Pressure (P)

Pressure is the most complex of the four properties of a gas that we can measure.

Pressure is defined (in physics) as a force applied over an area.

In English, that means that pressure depends on how much force (or push) is applied to a surface and also depends (inversely) to the area that force is spread over.

When dealing with gases the force is caused by molecules banging into the walls of the container and the area is the surface area of the container walls.

The first part of the pressure definition is easy to understand, the more force is applied the more the pressure. In simple terms, it hurts more to be stepped on by a baby elephant than by a sparrow.

The second part (about area) may not be as clear. As an example, think about carrying a plastic grocery bag with several cans of food in it. At first it doesn't seem so heavy, but as you carry it and the handles bunch up it begins to “cut” into your fingers. The bag has not gotten heavier, but you are applying that force to a smaller area.

In the same way, it hurts much more to be stepped on by someone wearing heels than it does to be stepped on by the same person barefooted. This is because although the force (their weight) is the same, it is spread over a much smaller area when the person is wearing heels. In fact, a 110 lb woman wearing a spike heel (¼ inch square) exerts a pressure of over 1700 lbs per square inch, enough to go through someone's foot. In fact, stewardesses on airliners are not allowed to wear heels because they can dent the aluminum floor of the aircraft.

Because both of these factors (force and area) influence pressure, any change to EITHER of them will change the pressure of a gas.

To really understand pressure, you need to know how we measure pressure.