Use breathing as an example to explain the relationship between volume and air pressure.?

Answer 1

Interesting question.

If we keep temperature and the amount of air constant, then volume and pressure are inversely related. So if we double the volume and keep the amount of air the same, the pressure halves. On the other hand, if air can move in and out, then the pressure is going to equalize and the amount of air will change with volume, rather than the pressure.

Now consider normal breathing. When we inhale, we expand our lungs - increase their volume. Air flows in because of the pressure differential - the original amount of air in the larger volume has lower pressure so the higher outside air pressure forces air into the lungs to equalize the pressure.

Similarly on exhale, we forcibly reduce the lung volume, creating higher pressure in the lungs, forcing some of the air out.

Now consider what happens when you are trying to blow up a balloon. The pressure in the balloon is higher than outside air pressure so in order to get air from the lungs into the balloon, it is necessary to exert greater force.