1. 1.7.Pressure
GCSE Physics
David Raju Vundi

2. Pressure Pressure is the amount of force applied to a known area.
We can calculate it using the equation
Pressure = Force (N) / Area (m2)
We can reduce pressure by reducing the size of the
force or increasing the size of the area.

3. Pressure and Density Density = Mass/Volume A property of the material.
Pressure = Force/Area
Depends on the height of the fluid.
Same in all directions.
Units are:
Force/Area = N/m2.
Pascals  1 Pa = 1 N/m2.
Atmosphere  1 atm = X 105 N/m2.

4. Pressure is due to the net force of the molecules in a fluid colliding with the walls.
A very large number of collisions happen each second.
Each collision exerts a tiny net force on the wall.

5. How to Measure Pressure
Barometer
measures atmospheric pressure
Aneroid Barometer
vacuum
Patm
PHg
Courtesy Christy Johannesson

6. vacuum
air
pressure
mercury (Hg)
barometer: device to measure air pressure

7. Barometer Empty space (a vacuum) Hg Weight of the mercury in
the column
atmosphere
(atmospheric
pressure)
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

8. The barometer measures
Water column
(34.0 ft. high
or 10.4 m)
Mercury filled
760 mm = 1 atm
Water filled
10400 mm = 1 atm
Atmospheric
pressure
Mercury column
(30.0 in. high
or 76 cm)
The barometer measures
air pressure

9. Barometers
Mount Everest
Sea level
Sea level On top of Mount Everest

10. Pressure Experiment 100g SAND
I think the bigger mass will sink further
I think a bigger area will stop the mass sinking
SAND

11. Measuring
Pressure

12. Torricelli Barometer
The mercury in the tube pushes down with its weight.
The bottom of the tube is open to the atmosphere.
The air pushes on the open surface of the mercury.
On an average day, the pressure of the air equals the pressure exerted by a column of mercury 760 mm high.
Above 760 mm, there is a vacuum in the tube.
Weight of mercury

13. Barometer Detail
Why doesn’t the diameter of the column of Hg make a difference?
Recall that Pressure = force/area.
The “force” is the weight of the mercury, but the pressure that results is that weight divided by the area of the column. So … a bigger column weighs more but also has a proportionally bigger area, and the two factors cancel one another out.
The pressure caused by the column of mercury pressing down is independent of the diameter of the column.

14. Manometer
A manometer is comprised of a bulb containing a gas and a U-shaped tube.
The U-shaped tube is partially filled with mercury. The weight of the mercury puts pressure on the gas.
If the U-tube is OPEN there is also air pressure acting on the gas.
The gas molecules put pressure on the mercury.
PHg

15. Closed Manometers
There is a balance between the weight of the mercury on the left (PHg) and the pressure of the gas on the right (Pgas).
The difference between the heights of the mercury on each side of the tube is a measure of the pressure of the gas.
Pgas = Dh
vacuum
PHg

16. Open Manometers
When gas pressure is greater than atmospheric pressure, the mercury is pushed toward the open end.
The balance is between the gas on the right, and the air plus mercury on the left.
Pair + PHg = Pgas
The weight of the mercury is measured as the height difference:
PHg = Dh
So Pgas = Pair + Dh
Pair
PHg

17. Open Manometers
When gas pressure is less than atmospheric pressure, the mercury is pushed toward the gas reservoir.
The balance is between the air on the left and the gas plus mercury on the right:
Pair = Pgas + PHg
The weight of the mercury is measured as the height difference:
PHg = Dh
So Pair = Pgas + Dh
Or Pgas = Pair- Dh
PHg
Pair

18. Sample Problems Find the pressure of the gas in each manometer. 1. 2.
20 mm
Pair = 790 mm 1.
h = 13 mm
PAIR = 753 mm 2.
PAIR = 765 mm
h = 27 mm 3.
Find the pressure of the gas in each manometer.
Pay attention to whether the manometer is open or closed!

19. Sample Problem Answers
1. Pgas+ Dh = Pair
Pgas= 790 mm - 20 mm = 770 mm Hg
2. Pgas = vacuum + 13 mm = 13 mm Hg
3. Pgas = Pair + Dh
Pgas = = 792 mm Hg