Pressure in gases [replacing lesson "Pressure in gases"]

Lina! Look how giant my balloon is! How much bigger do you think it can get? I don’t think that’s a good… … idea. Why did Leon's balloon burst?

When Leon blows into the balloon, he fills it with air. The air is made up of tiny particles that move in all directions — just like any other gas! As the particles move, they collide with each other and hit the inner surface of the balloon. When the particles of air hit the balloon’s inner surface, they exert a force on it. This creates pressure.

The amount of pressure depends on the force with which the particles hit the surface of the balloon. The more particles are inside the balloon and the faster they move, the more force they exert, and the higher the pressure rises. At some point, the walls of the balloon can’t withstand the pressure anymore and they tear - the balloon bursts! You can make the balloon burst by increasing pressure in other ways too. What happens if you squeeze a blown-up balloon really hard?

As you squeeze it, you reduce the space in which the particles inside the balloon can move. The particles collide and hit the balloon walls more often. This increases the pressure inside the balloon and makes the balloon pop. Heating the balloon can have a similar effect. This is because heat makes the air particles inside the balloon move faster.

And the faster they move, the more they hit the balloon’s inner surface, and the more pressure they create. So, if you leave a balloon that’s filled with air in the hot sun too long, it will burst. Whether you increase pressure by adding more air, reducing the space, or increasing the temperature, eventually the pressure reaches the point when the balloon can’t withstand it anymore. The balloon explodes! As soon as the balloon bursts, the air particles rush out through the burst hole and spread out.

Gases have no definite shape so they spread to fill the space they are in evenly. But the air outdoors is not contained by anything, does it still exert pressure? Yes, it does! Even though there are no external walls for the air to exert pressure on, the air particles in the Earth’s atmosphere have mass and are pulled by the Earth's gravitational force. It’s estimated that all the air in the Earth’s atmosphere weighs about 5 quadrillion tonnes in total!

This mass of air is distributed over the whole surface of our planet, but it still creates significant pressure. This pressure exerted by the mass of air in the atmosphere on the surface of the Earth is called atmospheric pressure. Gas pressure, including atmospheric pressure, is exerted equally in all directions. But... the air pressure at sea level is much higher than the air pressure on Mount Everest.

Why? The amount of pressure depends on altitude. At higher altitudes the air is thinner, so there aren’t as many particles colliding. There are also fewer layers of air above pressing downwards. So the higher you go, the lower the pressure is.

Pressure in a gas is the amount of force the gas exerts on a surface of an object. The more gas particles there are and the faster they move, the higher the pressure they exert. This is the case in a contained space, like a balloon, or outside, where the air molecules in the atmosphere, pulled by the gravitational force, exert atmospheric pressure on the Earth. Atmospheric pressure decreases with altitude.