# 1.8.1 Pressure

### 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:

• 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

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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