# Pressure

## Pressure

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# Pressure

Student Notes

Pressure is caused by a force that pushes against an object.

The pressure increases when the force increases or when the area (that the force is acting on) decreases.

The unit of pressure is the pascal (Pa).
You can also use the N/m2 or N/cm2.

Example 1:
The wind exerts a horizontal force of 1000 N on a wall of area 20 m2. Calculate the pressure at the wall.
Answer: P = F ÷ A = 1000 ÷ 20 = 50 pascals.

Example 2:
A lady weighs 800 N and is standing in a pair of stiletto heels, each of area 2 cm2.
An elephant weighs 27000 N and the total area of its feet is 1800cm2.
Which exerts more pressure on the ground – the lady in stilettos or the elephant?
Total pressure exerted by the lady’s (two) feet = 800 ÷ 4 = 200 N/cm2
Total pressure exerted by the elephant’s (four) feet = 36000/1800 = 20 N/cm2
The lady exerts ten times more pressure on the ground than the elephant.

Pressure in Liquids: the pressure in a fluid increases with depth.

Demonstration
Set up as shown.
The water coming out of the bottom hole travels the farthest, because it is under the greatest pressure.

Air has mass

Demonstration
Weigh an empty plastic bottle (with the top on) and note the mass.
Pump air into it using a special pump and weigh it again. Note the new mass.
Result: there was an increase in mass as a result of pumping air in, therefore air has mass.

Air occupies space

Demonstration
Glue some cotton wool to the bottom of a glass (inside).
Turn the glass upside down, submerge it completely into a large bowl of water and then remove it again.
Notice that the cotton wool is still dry.
Conclusion: the water could not enter the glass because there was air already in there.

Atmospheric Pressure

The atmosphere exerts pressure

The atmosphere is made up of giggling atoms and exerts a huge amount of pressure on all objects, including the earth.
Demonstration
Use a large metal drum and pump the air out using a vacuum pump. The drum will collapse (implode) because there is now a greater pressure outside acting inwards than there is inside acting outwards.

Atmospheric pressure varies with height

Atmospheric pressure decreases as the height above sea level increases (because there is less air overhead).

A barometer measures atmospheric pressure

Atmospheric Pressure and the weather

Areas of high atmospheric pressure are represented on a weather chart by the letter H and will normally have dry, clear, settled weather.
Areas of low atmospheric pressure are represented on a weather chart by the letter L and will normally have cloudy, windy, wet weather.

Why is low atmospheric pressure associated with bad weather?
Low atmospheric pressure means that it is easy for water to evaporate into the air (because the ‘blanket’ of atmospheric pressure pressing down on the water is lighter)
This results in more clouds and therefore more rain.

We will see when we study the chapter on Heat that water boils at 100 0C at atmospheric pressure. Can you guess why it will boil at less than 100 0C if the atmospheric pressure is low?

Exam Questions

 Area Force
• [2006 OL] [2009 OL][2010 OL]

Use the box on the right to give the formula for pressure.

• [2006 OL][2010 OL]

Name an instrument used to measure atmospheric pressure.

• [2009 OL]

If a metal block applies a force of 20 N on an area of 5 cm2, find the pressure being applied by the block.

• [2010 OL]

If the area of the face of a metal block is 30 cm2 and the force (weight) of the block is 90 N, find the pressure being applied by the block.

• [2010]

The diagram shows a tank full of water. The mass of the water in the tank is 48 000 kg.
Calculate the approximate pressure that it exerts on the base of the tank.

• [2009 OL]

The diagram shows a container with three spouts. The container is filled with water.
Jets of water pour out of the spouts.

Why does the jet of water from the bottom spout travel the furthest out from the container?

• [2010 OL]

A household water supply has a water tank in the attic.
The water pressure at the upstairs tap is lower than at the downstairs tap.
Give a reason why this is the case.

## Atmospheric Pressure

• [2007]

The diagram is an Atlantic weather chart.

• Use the chart to predict two weather conditions that you might expect for Ireland.
• Explain why low atmospheric pressure causes one of the weather conditions that you have given.

• [2010]

The apparatus shown in the diagram was used to investigate the expansion and contraction of a gas.

• What is observed when the flask is heated?
• What is observed when the flask is allowed to cool?
• Explain what you observe as the flask cools.

• [2009]

The diagram shows a model of the human breathing system.

• Name the part of the breathing system represented by the balloons.
• What is the part of the breathing system represented by the bell-jar?

Exam Solutions

• Pressure = Force ÷ Area
• Barometer
• Pressure = Force ÷ Area = 20 ÷ 5 = 4 Pa
• Pressure = Force ÷ Area = 90 ÷ 30 = 3 Pa
• The mass is 48000 kg and weight (which is the force) is mass × 10 = 480000. The area is 2 × 2 = 4 m2.

Pressure = Force ÷ Area = 480000 ÷ 4 = 120000 Pa

• Pressure increases with depth so the water at the bottom is under the greatest pressure.
• Pressure increases with height and the water in the upstairs tap is higher up (closer) to the tank in attic.
•
• Any two from: cloudy/ windy/ rain…
• Any one from: air rises/ air moves in/ water vapour condenses (cools)
•
• Bubbles of air come out of the bottom of the glass tube.
• The air in the flask expanded.
• The bubbles stop and water rises up the glass tube.
• Air in flask contracted therefore the air pressure is less than atmospheric pressure.
•
• The lungs
• The rib-cage

Other Test Questions

• Calculate the pressure exerted on the floor by a wooden box, which weighs 48 Newtons if the dimensions of the base of the box are 2m by 4m.

• An army tank can travel across wet land if it has tracks on, but not if it is on wheels. Explain why.
• Draw a labelled diagram of the apparatus you would use to show that pressure in a liquid increases with depth. Include in the diagram the results you would expect to see.

• Draw a labelled diagram of the set-up used to demonstrate atmospheric pressure.
• What is the relationship between weather and atmospheric pressure?

Pressure and boiling point

• What is the effect of increased pressure on the boiling point of water

• How would you demonstrate that pressure affects boiling point?
•
• Why does increased pressure affect the boiling point of water?
• Can you think of an application of this (a device which is built on this principle)?
• Can you think of anyone where this might be a problem?

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