Gas Pressure (DP IB Physics: SL): Revision Note

Calculating Gas Pressure

• Pressure is defined as

The force applied per unit area

• For example, when a drawing pin is pushed downwards:

  • it is pushed into the surface, rather than up towards the finger

  • the sharp point is more concentrated (a small area), creating a larger pressure

When you push a drawing pin, it goes into the surface (rather than your finger)

• When an object is immersed in a liquid, the liquid will exert pressure, squeezing the object

  • The pressure exerted on objects in fluids creates forces against surfaces

  • These forces act at 90 degrees (at right angles) to the surface

The pressure of a fluid on an object creates a force normal (at right angles) to the surface

• The equation for pressure is:

$P=\frac{F}{A}$

• Where:

  • P = pressure (Pa)

  • F = force (N)

  • A = cross-sectional area (m²)

• Pressure is measured in Pascals (Pa)

• This equation is only relevant when gas molecules exert a force perpendicular to the surface

Gas molecules bouncing off the walls of a container

• It is possible for someone to experience this force by closing their mouth and forcing air into their cheeks

• The strain on the cheeks is due to the force of the gas particles pushing at right angles to the cheeks

• This equation means that:

  • a force spread over a large area will produce a small pressure

  • a force spread over a small area will produce a large pressure

High heels produce a higher pressure on the ground because of their smaller area, compared to flat shoes