PressurePressure

2

Aim: To understand the concept of pressure.

You will learn about

• Understand atmospheric pressure

• How atmospheric pressure is measured.

• About barometers.

• The effect of pressure on weather.

• How to demostrate atmospheric pressure.

• How pressure varies with altitude

• How pressure varies in liquids

What pressures is.

KeywordsKeywords

Some examples of pressure

Pressure

Explain how the following work:

1) A sharp knife is easier to use then a blunt knife

2) A drawing pin

3) A woman with stilettos on might damage a floor more than an elephant would

Pressure is exerted whenever a force is applied over an area.

If the same force is applied in each picture, which arm exerts the highest pressure on the board?

1. 2.

What is pressure?

The arm applies a force to the board via a fingertip.

The force acts over a small area and so produces a high pressure.

1.

High and low pressure

The same force is now acting over a larger area – the palm has a greater surface area than the fingertip.

A lower pressure is produced.

2.

Pressure is measured in:Newtons per square metre (N/m2), which are also calledpascals (Pa).

Pressure can also be measured in:Newtons per square millimetre (N/mm2);Newtons per square centimetre (N/cm2).

pressure =area

force

p x a

f

Pressure is the force per unit area and is calculated using this formula:

Calculating pressure

Pressure

Pressure depends on two things:Pressure depends on two things:

How much force is applied, and

How big (or small) the area on which this force is applied is.Pressure can be calculated using the equation:

Pressure (in N/m2) = Force (in N)

Area (in m2)OR in N/cm2

F

AP

Calculating Pressure

A lady of weight 600 N standing on two stiletto heels of area 1.5 cm2. What is the pressure due to the heels.

Pressure (in N/cm2) = Force (in N)

Area (in cm2)=

600

2 x 1.5

= 200 N/cm2

Some pressure questions

1)A brick is rested on a surface. The brick has an area of 20cm2. Its weight is 10N. Calculate the pressure.

2)A woman exerts a pressure of 100N/cm2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on?

3)Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 1m2.

4)(Hard!) The pressure due to the atmosphere is 100,000N/m2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body?

• Pressure increases with depth.

• Pressure acts equally in all directions.

• Pressure is transmitted through liquids.

Pressure - In liquids

All hydraulics systems work because the pressure is the same throughout the system.

Showing pressure on liquids depends on depth

The lower the hole the further the water shoots out.

This shows the greater the depth the greater the pressure.

Gas Pressure

Air Pressure

Atmospheric Pressure

Can you feel air pressure?

When you take off or land in an aircraft, your ears may hurt or feel uncomfortable. This is because your eardrums can feel changes in air pressure as the aircraft moves quickly up and down.

But what is air pressure?

What is Air Pressure?

It is caused by the weight of all the air in the atmosphere pressing down on Earth.

It is also known as atmospheric pressure.

Air pressure changes with the height and also when air warms up or cools down.

Changes in air pressure cause changes in the weather.

Differences in air pressure

Low Pressure

When air rises, it leaves behind an area of lower pressure, because the upward- moving air is not pressing down so hard on the surface.

High Pressure

Areas of high pressure are formed where air is sinking back down, and so pushing down harder.

>> see the diagram

Since there are many areas of high and low pressure above the Earth’s surface due to uneven surface heating.

Air moves from high pressure to low pressure forming winds.

As a result, the greater the difference between the high pressure and low pressure areas is, the higher the wind speed is.

Demonstrating Atmospheric Pressure

Demonstrating Atmosphericpressure.

Highs and Lows

Pressure is different all over the world.

Lows are areas of low pressure with the lowest pressure at the center.

Lows usually bring wet, cloudy weather.

Highs are areas of high pressure with the highest pressure at the center.

Highs bring sunnier, and dry weather.

The way these move from day to day causes the changes in the weather.

Measuring Air Pressure

Air Pressure is measured in hectopascals (hPa) on a barometer.

The simplest kind of barometer is a mercury barometer and pressure is measured in mm (in) of mercury. 76 cm Hg = 1 atmosphere.

An aneriod barometer can measure move conveniently.

Uses of barometers

Aneroid barometers in houses to indicate weather changes.

Alimeters to measure height above sea level.

Hydraulic systemsan application of pressure

Hydraulic systems

Pressure is constant throughout this liquid