Pressure, volume and density
Pressure in liquids
If you dive deeper, what happens to the water pressure?
What just happened? You dived too deeply, and the water pressure pushed on your eardrum, so it ruptured. Ah, you mean how come the ear facing downwards ruptured, when it was the other ear that had the weight of all the water on it? Well, that's how pressure in fluids behaves. It presses in all directions, even upwards.
Down where you were, you had four meters of water on your head. Imagine carrying a four meter-high water barrel on top of your head. That's heavy, but the weight of that water doesn't just push down on your head from above. It pushes just as hard on your body from the sides and even from below. In a liquid that is at rest, the pressure acts equally in all directions. The ocean is large, yes? It's very big, but that doesn't matter.
No really, it doesn't. Look here. Imagine that you swam down to the same depth in these two swimming pools. In which one would you experience the highest pressure? Since you swam to the same depth, the pressure would be the same.
What about these two pools? Same depth, same pressure. As long as both pools are filled with the same liquid, it's only the depth, or the vertical distance between the surface and the point of measure that matters. Let's connect the pools with a pipe. Now, what do you think will happen with the pool on the right if we fill the left pool up to here? Let's try.
Since both containers are connected with a pipe, the water is pushed over to the other side and reaches the exact same height there. If we lower the right one, the water pours out. This is called communicating vessels - two containers, or vessels, which are connected to each other. They communicate - communicating vessels. As long as there's the same air pressure above the water surface in both vessels, both water surfaces will stay at the same level.
This is something you can benefit from every day when you use a water tap. This is a water tower. In a water tower, there's a large water tank placed high up. A pump is pumping water up to the tank. Once the water is up in the water tower, it doesn't need the pump anymore. The water is pushed out into the pipes by the force of its own weight.
The pressure at this point in the pipe is determined by this distance - the vertical distance to the surface. So the pressure here is equal to the pressure here. Up here, the pressure is lower, here too. The pressure in the pipe is enough to lift the water all the way up to here. But, now, we are at the same height as the surface of the water in the tower. So up here, the pressure is low.
If you build a taller building, you'll have to pump the water that extra bit. All of this, with water towers and communicating vessels, works fine when there is room for the water to move, so the water surface can go up or down as it wants. If you close the the vessels at the endpoints, it doesn't work quite the same anymore, but we'll talk more about that some other time.