  Picture 3.5. The particles in a pump collide with the walls. If the volume is halved, each particle collides twice as often. (The temperature (and therefore the speed) of the particles is the same throughout.    Making sense of Boyle’s law    Imagine some gas trapped inside a container  say a bicycle pump. The gas produces a pressure because its particles are colliding with the walls. Now think of just one of its particles, moving back and forth. Let’s say it makes 300 collisions every second;  now, we halve the width of the box
 this halves its volume.
 Now the particle has half as far to travel;
 so it makes twice as many collisions with the walls.
 Each collision is the same strength as before.
 So the pressure is doubled.
Note that the volume has to be reduced gently so as not to increase the temperature of the gas (see page 12). This sort of change is called isothermal.      
 Picture 3.6. Graph of Charles Law. Volume is proportional to absolute temperature.    Charles’ law    The final gas law relates the volume to the temperature (at constant pressure). We can set up an experiment to measure the effect of increasing temperature on a fixed mass of gas. We find that, as the temperature rises, so the volume increases. In fact        volume is proportional to absolute temperature   in symbols   Volume µ Temperature   we can write this as:   V = constant.T   or   V/T = constant      
 Picture 3.7. Lab apparatus for Charles' Law. Increasing the temperature (on the right) increases the volume.    Making sense of Charles’ law    The best way to think of Charles’ law is to think of the other two laws combining to try to keep the pressure constant.  So, if we double the temperature (with the volume constant)
 The pressure will double
 To get back to the original pressure, we need to allow the volume to double.
 So doubling the temperature will make the volume double whilst keeping the pressure constant.
    

