Pressure, Force & Area
- Pressure is the concentration of a force:
- If a force is spread over a large area it will result in a small pressure
- If it is spread over a small area it will result in a large pressure
When you push a drawing pin, it goes into the surface (rather than your finger) because the force on the surface is more concentrated
- Pressure is related to force and area by the equation:
- You can rearrange this equation with the help of the formula triangle:
Use the formula triangle to help you rearrange the equation
- The units of pressure depend on the units of area:
- If the area is measured in cm2 (and the force in N), then the pressure will be in N/cm2
- If the area is measured in m2 (and the force in N), then the pressure will be in N/m2
- Pressure can also be measured in pascals, Pa
1 Pa is the same as 1 N/m2
Pressure in Liquids: Basics
- When an object is immersed in a liquid, the liquid will exert a pressure, squeezing the object
- This pressure is exerted evenly across the whole surface of the liquid, and in all directions
Pressure, at a point in a liquid, acts equally in all directions
- The pressure in the liquid depends upon a couple of factors:
- The depth of the liquid
- The density of the liquid
- The bigger either of these factors, the greater the pressure
Extended Only
Pressure in Liquids
- The size of this pressure depends upon the density (ρ) of the liquid, the depth (h) of the object and the gravitational field strength (g):
- In the above equation:
- Pressure is in pascals (Pa) – where 1 Pa if the same as 1 N/m2
- Depth is in metres (m)
- Density is in kg/m3
- The gravitational field strength on Earth is approximately 10 N/kg
(You will always be given this figure)
Barometers and Manometers
- Barometers and Manometers are devices that measure pressure:
- A barometer is a device that is used to measure air pressure
- A manometer is used to measure other pressures
- A simple barometer consists of a column of mercury in an inverted tube, sat in a tray of mercury exposed to the atmosphere
A simple mercury barometer, used to measure atmospheric pressure
- The weight of the mercury in the tube is balanced by atmospheric pressure pushing down on the mercury in the tray
- If atmospheric pressure increases, a greater length of mercury can be supported in the tube
- If atmospheric pressure decreases then less mercury will be supported in the tube
- A simple manometer consists of a U-tube containing mercury
A simple manometer, consisting of mercury in a U-tube
- One end of the U-tube is open to the atmosphere
- The other end is connected to the pressure that is to be measured
- As the measured pressure increases, the mercury is pushed around the U-tube:
The greater the pressure, the further it is pushed