MENU
1. PRESSURE IN SOLIDS
2. PRESSURE IN LIQUIDS
3. PRESSURE IN GASES
4. MEASURING PRESSURE
Pressure in solids
An elephant will exert less
pressure than a person
wearing high heels. This is
because the weight of the
elephant is spread over a
larger surface area.
Sir jumps quickly to his feet
He’s got the point (-he’s got a scar!)
The pressure acting on his seat,
Is force per unit areaaaaaaaaagh!
Pressure is Force per Unit Area
Force ÷ Area = Pressure
Measured in N/m2 or N/cm2
N/m2 = 1Pa (pascal)
Pressure in solids
This man can lie on a bed of nails because by having many nails
he has increased the surface area so his pressure is less.
Increasing
pressure
Reducing
pressure
Reducing Pressure
These are all examples of where pressure in
reduced by increasing surface area
Increasing pressure
Pressure in solids
It acts in the direction of the force
only.
The smaller the area the higher the
pressure.
The larger the area the smaller the
pressure
Calculating pressure
Main
menu
Pressure in liquids
Pressure in liquids at depth
Pressure in liquids acts
in all directions.
The greater the depth the
greater the pressure, as
the mass (and therefore
the force exerted) is
greater.
Mini Lab: Pressure in Liquids
Aim: To show the relationship
between pressure and depth.
Hypothesis:
Materials: Can, hammer, nail, ruler,
tape and water
Procedure:
Variables:
» Independent:
» Dependent
» Control
Results:
Calculations:
Discussion: Use the data from the lab to show your findings.
P = mass x gravity / area
P = (density x Vol) x gravity / area
P = density x ( Area x height) x gravity / area
Areas cancel out.
P = density x height x gravity
Pressure is exerted
throughout the liquid and in
all directions.
The deeper the water the
greater the pressure
because the weight of the
water is greater
Calculating pressure at depth
Pressure = 10 x depth x density
(N/m2 )
( N/kg) (m) (kg/m3)
The hydrometer
This is used to measure
the density of milk and
other liquids. It floats to
different depths in
different liquids,
depending on their
densities
Upthrust and Pressure
All liquids exert an
upthrust because the
pressure inside the
liquid increases as
you go deeper. This
means that the
pressure on the
bottom of an object is
greater than on the
top, and so there is a
resultant force
upwards
Pressure in liquids
Liquids will find their own level
Pressure in Liquids
Higher
density
higher
pressure.
Using pressure in liquids
Hydraulic machines
It was easier to push on the small syringe that on the
big syringe. This is because a force acting over
small area creates a high pressure. This pressure is
then transmitted through the fluid and acts over a
large area. This results in a larger force on the
plunger of the large syringe.
Pressure in Liquids
Answers
Answer these
questions:
Why does the dam
wall need to be thicker
at its base?
Why does the boat
float?
Why must the diver
breathe out as he
surfaces?
The pressure at the bottom is
greater so the wall must be
stronger.
Upthrust from the water is
spread over a large area so
the upward force is greater
than the weight.
When the diver breathes at
depth the air is at a higher
pressure. As he surfaces the
water pressure decreases so
the air in his lungs would
expand and burst his lungs.
Main Menu
Pressure in Gases
We are living at the bottom of a ‘sea’
of air called the atmosphere, which
exerts a pressure on us (just as the
sea squeezes a diver).
Atmospheric pressure is 100 000N/m2
However, the pressure inside our bodies
is similar, so we don’t notice the
pressure of the air.
Collapsing Can Experiment
Before the pump is
switched on,
molecules are hitting
the outside and inside
with equal pressure.
After the pump is
switched on, there are
almost no molecules
inside the can and the
pressure of the
molecules outside the
can crushes it.
Magdeburg Hemispheres
Magdeburg hemispheres
The pressure of
the air outside
pushes the cups
closed when
there is a
vacuum inside.
When air enters the pressure outside and
inside are the same and they are easily
pulled apart
Gases can be Compressed
...Pressure increases
Exploding Marshmallow
At first the atmospheric pressure on the outside
of the balloon balances the air pressure inside
....
...when you pump out the air
A vacuum is created in the bell jar
air pressure in the balloon /
marshmallow is higher than the
pressure in the bell jar so the balloon
expands
Using Air pressure
Using pressure
Sucking creates
a negative
pressure in the
straw.
The high air
pressure acting
on the surface
of the liquid
pushes the
liquid up the
straw.
Using Air Pressure
The higher
pressure outside
the sucker holds
it in place.
As you push the
sucker down air
is forced out of
the cup creating
a negative
pressure
Using pressure – force pump
Plunger out –
Negative pressure water
pushed up by air pressure.
Plunger in –
Pressure exerted on liquid,
inlet valve forced closed, so
water pressure forces open
outlet valve
Main
Menu
PRESSURE SUMMARY
Pressure is the force per unit area
It is measured in N/m2 ( Pascals), N/cm2 , Bar.
The larger the area the more the force is spread so the less
the pressure.
The smaller the area the less the force is spread, so the
greater the pressure.
In liquids and gases the fluids will flow from an area of high
pressure to an area of low pressure until the pressure is equal
throughout.
Pressure in a liquid is greater at depth.
Pressure in liquids and gases are exerted equally throughout.
The same pressure at equal depths.