also descreased ..(?)..but this contradicts to the fact that the temp of the gas has increased inspite of the decrease in its internal energy?....

 

may be iam not clear....but i cannot put things together here.....

Edited February 19, 2013 by abvegto

[Enthalpy]

This expansion is not adiabatic. Energy provided by expansion is converted to heat at the porous material (in a nozzle, it would first become kinetic energy, then possibly heat depending on the surroundings).

 

This heat stays in the gas, so some W converts to Q. As an energy (find which one) is constant, so does the gas' temperature, meaning as well that the porous material absorbs no heat. Or turn the reasoning an other way.

 

As pressure was lost but no work extracted, something has been lost: the ability to provide future work with the available heat. No energy was lost but the entropy has increased.