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Why Does The System Pressure Decrease In An Isothermal Expansion?

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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))

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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.

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• 4 years later...

Why Does The System Pressure Decrease In An Isothermal Expansion? Because the System converts heat (Q), that is, entropy (s) into work (W): dQ=Tds, Q=T(s2-s1), Qin=W+Qout.
The operation of a heat engine has nothing to do with atmospheric pressure (with the exception of the barometer). Heat engine is a working medium that is inside the machine and with which the machine works (exchanges energy - work and heat - entropy). Each Heat engine works using a working medium whose state (p, V, T, s) changes during the thermodynamic cycle in which the machine works (Carnot's, Stirling's, Ericsson's, ...). Every efficient thermodynamic cycle takes place between "two isotherms" and two adiabats (Carnot) or isochores (Stirling) or isobars (Ericsson). Why should every efficient thermodynamic cycle have two isotherms? Because only during an isothermal change of state can the maximum amount of heat be transferred to a working medium and converted into work, and the maximum amount of work can be converted into heat! During an isothermal change of state, all state variables (p, V, s) of the working medium change except for temperature (T).
Whenever a mass (working medium) changes its state at constant temperature it means that by doing so, the same amount of energy is received from the environment and delivered to the environment. If the mass receives the work - it transmits heat (entropy), and if the mass receives the heat (entropy) the mass gives work, this is the PROCESS OF MAXIMUM CHANGE OF ENTROPY – MAXIMUM ENERGY FLOW!
Therefore, if we want to explain to someone what "Heat engine" and "Thermodynamic cycle" are, we cannot do it using the "Boyle-Mariotte" pV-diagram - as "physics professors" do today (that's why most people after finishing school don't have nor elementary knowledge of technical thermodynamics!). What is a thermodynamic cycle and how heat is exchanged and converted into work and work into heat is shown in Thermodynamics with a "Ts-diagram", i.e. the relationship between temperature and specific values of entropy, volume and pressure in a "working medium". Only the Ts-diagram can clearly show the difference between (for example) Stirling's and "Carnot's thermodynamic cycle"(https://austav.eu/stirling.html).

Edited by isaac

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