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Thermal Physics

p-V Diagrams

A "p-V diagram" is a graph showing changes in the pressure and volume of a sample of gas.

It is useful to be able to recognise various types of change of the state of a gas from a p-V diagram. Four examples are given below.

1. Change of p (and T) at constant volume; an isovolumetric change.

2. Change of V (and T) at constant pressure; an isobaric change.

3. Change in p and V at constant temperature; an isothermal change.

4. Change in p and V in an insulated container (no heating of the gas); an adiabatic change.

In practice, changes of state do not quite follow any of these ideal paths. However, approximate isothermal, adiabatic etc changes can occur.

In practice:

for an isovolumetric change, heat the gas in a fixed volume container (one made of a material having a low thermal expansivity)

for an isobaric change, trap a small quantity of the gas in a tube using a thread of mercury (or other liquid, as in experiment 6TP) and heat it slowly

for an isothermal change, compress (or expand) the gas slowly in a container of high thermal conductivity

for an adiabatic change, compress (or expand) the gas rapidly in a container of low thermal conductivity.

On a p-V diagram, an isothermal looks pretty much like an adiabatic. However, if an isothermal and an adiabatic have a point in common, then the adiabatic is the curve having the greater slope at that point.







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