A simple model of the atmosphere

1. The Earth is surrounded by a thin layer of air called the atmosphere, which is held in place by the Earth's gravity.
2. Air is a mixture of gases, mainly nitrogen and oxygen, and like all matter it has mass.
3. Compared with the size of the Earth the atmosphere is very thin, because most of the air lies within about $10km$ of the ground while the Earth's radius is about $6400km$.
4. A scientific model simplifies a real system so that its main behaviour can be explained.
5. In the simple model, the atmosphere is treated as a layer of air with a uniform density, so every cubic metre holds the same mass of air from the ground to the top.
6. The model also gives the atmosphere a definite top, above which there is no air.
7. With a uniform density of about $1.2{kg/m}^3$, the model atmosphere would need to be about $8km$ deep to produce the pressure measured at sea level.
8. The real atmosphere gets less dense with height and fades away gradually rather than stopping at a sharp edge, so the uniform model is an approximation.

Atmospheric pressure

1. Gravity pulls on the air, so every column of air above the ground has weight.
2. Picture a column of air standing on $1m^2$ of ground and reaching all the way to the top of the atmosphere.
3. That column has a mass of about $10000kg$, so its weight is about $100000N$.
4. This weight pressing down on $1m^2$ gives an atmospheric pressure at sea level of about $100000Pa$, which is $1.0\times 10^{5}Pa$ or $100kPa$.
5. The same pressure can be described in terms of molecules, because air molecules constantly collide with every surface and each collision exerts a tiny force on it.
6. The molecules move randomly in all directions, so atmospheric pressure acts in every direction, on the top, the sides and the underneath of objects alike.
7. You do not feel crushed because the fluids inside your body push outwards with a matching pressure, so the forces on the two sides of your skin balance.

Effects of atmospheric pressure

1. Atmospheric pressure only has a noticeable effect when there is a pressure difference across a surface.
2. If the pressure on one side of a surface is lowered, the atmosphere on the other side pushes harder, giving a net force towards the low-pressure side.
3. A suction cup stays on a smooth tile because pressing it flat pushes most of the air out from underneath, so the atmosphere outside pushes the cup against the tile.
4. Drinking through a straw works because you lower the pressure inside your mouth, and atmospheric pressure acting on the surface of the drink pushes the liquid up the straw.
5. When you breathe in, your chest expands and lowers the pressure inside your lungs, so atmospheric pressure pushes air in through your nose and mouth.
6. In every case the object or liquid is pushed by the higher pressure, because a lower pressure cannot pull.