jgourlay - 11-7-2011 at 04:38
Boyle's law is given as P*V = Constant. Note that there being no exponent in that formula, a P vs V graph should form a line. The typical caveat
being "ideal gas". Given the period of Boyle, it is doubtful he would have had any access at all to pure gases, let alone highly purified gases that
closely approximate "ideal": it is almost certain he was using air.
Amazon sells a cheap Boyle's law demonstrator: a stock syringe glued into a block with a board glued to the plunger. You stack weights on top and
read the change in volume as cc's on the side. I made this, and was testing it friday night . Given the materials and processes available to
Boyle/Hooke, it's difficult to argue they had superior instruments. Qualitatively it successfully demonstrates the obvious: gas is a compressible
fluid. There is also a clear problem: an airtight seal requires pressure between wall and plunger. This in turn creates a frictional force that must
be overcome. The first expectation from that would be that the change in volume from the first state (zero) to the second state (first weight) would
be less than the delta between the second and third state assuming the first and second weights are the same. This could be anticipated to cause a
portion of the force of gravity that should be converted into pressure causing downward force is getting soaked up to overcome friction. So one could
expect that second point on the graph would be a little off. But because of P*V=C all the subsequent points should fall on a line.
Given all that wind up, I'll skip to the issue. I did this several times, being very careful: it's not a straight line relationship. Each time, the
graph forms the prettiest little logarithmic curve what ever you want to see.
So....what am I missing/doing wrong? Why am I not getting Boyle's pretty straight line?
woelen - 11-7-2011 at 05:01
P*V is a constant, so P is inversely proportional to V.
A P versus V graph (or a V versus P graph) will be hyperbolic with a horizontal and a vertical asymptote.
If you draw P versus 1/V then you get your nice line.
jgourlay - 11-7-2011 at 05:14
Wow....I'm an r-tard.
How could I have missed that? I just looked at it and said, "no exponent, it must be a straight line"!
Geez....I need to delete this thread or the state is going to repo my professional certification. (Which I am now embarrassed to mention).
woelen - 11-7-2011 at 22:37
Don't feel embarrassed. Every person has his blonde moments 
Vogelzang - 12-7-2011 at 13:45
PV=nRT
Both sides are in units of energy, IIRC (you can check my math).
Vogelzang - 12-7-2011 at 13:49
P = F/A
A = l^2 = length squared
V = l^3 = length cubed
PV = F/l^2(l^3) = F times distance = energy
where:
F = force
P = pressure
V = volume
l = length (distance)
A = area
[Edited on 12-7-2011 by Vogelzang]