[gkangelexa]

i have a rather silly question involving PV = nRT and Charles's Law.
Charles's Law states that the volume of a fixed amount of gas maintained at constant pressure is directly proportional to the absolute temperature of the gas.

I'm wondering about how do you maintain a constant pressure of the gas?... because when you increase the temperature, the pressure of the gas also wants to increase because the molecules have more kinetic energy.

How, in general, do you maintain constant pressure of a gas?

[BluePill]

It's like a balancing act. Let's use a see-saw model. Let's use pressure as the fulcrum and the two people on the see-saw is Volume (V) and Temperature (T). When you increase temperature, there would an imbalance in the see-saw. To balance the see-saw, you could move the fulcrum (which is a scenario of constant volume) or you need to compensate for the change caused by temperature by increasing the volume as well.

[fledarmus]

Experimentally, you can do it by having the gas support a constant weight. If you have a floating lid on whatever chamber your gas is in, when you increase the temperature, the lid will rise, increasing the volume but maintaining a constant pressure.

You can also do it by maintaining atmospheric pressure. This is commonly done by inverting a graduated cylinder in a container of water (or some less volatile liquid, if you're worried about water vapor throwing off your calculations). A bubble of gas is let into the cylinder, and you raise or lower the cylinder until the water level inside the cylinder matches the water level outside. Now the pressure inside the cylinder is equal to the atmospheric pressure outside the cylinder. As you increase the temperature, the gas bubble will expand, and you will have to raise the cylinder further to keep a constant pressure.

Latex balloons are also fairly good for this - the pressure exerted by the balloon is very close to atmospheric, and it doesn't change very much with expansion of the balloon. With all the assumptions present in using the ideal gas law, you don't lose much accuracy assuming the pressure remains equal to atmospheric. Then as you increase temperature, the volume of the balloon will increase with the pressure remaining (relatively) constant, and as you decrease temperature, the balloon will shrink.