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  1. In my book "GKP Publications Thermodynamics (Physics Galaxy)", it is stated:

    "Each point on an indicator diagram represents a unique state of a gas. In a PV diagram, each point gives a specific value of pressure and volume of a gas, and if the number of moles is known, we can get its temperature using the gas law. Hence, a PV curve is the locus of the states of a gas during a thermodynamic process between initial and final states."

    From this, it seems the Y-axis represents the pressure of the gas, but I always thought it represents the external pressure. Which pressure is actually plotted on a PV diagram: internal pressure $(P_{gas})$ or external pressure $(P_{ext})$?

  2. Is external pressure $(P_{ext})$ always constant during thermodynamic processes, or are there cases where it varies?

  3. I’m struggling to visualize the PV diagrams for irreversible vs reversible isothermal processes and similarly for isobaric and isochoric processes. In a reversible process, is it always the external pressure $(P_{ext})$ that varies quasi-statically, or could some other parameter, like temperature, also vary quasi-statically?

  4. Are the PV diagrams illustrating Boyle's Law, Charles' Law, and Gay-Lussac's Law specifically for reversible processes?

I’ve searched extensively for answers but couldn’t find a resource that explicitly resolves these doubts. Clear and specific clarifications would be greatly appreciated!

Qmechanic
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Peter swift
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1 Answers1

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(1) By default (I.e. unless told otherwise) the pressure on a PV diagram is normally assumed to be both the external pressure and gas pressure. In other words the processes shown on the diagram are assumed reversible.

If it is stated that a process is irreversible then P is only the external pressure and the applicable equation of state for the gas can’t be applied except for the initial and final equilibrium states of the irreversible process.

(2) No. For example, in a reversible isothermal process the external pressure is intentionally varied in such a way that the product of the gas pressure and volume is constant.

(3) An isothermal and isobaric process can be made thermally or mechanically irreversible. Mechanical irreversibility generally involves sudden changes in external pressure. You need to be told this. A sudden drop in external pressure is a vertical line on the PV diagram which could be misinterpreted as a constant volume (isochoric) process.

Thermal irreversibility involves heat transfer over a finite temperature difference. Again you’d have to be told this because you may not be able to tell from the PV diagram.

An irreversible isochoric process only involves irreversible heat since no work is done.

(4) Generally those laws assume reversible processes. So unless you are told otherwise, you can assume the PV diagrams are for reversible processes.

Hope this helps.

Bob D
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