This is a topic that I am going to talk about in my physical science class. Might as well make a post about it, right?
Here is the deal. You are in a pool. You drop a quarter in the deep end and swim down to get it. I know the first thing you are going to ask: Why do I have a quarter in the pool? Does it matter. What matters is that your ears are killing you. Boy, that hurts. Why do your ears hurt and what can you do about it?
Pressure and depth
When you put a fluid in a gravitational field (like on Earth), the pressure in that fluid (or gas) increases as you go down. Why? There are a couple of ways to explain this. One cool way is to think about the gas as a bunch of particles (which it is) and treat each particle as though it were in projectile motion. I did this with the PhET simulator before – but maybe sometime I should come back to this.
Another way is to consider some fluid floating in a fluid. Suppose I take a chunk of air in a room with no wind or anything. That chunk of air just stays there. Here is a diagram.
If this rectangular cube thing of air is not moving and in equilibrium, then the total forces on it must be zero. The air outside of this rectangle of air pushes on this in all directions. The force on each side will be the air pressure on that side times the area of that side.
For the sides of this air rectangle, the pressure is not really important. Clearly, the forces from the sides are the same. The net force in the horizontal direction is zero. The problem with the vertical forces is that there is something extra there. Other than the force from the air above and the force from the air below, there is also the gravitational force pulling down. This means that in order for the net vertical force to be zero, the air from the bottom must push up more than the air on the top. The areas of these two things are the same, so the pressure on the bottom must be larger.
Here is a model of an ear:
In normal situations, the air inside your ear is at the same pressure as the air outside your ear. This means that the force on the two sides of the ear drum are the same and you are happy.
But what if you go underwater? In this case, there is water on the outside of this ear drum, but air on the inside. If you don’t do anything, the pressure inside will still be at atmospheric pressure. However, on the outside, the pressure will be greater. This means that the force from the inside air will not cancel with the pressure from the outside. Your ear drum doesn’t want to accelerate, so it stretches like a spring to produces a net force of zero. This stretching of the ear drum hurts.
What can you do?
You can fix this problem. Well, obviously people scuba dive at depths much deeper than a pool – their ears don’t hurt. The solution is to add air to the inside of your ear so that the pressure inside and outside are the same. This is called “equalization”. You can do this because your ears are connected to throat with the eustachian tube (that little tube on the side in my drawing). This tube is not huge so that it is mostly closed. You can force air through this tube by holding your nose and gently trying to blow out your nose. So, while you are going down to the bottom of the pool, you can do this nose-thing. If you are an expert (like me), you can do this without even holding your nose.
How do you get the air out? Well, it usually comes out on it’s own. When you come back to lower depths, the greater air pressure in your ear forces the air back out through the eustachian tube. Actually, if you have a lot of mucus in your head sometimes this air doesn’t come out. This is known as a reverse block. Really, at this point you have no option but to keep going up.
There are other air cavities in your head that could cause a problem. In particular, your sinus cavity. For most people, air can easily flow in and out of this making equalization a non-issue. If you have a cold, mucus can block these passages and it can hurt in that area too.
These ideas don’t just apply to scuba diving and swimming in pools. When you fly in a plane, the air pressure decreases. I am sure you can feel this in your ears. Sometimes they recommend chewing gum in a plane. This makes your jaw move more and possibly open up the eustachian tube so that your ears can equalize.
More on scuba
Have you ever looked at scuba masks? Here is what most people think of (image from wikipedia):
The key feature to observe is the nose pocket. This makes it very easy for a diver to hold his/her nose closed for equalization. Check out this old-style oval shaped mask:
Notice that it also has these nose pockets. Oh, I have no idea why this girl is inside with a flooded mask – but it was the only picture I found of this kind of mask. If you ever get a chance to use one of these suckers, you should. Even though they are old, they are very comfortable.