Curriculum Shule Direct Kenya Secondary School Form 1 Physics Kenya

Physics Kenya

Pressure
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Pressure

Pressure
Definition of pressure
Pressure is defined as the force per unit area. Mathematically, the force discussed here is the force which acts normally or perpendicularly to a unit surface area.
That is to say, pressure depends on the force applied and the surface 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).
However, due to the fact that measurement of pressure may result to very large numbers or small numbers there are other units used in measurement of pressure. These are atmosphere(atm), Millimetres of mercury (mmHg), torr and bar.
Whereby; -
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 in solids
Dependence of Pressure on Surface of Contact
The pressure in solid depends on the surface area of contact. A force (F) applied by a small area exerts a higher pressure as compared to when it is applied by a large surface.
This is well explained by the formula P =F/A meaning that when the value of the area is small then the value of pressure will be large and when the area is large for the same amount of force, the value of pressure will be small. That is to say, increase in area causes the decrease in pressure for the same amount of force.
That is why; -
  • Feet of elephant cannot sink into soft soil even is very heavy due to large surface area over elephant feet.
  • Sharp edges of knife cut easily than blunt knife because a sharp knife has a small surface area.
  • A tractor and other heavy trucks works on soft ground cannot sink due to large surface area of the tyres (wide tyres).
  • We feel great pain if we carry a bucket of water with a thin handle than when we use a wide (enhanced) handle.
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
  • Data:
  • Force = 30N
  • Largest base area = ?
  • AL = 10mx5m = 50m2
  • P = F/A = 30N = 0.6N/30m2
  • Pressure = 0.6N/m2
  • Force (F) = 30N.
  • Small base are = ?
  • As = 5mx4m
  • As = 20m2
  • P= F/A = 30 N/20M2 = 1.5 N/M2
  • Pressure = 1.5 N/M2
Example 2
A tip of needle has a cross- sectional area of 1x106m2. 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:
  • Data:
  • Area (A) = 1x106m2
  • Force (F) = 20N
  • Pressure =?
  • P = F/A
  • P = 20 = 2.0x107 N/M2
  • The pressure extended by the needle lip is 2.0x107N/m2
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:
Dimension = 1.2m,1.5m,1.0m.
Density = 7000kg/m3
  • Maximum pressure (Pmx) = ?
  • Minimum pressure (Pmn) = ?
  • Volume = 1.2x1.5x1=18m3
  • Density = mass/ Volume
  • 7000kg/m3 = mass/18
  • Mass = 7000x18 = 126000Kg
  • 1kg = 10N
  • 126000Kg = ?
  • F = 1260000 = 1.26 x 106
  • Force of metal = 1.26 x 106
  • A1 = 1.2 x 1.5= 18m2
  • A2= 1.2m2= 1.2mx1m=1.2m2
  • A3= 1.5 x 1m= 1.5m2
  • Amx = 18m2
  • Amin = 102m2
  • Pmn = F/Amx =1.26 x 106N/18M2 = 1.26X106/1.8 X101
  • Maximum pressure = 1.05x106N/m2/maximum pressure= 0.7x105N/m2
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.
Factors affecting pressure in fluid (Experimental treatment required)
Derivation of P = ρgh
Atmospheric pressure
Atmospheric pressure is as a result of the weight of this layer of gases, the act at atmospheric pressure is surrounded the layer of air consisting of a mixture of gases.
Note. The atmospheric pressure on the earth’s surface and objects on the earth is a approximately 1.01x105N/m2.
The experiment which demonstrate the existence of atmospheric pressure.
Umbler experiment
When you fill a glass timber with cord and gentle turned upside down. It will be seen that water will not poured down.
Pulling the plunger is not easy. This is so because all the air is squeezed out from the table when the surrounding pressure being high the plunger sticks.
Crushed bottle
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.
Simple mercury barometer, manometers
Applications of pressure: drinking straw, syringe, siphon, hydraulic press, hydraulic brakes, bicycle pump, force pump, lift pump
A siphon
A siphon is applied in areas and devices that we use everyday
  1. 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.
  2. It is used in special rain gauges called siphon rain gauge which are able to automatically drain out excess water.
  3. A siphon cup is a reservoir attached to a gum.
  4. It is used is some drainage systems to drain water to one point.
The lift pump
A lift pump is used to raise water from underground sources. This is a pump that is used to lift the liquid, rather than force liquid up.
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:
  1. They can be fitted with hypodermic needles and used to administer injections.
  2. They are used to measure liquids and gases in a laboratory.
  3. They are used to apply certain compounds such as a glue or lubricant.
Bicycle pump
This is a type of force pump that consists of a hollow metal cylinder and a movable piston.
Problems on pressure
Listening to this topic

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