Measuring pressure

• Aims• Describe the simple mercury

barometer and its use in measuring atmospheric pressure

• Use and describe the use of a manometer

The manometer• A manometer measures pressure

difference.

Atmospheric pressure

• Gasses are also fluids• The atmosphere is made of gasses• The atmosphere exerts a pressure• Equal to

• Density changes so must use average

Using atmospheric pressure

Pressure from the air

The mercury barometer

• Torricelli filled a tube with mercury and inverted it into an open container of mercury. • Air pressure Po acting on the mercury in the dish can support a column of mercury 76 cm in height.

Standard atmospheric pressure

The pressure that will support a column

of mercury of 760.0 mm heigh is known

as standard atmospheric pressure or

1 atmosphere (1 atm).

1 atm = 76 cmHg = 1.013 x 105 Pa = 103 millibar (mb)

Pressure from the air

• As the air is removed from the inside the external pressure crashes the can.

Uses of pressure in fluids

Hydraulic jack

• The hydraulic press is an important application of Pascal’s Principle.

Hydraulic Brakes

1. Brake pedal.2. Master cylinder.3. Break fluid.4. Slave cylinder.5. Large piston.6. Brake disc.7. Tyre.

Why are the walls of a dam wider at the bottom than at the top?

A. It is cheaper to build

B. Gravity increases with depth

C. Pressure increases with depth

D. Looks nice

A pressure of 600 Pa acts over an area of 2m2. What force creates this pressure?

A. 1 200 N

B. 300 N

C. 1 200 Pa

D. 300 Pa