Liquids and gases exert pressure equally in all directions. Pressure increases with depth in liquids and gases due to the weight of the fluid above pressing down. Magdeburg hemispheres demonstrate that air pressure keeps spheres stuck together when the air between them is removed. The formula for calculating fluid pressure is Pressure = Density x Gravity x Height, where increasing height results in increasing pressure.

1. Pressure in fluids
Liquids and gases also exert pressure…..

2. Pressure in liquids
 Pressure in liquids acts equally in ALL
directions, as long as the liquid is not
moving.

3. Pressure in gases
 Gases also exert pressure on things
around them.
 The atmosphere exerts about 100 000
Pa on your body!
 Like liquids, pressure in gases acts
equally in all directions, so long as the
gas is not moving.

4. Magdeburg hemispheres
 Magdeburg
hemispheres
demonstrate air
pressure very well.
 When the air is
sucked out, the air
pressure acting on
the outside will
keep the spheres
stuck together.

5. Pressure and depth
 Pressure in a liquid
increases with
depth.
 Think about a
column of water.
 The force at the
bottom of the
column…..
 …….is equal to the
weight of all the
water above it.

6. Calculating Pressure
 The volume of
water (V) is equal
to the area of its
base (A) multiplied
by the height of the height h
column (h)
V=Axh
area A

7. Calculating Pressure
Force (F)
 The mass of the
water can be
calculated using
the Volume and the
density of the height h
water:
mass = A x h x r
 The force (F) is
equal to the weight area A
of the water
multiplied by
gravity (g):
F=Axhxrxg

8. Calculating Pressure
 Finally we know that pressure, (P) is
equal to the force, (F) divided by the
area, (A). Axhxrx
Pressure (p) =
g A
 The area of the column cancels
out, leaving us with:
p=hxrxg
YOU NEED TO KNOW THIS!

9. Calculating pressure
 We can use this equation for
calculating pressure differences in
other liquids and gases, as long as we
know the density.
 Turn to page 168 in your text books
for some example questions on
pressure, (there is a worked example
on page 167 to help you).