This essay is meant to give anybody an understanding of gases and pressure. That means that the essay will start off on a very basic level, and work up. So, if you already have some understanding of gases, you'll probably want to skim through this essay until it starts talking about topics that you don't understand.
So what is a gas? Well, the type of gas that I'm going to describe is not the type of gas that you put in a car, which is really gasoline, but is rather a phase of matter. So, I guess I'd better explain what phases and matter are. I'll start off by explaining matter. Matter is the stuff that makes up the universe. Anything that you can touch, or feel, is matter. Your computer is matter. The air that you're breathing is matter. Water is matter. You yourself are matter.
Matter can exist in four different phases: solid, liquid, gas, or plasma. Basically, solids are anything that has a definite shape, such as a rock, or a brick. A solid doesn't necessarily have to be hard, though. A rubber band, a pillow, these are both solids as well.
A liquid is anything that will take the shape of the container it's in, but not change the amount of space it takes up. In other words, you can take a quart (or a liter, for those of you used to metric) of a liquid, and pour it into any container. The liquid will take the shape of the container, but it will never be more or less than a quart (If you pour the liquid into a container smaller than a quart, the liquid will overflow, but the combined liquid in the container and the spilled liquid will still be a quart). When I use the word, "container," it does not have to be something small and man made. The ocean floors of the earth are giant containers. Water is the most common liquid on Earth. Other examples of liquids include milk, alcohol, and gasoline.
A gas is similar to a liquid, in that it takes the shape of the container it's in, but a gas will always fill the container. Air is a gas. This whole essay will describe gases, but for a first understanding, look at it this way. Say you have a bottle with a hose sticking into it. The bottle is all sealed up, so that the only way for anything to get into or out of the bottle is through the hose. If you blow really hard on the hose, you will push more air into it, but the air is still taking up the same amount of space. If you suck really hard on the hose, you will suck out some of the air, but the air left will take up less space.
Plasma is a very extreme phase of matter that only occurs at very high temperatures. It is not like anything we ever experience, because we could not survive in the same conditions where plasma exists.
To further understand gases, it helps to understand matter a little further. At a very small level, matter is composed of atoms. Atoms are the basic building blocks of the universe. Atoms are the smallest particles that have the basic properties of matter. Atoms are made up of even smaller particles, protons, neutrons, and electrons. The number of protons, neutrons, and electrons in the atom determines what type of atom it is. For example, hydrogen, the simplest atom, has one proton and one electron. Oxygen has eight protons, eight neutrons, and eight electrons. There is much more that could be explained about atoms, but it doesn't need to be understood to understand how airplanes work, so it won't be discussed here.
Just like different particles come together to make up atoms, atoms come together to make up something called a molecule. For example, two hydrogen atoms together make a hydrogen molecule. Two hydrogen atoms and an oxygen atom together make a water molecule. The bonds that hold molecules together are very strong. But there can be weaker bonds between different molecules. It is these weaker bonds between the molecules that make the different phases of matter. If the bonds between molecules are fairly strong, holding the molecules close together and keeping them pretty much in the same position, there is a solid. If the bonds are weaker, so that the molecules are still close together, but are free to move around, there is a liquid. If there are no bonds, so that the molecules are free to move around and can spread out as much as possible, there is a gas. Air is composed of many different types of molecules. There are nitrogen, oxygen, carbon dioxide, water, and many other types of molecules in the air.
Now that you have an idea of what a gas is, let's start talking about a specific property of gases, pressure. To understand pressure, begin first by imagining that you have a tennis ball. The tennis ball is kind of like a gas molecule. If someone throws the tennis ball at you, the ball will do two things. One, it will push on you a little bit. The other is that it will bounce off. This is what gas molecules do when they collide with something, they push on it a little bit, and bounce off, continuing in a different direction. Now, instead of imagining someone throwing the tennis ball at you, imaging throwing the tennis ball at a wall (This won't hurt as much). The same thing happens to the wall as to you when the ball hit you. The ball pushes on the wall and bounces off. The harder you throw the ball, the more it will push on the wall when it hits it. Also, if you and a friend both have a ball, and you each throw your ball at the wall so that they hit at the same time, the combination of the two balls will push the wall harder than one ball would. In gases, this push is known as pressure. The more molecules that hit an object, or the faster the molecules are going, the higher the pressure will be. If you close your mouth and hold your nose and try to exhale, this is exactly what the pressure you feel is. It is all the gas molecules bouncing around inside your head.
Now, go back to imagining a wall. Imagine that we put the wall on castering wheels, so that it is free to move in any direction. If the wheels are greased very well, it will take very little to move the wall. If we throw a tennis ball at it, when the ball bounces off, the wall will move a bit, too. The more balls at a time that we throw at the wall, and the harder we throw them, the faster the wall will move. Now, put some of your friends on the other side of the wall, and have both groups start throwing tennis balls at the wall. The ball will move towards the side that isn't being hit as hard. For gases, this means that if an object does not have equal pressure all around it, it will be pushed in the direction of the least pressure.