CIE A Level Physics复习笔记4.2.2 Pressure

• Pressure tells us how concentrated a force is, it is defined as the force per unit area

Pressure is equal to the force per unit area

• This equation tells us
  • 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

Different pressure is exerted for the same force on different areas 

• 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 where 1 Pa is the same as 1 N/m²
• Pressure, unlike force, is a scalar. Therefore pressure does not have a specific direction

U-tube manometer

• A manometer is an instrument to measure pressure and density of two liquids

• In Figure 1: The level of liquid is equal because the atmospheric pressure (P_atm) is the same
• In Figure 2: If the pressure on one side rises, the liquid will be forced down making the liquid in the other limb rise. The difference between the two levels gives the pressure difference between the two ends of the tube
• In Figure 3: The U-tube now has two different liquids. The density of the blue one is larger than that of the orange one. The pressure at each point is due to the atmospheric pressure plus the weight of the liquid above it