Why pressure decreases during isothermal expansion?

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  • #1
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Initially, the pressure of the gas inside the cylinder is higher than the atmospheric pressure and the piston is held in the contracted position by some means.When the piston is released, it expands and the pressure and temperature of the system decreases but the volume increases.In order to maintain constant temperature, energy is added to the system.So the pressure reduction is reduced and finally it reduces just by an amount required to nullify the increase in volume.
But how to explain the process when the system and the surrounding have the same pressure(atmospheric pressure) initially?(won't the system pressure increase to push the piston in this case during isothermal expansion?)
 

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  • #2
Simon Bridge
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You have described a situation where the final state has larger volume and higher pressure than the initial state (higher pressure required for equilibrium as noted).
So plot both situations on a PV diagram, and see if there is any way that getting from the initial to the final state can be done isothermally ... ie: can the initial and final states lie on an isotherm?

If not, the the situation described cannot be done isothermally ... if it can, then the isotherm in question will explain how this can happen.
Happy hunting.

Alternatvely, provide a reference that illustrates what you are trying to talk about.
 
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  • #3
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You have described a situation where the final state has larger volume and higher pressure than the initial state (higher pressure required for equilibrium as noted).
So plot both situations on a PV diagram, and see if there is any way that getting from the initial to the final state can be done isothermally ... ie: can the initial and final states lie on an isotherm?

If not, the the situation described cannot be done isothermally ... if it can, then the isotherm in question will explain how this can happen.
Happy hunting.

Alternatvely, provide a reference that illustrates what you are trying to talk about.
Is there a possibility that an expansion process with initial system and surrounding pressure equal(initially before expansion system pressure=surrounding pressure) can never be isothermal?
 
  • #4
Simon Bridge
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Is there a possibility that an expansion process with initial system and surrounding pressure equal(initially before expansion system pressure=surrounding pressure) can never be isothermal?
Did you do exercise recommended in post #2?
You can do that for the situation that the internal and external pressures are the same ... plot the initial pressure and volume on the PV diagram, then sketch in the isotherm that passes through the initial point ... are there any points on the isotherm that have a higher volume than the initial volume? If "yes" then it is possible for there to be an expansion from that initial state that is isothermal.

If the initial system has equal internal and external pressure, then it won't spontaneously expand. There needs to be something else happening... like heat entering or leaving the system or some external force pulling on the sides of the container.

This is a bit of a non sequitur from post #1 though ... which talks about a spontaneous expansion from a higher pressure state.

It is not clear what this focus on equilibrium with external pressure is about.
 
  • #5
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Did you do exercise recommended in post #2?
You can do that for the situation that the internal and external pressures are the same ... plot the initial pressure and volume on the PV diagram, then sketch in the isotherm that passes through the initial point ... are there any points on the isotherm that have a higher volume than the initial volume? If "yes" then it is possible for there to be an expansion from that initial state that is isothermal.

If the initial system has equal internal and external pressure, then it won't spontaneously expand. There needs to be something else happening... like heat entering or leaving the system or some external force pulling on the sides of the container.

This is a bit of a non sequitur from post #1 though ... which talks about a spontaneous expansion from a higher pressure state.

It is not clear what this focus on equilibrium with external pressure is about.
My doubt is that,In a piston cylinder arrangement, the piston can be extended only if the pressure of the gas inside is higher than the atmospheric(surrounding) pressure.In case of isothermal expansion of ideal gas, initially the piston is at rest(gas pressure is equal to the atmospheric pressure) and as energy is given to the system(heat enters the system) the piston moves.Doesnt this mean the pressure of the gas increases above the outside pressure?(but I have learnt that in an isothermal expansion, the pressure never increases, it decreases with increase in volume(hyperbolic relation))
 
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  • #6
Simon Bridge
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Ah right ... well since the initial equilibrium situaton will not expand by itself, and adding heat will produce expansion without it being isothermal ... there must be another way to get an expansion: without putting heat into the system.

ie. immerse the system in a large heat bath that is at a constant temperature ... say it has a very very high heat capacity.
Start out at thermal equilibrium ... the grab the piston and pull it out very slowly so the system remains in thermal equilibrium at all times.
That is your isothermal expansion.
 
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  • #7
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Ah right ... well since the initial equilibrium situaton will not expand by itself, and adding heat will produce expansion without it being isothermal ... there must be another way to get an expansion: without putting heat into the system.

ie. immerse the system in a large heat bath that is at a constant temperature ... say it has a very very high heat capacity.
Start out at thermal equilibrium ... the grab the piston and pull it out very slowly so the system remains in thermal equilibrium at all times.
That is your isothermal expansion.
So for an isothermal expansion, the pressure of the surrounding should be reduced and the cylinder should be in contact with a constant temperature bath.If just one of these conditions is applied (i.e. if the external pressure is reduced without the constant temperature bath or if heat is applied without reducing the external pressure) the process won't be isothermal.Please tell me whether my interpretation is right.
 
  • #8
Simon Bridge
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So for an isothermal expansion, the pressure of the surrounding should be reduced and the cylinder should be in contact with a constant temperature bath.If just one of these conditions is applied (i.e. if the external pressure is reduced without the constant temperature bath or if heat is applied without reducing the external pressure) the process won't be isothermal.Please tell me whether my interpretation is right.
You are incorrect.
By definition an isothetmal process is where the system temperature remains constant. Any means of doing that is fine. It is not restricted to my example.
 
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