PRESSURE

PressurePressure is the distribution of force over a certain area

P=F/A

Force is measured in Newtons (N)

Area is measured in m2

Pressure is measured in N/m2

1 N/m2 is also called 1 pascal (Pa)

1000 Pa is called a kilopascal (kPa)

On Earth's surface, a mass of 1 kg exerts a force of approximately 9.81 N [down].

Therefore, if you weigh 57 kg (125 pounds), you will exert a force of about 560 N (57x9.81).

There are many, many different physical situations that we

encounter every day in which pressure is an important element.

Walking

Peeling an apple

Hitting a nail

EXAMPLES:

Using the formula for Pressure, explain how a thumbtack works.

(hint: there are two surface areas – the top of the tack and the pin)

Using the formula for pressure, explain why it would hurt your foot more to wear this shoe:

As opposed to this shoe:

Why would it be better to get an injection with a sharp syringe rather than a dull syringe?

“Bed of nailS” versus a “bed of a nail”

http://www.youtube.com/watch?v=fqCdIbVmaqM

Snowshoes

Complete sheet on Pressure questions

Calculations

Force(N) = mass(kg) x gravity(9.8 m/s2)

What pressure is exerted by 150 mL(g) of water with a surface area 0.001 m2?

What pressure is exerted if we double the mass of the water?

Compressible and

IncompressibleFluids

A fluid is any substance that has the ability to flow, or take theshape of its container.

Therefore, both liquids and gases are considered fluids

Confined fluids are fluids that are in a closed system.Systems can be pneumatic (gases) or hydraulic (liquids).

Examples:

Air or water mattress

Circulatory System

Braking Mechanisms

Respiratory System

Hydraulic Braking System

When you push on the brake pedal, force is applied to the

master cylinder piston.

The master cylinder containsbrake fluid, which is forced into the

wheel cylinders.

This fluid then puts pressureon the wheel cylinder pistons,which push against a brake

drum, stopping the car’s tires.http://www.meineke.com/services/brakes.asp

Pneumatic Braking System

• Much larger and more complicated than hydraulic system. • Uses compressed air instead of brake fluid to apply brakes. • Compressed air is very pressurized, so these types of brakes can apply a lot of force (used in buses, 18-wheel trucks etc.)

Using your knowledge of the PARTICLE THEORY (and whatyou learned in science last year), what is the major difference

between using a pneumatic or hydraulic system?

Liquids are virtually INCOMPRESSIBLE – they cannot be compressed (their volume cannot be reduced by much)

Gases are COMPRESSIBLE – their volume can be reduced

In a liquid, thespaces between

particles are very small.When force is applied to a liquid, only a very

small decrease in volumetakes place. Thus they transmit force better

than gases.

In a gas, the particles arefar away from each other.When a force is applied

to a gas, the particles getpushed together and the

gas is compressed. This iswhy there is a delay in

using air filled systems – ittakes time to compress

the air.

Because gases are compressible and their volume can be changed,

they behave in very predictable ways……….

Gas Laws:

Boyle and

Charles/Gay-Lussac’s Laws

Boyle’s Law

In the 1700s, scientist Robert Boyle (see above) investigated the relationship between the volume of a gas and its pressure.

There are four different variables that can be changed when dealing with a sample of gas:

Pressure Volume

TemperatureAmount (number of particles)

Boyle kept the amount of gas and the temperature of the gas the same,

and examined the relationship between pressure and volume.

To do this, he used a piece of equipment called a manometer

A common, simple manometer consists of a U shaped tube of glass filled with some liquid.

Typically the liquid is mercury because of its high density.

Sealed end filled with gas

End opento atmosphere

Mercury

The more mercury added, the greater thepressure put on the sealed gas, whose

volume decreases.

He found that as the volume of a gas decreased, the pressure on the gas increased as long as the temperature of the gas and the amount of gas is kept

constant.

OR

P1V1=P2V2This is known as Boyle’s Law

A graph of pressure vs. volume ofa gas at a constant temperature

would look something like this

As you decrease the volume, you increase the pressure at a constant temperature.

If the temperature increases of a gas, the pressure of the gas will increase due to an increase in the number of collisions.

Practical Applications of Boyle’s Law: Who CaresAbout Gas Pressure and Volume?

When you inhale, your diaphragm moves downwards,

_______ the volume of your lungs.

This causes the pressure in your lungs to __________.

Since air moves from high to low pressure, air ________ your

lungs.

When you exhale, your diaphragm moves upwards, _______ the volume of your lungs. This cause the pressure in your lungs to _________, forcing air _______ of your lungs.

Breathing – fill in the blanks

While you are diving in the ocean, small bubbles of gas enter your circulatory system through small

ruptured veins in your lungs (caused by underwater pressure). If you ascend (return) to the surface of the water too quickly,

what will happen to the air bubbles and why?

Air in jugular veinseen during autopsy

Can result in an air embolism, in which the tiny bubbles of gas increase in volume as the underwater pressure

decreases, causing large, potentially fatal bubbles in the circulatory system.

We need oxygen to breathe. The concentration of oxygen in air is about 21%.

You go on a hiking trip up Mt. Kilimanjaro, which is 4600m tall. About half way to the top, you become short of breath, dizzy,

and develop a severe headache. You have altitude sickness because your body is not getting enough oxygen,

even though the oxygen level on the mountain is still 21%!

What is the cause? (hint: the atmospheric pressure at 3600m is half that at sea level)

You have a balloon filled with air to near-bursting at sea level.

If you were to take this balloon to the top of a very high mountain, ( 3000 m) what might happen and why?

(assume the temperature is kept the same)

While the SI unit for pressure is the pascal (Pa), when discussing gases, pressure is measured in atmospheres (atm).

Atmospheres are related to atmospheric pressure.

1 atm = 1.0123x105 Pa (101 325 Pa)

If 50mL of oxygen gas is compressed from 20 atm of pressure to 40 atm of pressure at constant temperature, what is the new volume? (remember P1V1=P2V2)

Complete Boyle’s Law Worksheet

P1V1=P2V2

P1 = 20atmV1 = 50mLP2 = 40atmV2 = x

Charles/Gay-Lussac’s Law

Relationship between temperature and volume of gases.

At constant pressure, as the temperature of a gas in increases, the volume also increases proportionally.

Jacques Charles Joseph Gay-Lussac

V1/T1=V2/T2

The Kelvin is the SI unit of temperature

Zero Kelvin is called “absolute zero” and is the coldest temperature possible0 K = -273.15ºC

To use the equation for Charles/Gay-Lussac’s Law, ALL TEMPERATURES MUST BE CONVERTED INTO KELVIN

To convert from Kelvin to Celsius: °C = K − 273.15

To convert from Celsius to Kelvin: K = °C + 273.15

Convert from Celsius to Kelvin:

50°C - 62 °C --46°C -480°C -

Convert from Kelvin to Celsius

273.15 K -51 K -0 K -560 K -

Hot Air Balloons: Air is heated inside the balloon, causing it to expand.

As the air expands, the balloon gets bigger, and eventually the balloon is able

to overcome gravity and rise.

What will happen if you put a balloon in a car on a hot day?

On a cold day? Why?

Standard hot air balloon

Make your own!

A gas occupies a volume of 100mL at 300K.

At what temperature will the gas have a volume of 200mL?

(remember V1/T1 = V2/T2)

V1 – 100mL 100/300 = 200/xT1 – 300 K x= 600 KV2 – 200mLT2 – x

Complete Charles/Gay-Lussac’s Law Worksheet

Cheat Sheet

Force - NArea - m2

Pressure - N/m2

1 N/m2 = 1 pascal (Pa)1000 Pa = 1 kilopascal (kPa)

1 kg = 9.81 N 1atm = 1.10132x105 Pa

P=F/AP1V1=P2V2

Kelvin to Celsius: °C = K − 273.15Celsius to Kelvin: K = °C + 273.15

V1/T1 = V2/T2