OLD - 1.1 Introduction to Pressure

Pressure can mean a lot of different things. We can be under pressure to perform, we can be peer pressured, we can pump tyres to a certain pressure, we can measure our blood pressure.

Pressure is all about pushing and pulling. Today we aren’t looking at metaphoric pressure that we might feel when we’re nearing a deadline. We will be looking at pressure as it relates to our environment (air and water) and how it is relevant to submarines.

Pressure is fairly unique and can sometimes be described as a force but is actually a measure force acting on a given area. When we think of a force we might think of gravity, or something behind kicked or hit. Pressure is a bit different, it’s related to gravity but isn’t a force the same way gravity is.

When we stand outside we can’t really feel the pressure of the air around us. Or when we jump into a pool. But! We can definitely feel when pressure changes - you would have felt a change in pressure if you have ever been on a plane or gone up a tall hill (higher altitude). The same strange thing that happens with your ears when you go very high up, also happens when you go very far down, either lower in altitude or under water.

When our ears feel pressure they aren’t measuring pressure in the same way we would measure the temperature using a thermometer. What our ears are doing is telling us that there has been a change in pressure.

This change in pressure is what we will be looking at and what matters to us with our submarine.

When there’s a difference in pressure we call this a pressure differential.

The difference in pressure from where we start is what is most worth looking at when it comes to subs.

Pressure is measured by dividing the amount of force applied in a certain space. With a gas we can measure how many times the particles hit the walls of their container, this is essentially what a pressure gauge will display.

If we think of a container that is 100 units cubed that is filled with gas. Let’s assume it has a pressure reading of 50 units (usually we measure pressure in PSI or atmospheric units).

If we compress the air in that container, and the 100 units cubed is compressed into 50 units cubed (half the volume) then we will get double the pressure.

Gases and physical objects work in slightly different ways but we still measure pressure by considering the amount of force applied and dividing that over the area we’re working with.

Many of us would have seen an image like that to the left. What it’s showing is different pressure systems and it predicts how the weather will behave.

High and Low pressure systems are caused by different temperatures in the atmosphere and are effected by heat, altitude and air moisture.

What is being shown in the image to the left is the atmospheric pressure as measured by a barometer. What is being displayed in the numbers is the weight of the air above the ground which is pushing down. We don’t really feel it unless we go to extreme locations but it effects weather and will definitely effect your submarine when its underwater.

If we were to climb Mount Everest we would find there isn’t much air pressure at all. Again, you wouldn’t necessarily feel the lack of weight of the air, but you would certainly feel the lack of oxygen in the air. Less pressure also means there are less air molecules pushing down as they are pulled by gravity.

Pressure is a measure of force for a given area. The image on the left shows the mathematical representation of pressure where

P is pressure

F is force (in Newtons)

and A is area (in Meters squared).

In the equation above Pressure is a function of Force and Area. That means, changes to force and area will effect pressure. The reason the units for force and area are useful is because 1 newton per square metre is the same as one Pascal, which is a common unit for pressure.

When comparing pressures the units don’t really matter as long as they are kept constant.

www.sciencepart.com/feel-pain-stepping-lego/

Questions

If force stays the same, how will a change in area effect pressure?

If force stays the same, what will happen with a very small area and what will happen with a very large area?

In the image of the submarine we can see a few Ballast Tanks. A ballast tank is used to maintain balance and control depth.

In our submarines we only have two Ballast Tanks, the rear trim tank and the forward trim tank.