Charles' law states that volume and temperature are directly proportional at constant moles n and pressure p. It's named after Jacques Charles, and it illustrates how the volume of a container is greatly affected by temperature. Here, to show this direct proportionality between volume and temperature, we just say V is directly proportional to T when our moles n and pressure p are constant or fixed. If we were to illustrate this with movable pistons, if we take a look here, we'd say that in this first image, our volume is low, and we haven't applied any temperature or heat to this container, I mean, so the temperature would be low. Here, I'm applying a flame to this. This is going to cause a higher temperature. And what's happening here is because the piston is movable and the pressure is constant, our gas particles are gaining enough outside energy to basically hit all corners of this container, including the movable piston upward, and that's what causes the volume to also expand. So our volume is high. How do we illustrate this direct proportionality between volume and temperature? Well, we'd say here that they're both increasing or decreasing together. You can illustrate this by a line that's going up over time as they both increase. The adjusted formula or Charles' law formula would just become:

Here we'd say that our initial volume is V₁, our initial temperature is T₁, final volume is V₂, and our final temperature is T₂. So remember, when it comes to Charles' law, we say that volume and temperature are directly proportional, which means they both can increase or decrease together if our moles n and our pressure p are held constant or fixed.