1. UNIT 13 – Part 2 Under Pressure
Chapter 3

2. What is Pressure?
In science, the concept of pressure involves both force and the surface area on which that force is exerted.
Pressure is the result of a force applied in a perpendicular fashion to a surface.

3. Variables
Force
Surface Area

4. What is Force? Causing an object to
Accelerate
Slow down
Change course
Stop
Change its shape
Requires a certain amount of force to be applied.
Force can be produced through an action or an attraction.

5. What is Surface Area?
Surface Area is the facial dimensions of an object.

6. Force and Pressure
The Greater the mass (means greater force) will end up exerting a greater pressure.
Variation in Force
Result
If the force increases
Pressure increases
If the force decreases
Pressure decreases

7. Surface Area and Pressure
When force is distributed over a greater surface area then less pressure is applied.
Variation in Surface Area
Result
If the surface area exposed to a force increases
Pressure decreases
If the surface area exposed to a force decreases
Pressure increases

8. Calculating Pressure P = F A Pressure is measured in pascals (Pa)
Pressure corresponds to the force applied per unit of surface area. The formula is:
P = F A
P= Pressure (Pa)
F= Force (N)
A= Exposed Surface Area (m2)

9. Pressure Exerted by Fluids
Particles in fluids are in constant motion and therefore pressure is exerted equally in all directions.

10. Pressure exerted by Incompressible Fluids
Incompressible fluids such as water, the force exerted comes from the mass of the fluid that is situated above the object and not the total quantity of fluid.
U-Shaped
manometer
Depth gauge

11. A depth of 1m has the same pressure whether you’re in a pool or a lake.
So, the more fluid above an object, the greater the force exerted by the fluid and the greater the pressure.

12. In Summary
The pressure exerted on an object by an incompressible fluid depends on:
The depth of the object in the fluid
The density of the fluid

13. Pressure exerted by Compressible Fluids
Particles in a compressible fluid (gas) are constantly moving. When the particles meet an obstacle (another particle or say the edge of the container) they change direction.
During each collision , the fluid particles exert a force on the obstacle.

14. Pressure and Compressible Fluids
Pressure depends on the number of collisions involving fluid particles.
The more collisions = the greater the pressure.
There is a direct relationship between pressure and the number of collisions.

15. Factors that Affect the number of Collisions
Number of particles: the more particles, the more they collide.
Temperature: According to the particle model, the speed of the particles is directly related to the temperature. The faster the particles move, the more they collide.
Volume of fluid

16. Instruments
Pressure Gauge
U-Shaped
manometer
Dial-Faced Gauge

17. Volume and Compressible Fluids
When the volume of a gas decreases the number of collisions goes up. This is because the particles have less space to move around and the frequency of collisions increases.
The opposite is true for an increase in volume.
Variation in volume
Result
If volume increases
Pressure decreases
If volume decreases
Pressure increases

18. Boyle’s Law In 1661, he formulated the relationship between the volume
and the pressure of the gas.
*At a constant temperature; the volume of a compressible fluid is inversely proportional to pressure. If the pressure increases, volume decreases, and vice versa.*

20. Relating Boyle’s Law to the human ear
When the external pressure decreases, for example when we fly high in an airplane, the eardrum swells. The eardrum returns to normal when the pressure is the same on both sides of the drum.

21. Atmospheric Pressure

22. Atmospheric Pressure
Atmosphere is the layer of air that surrounds the earth.
This air is a mixture of different gases and it exerts pressure on us.
Although unnoticed, the pressure in the atmosphere is considerable.

23. Atmospheric pressure varies with altitude
Atmospheric pressure varies with altitude. As altitude increases, atmospheric pressure decreases because there are less particles in the air and so less collisions.
Instrument used to measure
atmospheric pressure is called
a barometer.

24. Principles For the variations in the pressure of fluids.
Fluids naturally move from a zone of high pressure toward a zone of low pressure.
See pg 81 figure 3.35

25. Pressure applied to the surface of a fluid inside a closed container gets uniformly distributed to every part of the fluid.
See pg 82, figure 3.36

26. A transfer of pressure in a fluid can increase the force involved.
See pg 82, figure 3.37 (hydraulics)

27. Natural Mechanisms

28. Blood Pressure See page 83 figure 3.38
The heart acts like a pump and provides the necessary changes to help the blood circulate through the body.
Blood pressure in the arteries is measured with a blood pressure cuff (sphygmomanometer)

29. Breathing See pg 83 figure 3.39
When we breath in, our thorax swells and the volume of the lungs increases. This increase in volume lowers the pressure in the lungs relative to that of the air and vice versa.
When breathing in and out, the volume of the lungs changes, letting air in and out of our body.

30. See pg 84 figure 3.40
Wind
This pressure changes with altitude and with weather conditions.
Wind is the movement of air from a high pressure zone toward a low pressure zone of depression.