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: Pressure = Force (f)/Area (A) P = F/A
Where
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).
1Pa = 1 N/M2
The other units of pressure are atmosphere, torr bar and mmHg.
 1 atmosphere = 780 mmHg
 1 atmosphere = 1 105 N/M2 = 1 bar (used by meteorologists)


Note: for a given amount of force, the smaller the area of application the greater the pressure exerted.


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.


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-defense. 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

Note: Pressure in liquids =Force/Area
But force= hApg/A
=Hsg
Hence pressure in liquids is given as hsg.

H= Height of the liquid column
A= Area of the base


The pressure at any point in a liquid at rest then depends
1. Depth
2. Density of liquid

Note: It does not depend on the area


Pressure in liquids is characterized using the following parameters:
1. Pressure in a liquid increase with depth
2. Pressure in a liquid acts equally in all directors
3. Pressure in a liquid increases with increase in density of the liquid.

Mercury exerts more pressure than an equal volume of water. This because mercury is denser than water.


The Variation of Pressure with Depth in Liquids

Demonstration of water spurting from holes at different heights.

As pressure at point A is low due to small height of water above it but at point B the height increase and the distance of water increase while at point C the height is greatest therefore the level of water distance will be large distance.
Demonstration of pressure in a communicating vessel

The shape of vessel found in point A, B, C and D is different but the pressure is the same due to the equal height (L) of the liquid above the points.


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.

Note: in all side of vessel will experience equal pressure.
The  hydraulic  press  works  on  Pascal's  principle,  when  using  this  principle  a  small  force converted into a large force and vice versa.

From Pascal's Principle

Note: The distance moved (d) is inversely proportional to the cross sectional area.

Application of Hydraulic Press

Hydraulic press is used in industries to express bulk items.



Hydraulic brake system
When pressure is applied to the brake pedal, it pushes the piston in the master cylinder forward creating a pressure in the brake fluid. 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).