Pressure, volume and density
Pressure in gases
Why aren't we pushed down by all the air above us?
You've probably done this at some point many years ago, and it's not just childish play. It's a good way to experiment with pressure and gases. This is how it works. The air around you is a gas a mix of mainly nitrogen and oxygen. The gas has a mass and is pulled downwards by gravity.
Above your head, you have a column of air that extends more than a 100 kilometers upwards. That air column presses down on you from above and from the sides and from below. It even presses from inside since you inhale the air. Just like pressure in liquids, pressure in gases acts in all directions even upwards. The air pressure at sea level is about 100,000 pascals or 760 millimeters of mercury or 1 atmosphere.
The particles in a gas are constantly moving randomly in all directions. If we capture the gas inside a container, the particles knock against the walls of the container. Pressure is how many particles knock against each section of the wall and how hard they knock. If we squeeze the container so the same amount of particles get less space to move around, knocks per section of the wall. then we'll get more The pressure increases and if we heat the container, the particles will move around faster and knock against the walls with greater frequency, increased pressure.
A bicycle tire is full of air. There is a higher pressure inside the tire than outside of it. The air molecules inside the tire are compressed and together they push harder from inside the tire than the atmosphere and your body weight pushes from outside onto the tire. To fill the tire with air that has a higher pressure than the air on the outside, you need a pump. When you pump the tire, the air molecules are pushed together.
They are compressed inside the pump. If you compress the air in the pump more than the air in the tire, the vent opens and air particles rush into the tire. The pressure spreads. If particles are crammed together in one place, they will try to spread out and move to where there's more space. Until they are all evenly spread out, the same pressure.
This way, pressure behaves similarly in gasses and in liquids. Pressure spreads out until it is evenly distributed in a container. When working with pressure, we are often interested in the difference in pressure rather than the pressure as such. The pressure inside this can is higher than the air pressure outside of it. When working with pressure relative to some other neighboring pressure, we call it gauge pressure. No more gauge pressure, it evened out.
In this can, the pressure is lower than in the atmosphere outside of it. We call this negative pressure, or negative gauge pressure. If we remove the rest of the air in the can so there are almost no air particles left, we have a vacuum. Now, the atmospheric pressure pushes hard on the can with nothing inside the can to push back. If the can is soft it will collapse.
Just like that - negative pressure. Yes, thank you very much. That's enough.