### 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
**cm**(and the force in^{2}**N**), then the pressure will be in**N/cm**^{2} - If the area is measured in
**m**(and the force in^{2}**N**), then the pressure will be in**N/m**^{2}

- If the area is measured in
- Pressure can also be measured in pascals, Pa

**1 Pa is the same as 1 N/m**^{2}

### 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

### Pressure in Liquids

- The size of this pressure depends upon the density (
) of the liquid, the depth (*ρ*) of the object and the gravitational field strength (*h*):*g*

- In the above equation:
- Pressure is in
**pascals**(Pa) – where 1 Pa if the same as 1 N/m^{2} - Depth is in
**metres**(m) - Density is in
**kg/m**^{3}

- Pressure is in
- 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