PRESSURE
Pressure
Pressure 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.
EXAMPLES:
Walking
Peeling an apple
Hitting a nail
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
Incompressible
Fluids
A fluid is any substance that has the ability to flow, or take the
shape 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.
This fluid then puts pressure
The master cylinder contains
brake fluid, which is forced into the on the wheel cylinder pistons,
which push against a brake
wheel cylinders.
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 what
you 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, the
spaces between
particles are very small.
When force is applied
to a liquid, only a very
small decrease in volume
takes place. Thus they
transmit force better
than gases.
In a gas, the particles are
far away from each other.
When a force is applied
to a gas, the particles get
pushed together and the
gas is compressed. This is
why there is a delay in
using air filled systems – it
takes 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
Amount (number
of particles)
Volume
Temperature
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.
End open
to atmosphere
Sealed end
filled with gas
Mercury
The more mercury added, the greater the
pressure 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=P2V2
This is known as Boyle’s Law
A graph of pressure vs. volume of
a 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 Cares
About Gas Pressure and Volume?
Breathing – fill in the blanks
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.
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.
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?
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.
Air in jugular vein
seen during autopsy
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)
P1V1=P2V2
P1 = 20atm
V1 = 50mL
P2 = 40atm
V2 = x
Complete Boyle’s Law Worksheet
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.
Joseph Gay-Lussac
Jacques Charles
V1/T1=V2/T2
To use the equation for Charles/Gay-Lussac’s Law,
ALL TEMPERATURES MUST BE CONVERTED INTO KELVIN
The Kelvin is the SI unit of temperature
Zero Kelvin is called “absolute zero” and is the coldest temperature possible
0 K = -273.15ºC
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:
Convert from Kelvin to Celsius
50°C
62 °C
-46°C
480°C
273.15 K 51 K
0K
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.
Make
your own!
Standard
hot air
balloon
What will happen if you put a balloon in a car on a
hot day?
On a cold day? Why?
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
T1
V2
T2
–
–
–
–
100mL
300 K
200mL
x
100/300 = 200/x
x= 600 K
Complete Charles/Gay-Lussac’s Law Worksheet
Cheat Sheet
Force - N
Area - 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/A
P1V1=P2V2
Kelvin to Celsius: °C = K − 273.15
Celsius to Kelvin: K = °C + 273.15
V1/T1 = V2/T2