# Pressure law (how they combined to get pv/t)

1. Dec 29, 2004

### Mo

Well .. i guess the question is in the title.I basically dont understand how they combined :

P/T = Constant

with

P . V = Constant to make P . V / T = Constant

i can guess

P/T = P . V which means P = P . V. T .. ummm help please!

Regards,
Mo

Last edited: Dec 29, 2004
2. Dec 29, 2004

### FulhamFan3

there is one more law your missing.

V/T=constant

don't assume all the constants are equal either. Each one of those constants are different from the other.

If we know that P varies Directly with T and that V varies directly with T than PV varies directly with T.

So you get PV/T=constant. The constant in this equation absorbs the other three. Yeah I know it's a bad explanation. I understand it in my mind perfectly but I don't know how to explain it any better for now.

3. Dec 29, 2004

### Integral

Staff Emeritus
You need to consider the complete expression of the Ideal Gas Law to understand the constants.

PV = nRT

So when you say PV = Constant you are saying that nRT, is a constant. The only true constant here is R, n is the number of moles of material present. Clearly the statement PV = C is only true in very special conditions. That is when the amount of material (number of atoms) present is constant and the Temperature is constant.

To say the $\frac P T = C$ is to say that $\frac {nR} V =C$.

4. Dec 29, 2004

### Mo

Ah yes.Thank you for your answers.The questions i am attemtping are rather easy and so they dont consider the amount of moles and molar mass and all of that stuff. Next section :)

Regards,
Mo

5. Dec 29, 2004

### FulhamFan3

Integral. What your doing is reverse engineering a problem. Charles' Law and Boyle's Law were found before the ideal gas law and he wants to know how they combined that to get $\frac {PV} T = C$.

It's like where you know the force of gravity is directly proportional to the mass and inversely proportional to the distance squared.

How do you combine that to get $$F_g = G \frac {m_1 m_2} {r^2}$$?