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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 recognize 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, very nearly 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.  
   
NB  
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.  
This is easily explained:  
When we, for example, compress a sample of gas isothermally, the pressure increases because the volume decreases.  
However, if we do the compression adiabatically, the pressure increases because the volume decreases but also because the temperature increases.  
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