TOPIC 7: PRESSURE | PHYSICS FORM 1
Concept of Pressure
Pressure is defined as the force per unit area. OR Pressure is the force acting normally (perpendicularly) per unit surface area.
It is calculated by the formula:
- P – Pressure
- F – Force
- A – Area
The S.I Unit of Pressure
The SI unit of Pressure is Newton per square metre (N/M2). This unit is usually referred to as the Pascal (Pa).
- 1 atmosphere = 780mmHg
- 1 atmosphere = 1 105 N/M2 = 1bar (used by meteorologists)
When a man lifts a bucket of water by its handle that is made with a thin metal, he would experience some discomfort but if the bucket was made with a thicker handle the discomfort will be much less if any.
This is because the area over which the force is applied is larger.
Pressure due to Solids
Dependence of Pressure on Surface of Contact
The pressure in solid depends on the surface area of contact. A force (F) applied onto a small area exerts a higher pressure as compared to when it is applied onto a large surface.
Pressure in solid = Force applied/Area of contact.
Example 1
A block of wood that weighs 30N and measures 5m by 10m by 4m. If it was placed on a table with the largest possible area (5mx10m) in contact with table, exerts less pressure than it would when placed with its smallest possible area (5mx4m) in contact with table.
Solution
Example 2
A tip of needle has a cross- sectional area of 1×106m2. If doctor applies a force of 2N to a syringe that is connected to the needle, what is pressure is exerted at the tip of the needle?
Soln:
Example 3
A rectangle metal block with sides 105m by 1.0m by 1.2m rests on a horizontal surface. If the density of the metal is 7000kg/m3. Calculate the maximum and minimum pressure that the block can exerts on the surface.
(Take the weight of 1kg mass to be 10N)
Solution:
The Applications of Pressure due to Solids
It is used to make different objects like screw, nails, pins, spears and arrows. This item is given sharp points to increase their penetrating power.
It helps some of living organisms for self-defence. A fish uses its sharp fins to protect itself.
When one walks on shoes with sharp pointed heels they exert greater pressure on the ground than when wearing flat shoes.
It helps in construction of railways. During the constructions of railways, wide wooden or concrete sleepers are placed below the railways tracks. This provides a larger surface area over which the weight of the train acts. This is a safety measure to train tracks.
Buildings are constructed with wide foundations to ensure that the weight of the building acts over the layer area.
Pressure in Liquids
A liquid will exert pressure on an immersed object as well as on the walls of the container holding it. Note that the pressure exerted by liquids is due to the weight of the liquid. Also increase in water level in liquid cause increase in pressure.
The Characteristics of Pressure in Liquids
Describe the characteristics of pressure in liquids
Note: Pressure in liquids =Force/Area

H= Height of the liquid column
- Depth
- Density of liquid
Note: It does not defend on the area
- Pressure in a liquid increase with depth
- Pressure in a liquid acts equally in all directors
- Pressure in a liquid increases with increase in density of the liquid.
The Variation of Pressure with Depth in Liquids
Examine the variation of pressure with depth in liquids
Demonstration of water spurting from holes at different heights.

Demonstration of pressure in a communicating vessel

Problems Involving Pressure in Liquids
Solve problems involving Pressure in Liquids
Example 4
A cube of side 2cm is completely submerged in water so that the bottom of the cube is at a depth of 10cm. use g=10m/s2 and s=1000kg/m3
- What is different between the pressure on the bottom of the cable and the pressure on its tap?
- Determine the difference in the force on the top and bottom.
- What is the weight of the water displaced by the cube?
Solution;
Example 5
Calculate the pressure at the bottom of tank of water 15m deep due to the water above it is (s=1000kg/m3).
Formula
The Principle of a Hydraulic Pressure
Explain the principle of a hydraulic pressure
It state that “Any external pressure applied to the surface of an enclosed liquid will be transmitted equally throughout liquid.”
Illustration of Pascal’s Principle.


Example 6

Application of Hydraulic Press

Hydraulic brake system
This pressure is transferred to the slave cylinders where it is multiplied and pushes the brake shoes against the brake dram that is attached to the wheel of the vehicle.
It is used in industries in the forming of metals.
Since the hydraulic press act like a lift, it is used for lifting heavy loads.
Measuring the pressure of liquids using a manometer. Manometer is the device for measuring pressure (commonly gas).
Atmospheric Pressure
The atmospheric pressure on the earth’s surface and objects on the earth is a approximately 1.01×105N/m2.
The experiment which demonstrate the existence of atmospheric pressure.



When putting hot water in, the bottle is cooled under cold water the steam condenses; leaving partial vacuum inside the bottle consequently the greater atmospheric pressure outside the bottle crushes it in wards.
Applications of Atmospheric Pressure
Identify the applications of atmospheric pressure

A siphon is applied in areas and devices that we use everyday
> It is used in the toilet flushing cisterns (chain and ball tank). The flush is triggered by a handle that operates a simple diaphragm like piston pump that lifts enough water.
> It is used in special rain gauges called siphon rain gauge which are able to automatically drain out excess water.
> A siphon cup is a reservoir attached to a gum.
> It is used is some drainage systems to drain water to one point.

A syringe is a simple piston pump that lifts a tube. The plunger that lifts can be pulled and pushed through inside a cylindrical tube or barrel.
This enables the syringe to take in or expel fluid through the opening (nozzle) at the end of the tube.

Uses of syringe:
> They can be fitted with hypodermic needles and used to administer injections.
> They are used to measure liquids and gases in a laboratory.
> They are used to apply certain compounds such as a glue or lubricant.
Bicycle pump

Measuring Atmospheric Pressure
Atmospheric pressure is measured by using barometer;
Types of barometer:
- The simple barometer
- Fortin barometer
- Aneroid barometer
Disadvantages of Fortin barometer:
> Mercury is very expensive to use and is very toxic. For these reasons, an aneroid barometer is usually preferred.
> It is not portable as it is generally big and contains liquid.
> It must be mounted in a vertical position