Gas Pressure

The concentration of gases is most often expressed as Gas Pressure. Pressure is defined as force per unit area. The equation for pressure is:

Pressure-defn

P = pressure

F = force

A = area

Gas Pressure is the force gases exert on their container per unit area.

This equation can be rewritten to solve for either Force or Area as well.

F=pxa

A=FP

Atmospheric pressure is a specific type of gas pressure referring to the pressure the atmosphere exerts on us.

At higher altitudes atmospheric pressure decreases.

Let’s consider the relationship among these three variables, pressure, force and area. Nails are easier than screws to hammer into wood because the area of contact between the nail and the wood is reduced to a small point. The force the hammer exerts on the nail is applied to an extremely small area, at the point of contact, which allows the hammer to apply a great amount of pressure. If a screw were hammered into a piece of wood with an identical force, the wider point of contact between the base of the screw and the piece of wood means that the screw exerts a smaller force.

hammer-screw

Looking at these two rectangular prisms below, we notice that the same force of gravity is acting upon them, because they each have the same weight.

pressure-blocks-area

However, the pressure each exerts is different.

Pressure-defn

The blocks on the right exert more pressure because the same force is being applied over a smaller area.

Sample Problem. 

If a rock exerts a force of 25 newtons over an area of 5.00 m2, how much pressure is the rock putting on the ground?

pressure-5

The rock exerts a pressure on the ground of 5.0 N/m2.

Sample Problem.

The pressure of a nail was measured at 350 Pascals (Pa). What force is exerted by the nail if the surface area is 0.17 m2?

Wanted:

Force = ?

Given:

P = 350 Pa

A = 0.17 m2

We rewrite the equation to solve for force:

pressure-60

A nail exerting a pressure of 350 Pa over an area of 0.17 m2 exerts a force of 60. Pam2.

Sample Problem.

What area is covered by a box that exerts 40 newtons of force and a pressure of 20 Pa?

Wanted:

Area = ?

Given:

F = 40 N

P = 20 Pa

We rewrite the equation to solve for area:

pressure-2

The box covers an area of 2 N/Pa.