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What is Pressure?

Everyone knows what happens if you put too much air into a balloon. If the air pressure inside the balloon is too much greater than the pressure outside, the balloon bursts. But what is this thing called pressure?

Have you read the page on the Ideal Gas Law equation yet? Although this equation doesn't tell you why; it states that the amount of pressure on the walls of a container is directly proportional to both the amount of gas inside, and the temperature of that gas (which incidentally, explains why pressure cookers work).

If you haven't yet read the page explaining what temperature is, read it now.

As you know, gas molecules have kinetic energy - they fly around the room, at a speed which specifies the gas's temperature. When they hit walls or other objects, they bounce off. (The same goes for molecules in liquids, although they move around in a slightly different fashion.) When they hit the walls of the container however, they exert a force of those walls - just as a ball would if you threw it at a window. The amount of resulting force on a wall or other object, acting on each unit of surface area of that object, is called pressure. It's that simple.

For example, if you measure force in pounds and surface area in square inches, then you could measure pressure in pounds per square inch, or PSI for short. PSI is a common unit of pressure for things like car tyres - and of course there is a rated maximum number of pounds of force that each square inch of rubber tyre can handle without running the risk of it bursting - it's printed on the side of the tyre.

The units that physicists more often use however are kilopascals (kPa). Pascals (Pa) are actually units of newtons (of force) per square meter (area). And a kilopascal is simply a thousand pascals. So 23 kPa is actually 23000 N/m² of pressure.

(A solid object, such as a person sitting in a chair, can also exert a force-per-unit-area on another object; and this is measured in kPa or PSI in just the same way. But if the object doing the pushing is a solid mass, then it's called stress instead of pressure.)