﻿ Pressure

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## Pressure

Applications of Atmospheric Pressure

Some machines and devices use atmospheric pressure to operate. These include: drinking straw, syringe, siphon, hydraulic press, force pump and lift pump. The working of these machines and devices is demonstrated in the sections that follow.

Applications of Atmospheric Pressure

Some machines and devices use atmospheric pressure to operate. These include: drinking straw, syringe, siphon, hydraulic press, force pump and lift pump. The working of these machines and devices is demonstrated in the sections that follow.

Drinking Straw

When you suck at the end of a straw, air is removed from the straw; and this lowers the pressure inside.  Consequently, the greater atmospheric pressure outside pushes the drink into the mouth.

Objectives

#### By the end of this lesson you should be able to:

• Define pressure and state its SI units
• Determine pressure exerted by solids
• Describe experiments to investigate factors affecting pressure in fluids
• Derive the formula P = hpg

Definition of Pressure

Pressure is the force acting normally per unit area.

As a quantity, therefore, pressure is expressed as

Pressure = Force/Area

Pressure in Solids

Consider a rectangular building block measuring 30cm x 20cm x 15cm and weighing 150N. The block rests on a table with its 30cm-edge vertical. Calculate the pressure exerted on the table.

Pressure in solids increases as the area of application of force (or area of contact) decreases.

Exercises

Quizzes

Demonstrating Pressure

The animation below shows a glass block falling on sand. In each case, the block is released from the same height, so it exerts the same amount of force on landing. How does the amount of penetration vary with the area of contact?

Observation

The same object penetrates deeper into sand when it lands on its smaller surface. That is, the smaller the area of contact, the greater the penetration.

Conclusion

The pressing (or penetrating) effect of a force increases as area of contact decreases. This pressing effect is therefore expressed in terms of the ratio force-to-area, a quantity known as pressure.

Pressure in liquids

Floating cork

Drag a cork below the surface of water in a beaker and then release it. State what happens to the cork.

The liquid thus exerts an upward force on the cork. This is due to pressure exerted on the cork by the surrounding liquid.

Horizontal forces on the cork balance each other but vertical forces give raise to a resultant upward force on the cork. Hence the cork moves upwards.

Factors Affecting Pressure in Liquids

Height (h) of liquid

The animation shows a thistle funnel moving gently down a liquid column. Note how this movement effects the reading on the pressure gauge. State your observations.

Conclusion

Pressure in a liquid increases with the depth of liquid.

There are other factors that affect pressure in liquids but are not demonstrated in this set-up. They are density of liquid and the gravitational constant, g.

Pressure Formula

Derivation of pressure formula

Exercises

Atmospheric Pressure

The term atmosphere means the sea of air surrounding the Earth. It is bound around the earth by the earth's gravity. The weight of air column acting on any given surface is the cause of atmospheric pressure.

Activity1: Demonstration of Atmospheric Pressure

The animation below shows an inverted glass vessel being moved gently into water.

Observation and explanation

Water does not enter the vessel. This is because of the presence of air that exerts atmospheric pressure in the vessel.

Exercises

Pressure in Gases

Gases also exert pressure.

Activity 3:Crushing Can

Observation

The can crashes inwards.

Explanation

As the water gets heated, the steam generated pushes out the air occupying the can. During cooling the steam condenses, creating a partial vacuum. The atmospheric pressure is now greater than the pressure inside the can.

Pressure in Gases

Gases also exert pressure.

Activity 3:Crushing can

Drag pump to the can and pump air out of the can. Observe what happens. Record your observation in the given text box.

Simple Mercury Barometer

A glass tube of height 1m is filled with mercury then inverted over a trough containing mercury. What do you observe?

The level of mercury in the tube falls upto a height of 760 mm above the level of mercury in the trough.

This shows that atmospheric pressure can support a mercury column of length 760 mm. This value, however, applies to the sea level only, since atmospheric pressure varies with altitude.

A Syringe

A syringe consists of a piston and a cylinder. When the piston is pulled, pressure inside te cylinder decreases. The greater atmospheric pressure outside then pushes the liquid into the cylinder. Observe the accompanying animation, which demonstrates the working of a syringe.

Siphon

A tube can be used to empty a beaker.  The tube is first filled with the liquid. The liquid continues to run out so long as the end E is lower than the level of water in the beaker. Pressure at A is the same atmospheric pressure at E.

E = Atmospheric pressure + hpg

The siphon works only when:

• E is lower than the level of water in the beaker
• The tube is first filled with the liquid, without any bubble in it
• The tube does not rise above the barometric height of the liquid from the surface A
• One end of the tube is inside the liquid to be emptied

Hydraulic Press

This device is used to compress (or compact) materials such as cotton and bales of clothes so that they occupy a smaller space. This is important, especially if such materials are to be transported over long distances. The animation below shows how the device works.

Hydraulic Brake

Bicycle Pump

Automatic water flushing

In automatic water flushing tank water is allowed to pour into the vessel continuously.  When it covers the tube it starts pouring automatically.  Click on Vessel underlined to get the water into the vessel.

Force Pump

A force pump is used to lift water to greater heights than is possible with a lift pump. Unlike a lift pump, whose operation depends on atmospheric pressure, the operation of a force pump depends on:

• Size of force used during the downstroke.
• Ability of the pump and its working parts to withstand pressure of the long column of water.

The diagram below shows the operation of force pump.

Upstroke
During the upstroke, air above valve B expands and its pressure reduces below atmospheric pressure. The atmospheric pressure acting on the water in the well pushes the water upwards, thereby opening valve B. Pressure above valve C is atmospheric and this valve does not open during the upstroke.

Downstroke

Valve B closes, the increase in pressure in the cylinder opens valve C and forces water into the chamber. As water fills the chamber, air is trapped and compressed in the upper part  closing valve C. This causes water to rise up in pipe P.Click the play button to observe the working of a force pump

.

Lift Pump
A lift pump is used to raise water from a well.  It operates as follows:

Upstroke

When the piston is pulled up, valve O opens and water flows into A.

Downstroke When the piston is pushed down, valve P opens and water flows from A to B.
During the next upstroke, valve O opens but P closes so that the water above it (P) is pushed out through the tap.

Click on the PLAY button to see it working.

Quizzes

•

Pressure

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Pressure

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