Why must pressure remain constant in Charles's Law (on temperature and volume)?

Question

Doing research for a project, and when I got to the variables for the law to be accurate, I was confused on why the pressure cannot change to make this law a fact

Answer 1

Charles' law states that at constant pressure, V/T = constant (or T/V = another constant).  If the amount of gas does not change,

 

P₁V₁/T₁ = P₂V₂/T₂

 

If the pressure is constant, then

 

P₁ = P₂ ⇒

 

and hence

 

V₁/T₁ = V₂/T₂.

 

You can see that to get constant V/T, the pressure must be constant, in addition to the amount of gas.

Answer 2

I don't mean to sound like I'm being sarcastic, but the reason pressure cannot be changed is that the law says so. Although gases obey the equation PV = nRt, which you would think means you can change almost anything and adjust the other values, Charles was focusing on a gas in which only the temperature and volume were allowed to vary. Everything else remains fixed in this case.