# Why Does The System Pressure Decrease In An Isothermal Expansion?

Thermodynamics

### #1 Mohankpbk

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Posted 26 May 2018 - 05:53 AM

In a piston cylinder arrangement, the piston can be extended only if the pressure of the gas inside is higher than the atmospheric 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 is given to the system) the piston moves.Doesnt this mean the pressure of the gas increases above the outside pressure?(But I have learnt that pressure never increases in an isothermal expansion, it decreases with increase in volume(hyperbolic relation))

### #2 exchemist

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Posted 28 May 2018 - 08:07 AM

In a piston cylinder arrangement, the piston can be extended only if the pressure of the gas inside is higher than the atmospheric 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 is given to the system) the piston moves.Doesnt this mean the pressure of the gas increases above the outside pressure?(But I have learnt that pressure never increases in an isothermal expansion, it decreases with increase in volume(hyperbolic relation))

You need to keep in mind that isothermal expansion is one of those idealised simplifications, used in physics to reduce the number of variables in considering a problem. The expansion has to be infinitely slow, too!

You can imagine it as an infinitesimal pressure increase, causing an infinitely slow motion of the piston. The opposite extreme is an adiabatic process in which the gas does not exchange heat with the environment at all. Real cases will always be somewhere in between these extremes, making them fiendishly difficult to model mathematically.