Thermodynamics - Boyle's & Charle's Law

[Jimmy87]

Hi,

If we have the equation PV = nRT then we can see that P is proportional to 1/V if all the other variables are kept constant (Boyle's Law) and P is proportional to T (in kelvin) if all other variables are held constant (Charle's Law). Since Boyle's Law takes on the form y = mx then it produces a straight line graph through the origin if you plot P against 1/V. Doesn't going through the original though mean that at zero pressure 1/V is equal to zero? Is that right> Also, is there a convention as to which way round you plot them i.e. P against 1/V or V against 1/Pas they would both give a straight line graph and confirm Boyle's Law? I'm sure I read somewhere that in order to be in direct proportion they have to be plotted a certain way round. Likewise, does it matter if you plot T against V or V against T for Charle's Law.

 

[Simon Bridge]

The theoretical equation, by itself, does not tell you everything about the situation. You also need the limitations where the equation remains valid.

Doesn't going through the original though mean that at zero pressure 1/V is equal to zero? Is that right?

Yes. But for $1/V \to 0$, what must happen to V? What does that mean for an experiment?

Also, is there a convention as to which way round you plot them i.e. P against 1/V or V against 1/Pas they would both give a straight line graph and confirm Boyle's Law?

There is a convention that the quantity you measure goes on the vertical axis while the quantity you control is on the horizontal.
In Boyle's Law experiments, it is usually easier to control the volume so 1/V will almost always appear on the horizontal. IRL however, experimental physicists do not always adhere to that convention - preferring to draw their graph whichever way up makes the subsequent calculations easier.

I'm sure I read somewhere that in order to be in direct proportion they have to be plotted a certain way round.

Not heard that one - but you can check easily.

Likewise, does it matter if you plot T against V or V against T for Charle's Law.

Why not check for yourself? Simple enough, you have the equation that describes the Law.

 

[Suraj M]

Doesn't going through the original though mean that at zero pressure 1/V is equal to zero? Is that right>

This is what happens if you allow a gas to expand into a vacuum (zero pressure)then you can imagine what the volume will become as there is no opposing force. so $\frac{1}{v}≈0$

 

[Jimmy87]

The theoretical equation, by itself, does not tell you everything about the situation. You also need the limitations where the equation remains valid.
Yes. But for $1/V \to 0$, what must happen to V? What does that mean for an experiment?
There is a convention that the quantity you measure goes on the vertical axis while the quantity you control is on the horizontal.
In Boyle's Law experiments, it is usually easier to control the volume so 1/V will almost always appear on the horizontal. IRL however, experimental physicists do not always adhere to that convention - preferring to draw their graph whichever way up makes the subsequent calculations easier.Not heard that one - but you can check easily.

Why not check for yourself? Simple enough, you have the equation that describes the Law.

Thanks for the answers. To answer your question about 1/V tending to zero does that mean V is infinite? So as you increase the volume, the pressure keeps dropping until you reach zero pressure where the volume will be infinite, is that the right way to think about it?