Transcript Pressure - Island Physics

Solids, Liquids and Gases
Syllabus
Title
Lesson 01: Density
Lesson 02: Brownian Motion
Lesson 03: Pressure, Temperature and Volume
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Solids, Liquids and Gases
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Density
08/04/2015
Aims:
• To experimentally find the density of different materials
• To calculate the density of different materials
Starter:
Which has more mass honey or beer?
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Density
 Definition:
Density is a measure of how much mass of a
substance is contained in a fixed volume.
 Symbol:
 ρ (rho)
 Equation:
 ρ = m or m = V ρ
V
m
 Mnemonic:
 my violent rhino
 Units:
 kg/m3
ρ

V
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What is the Mass of Air in the Room?
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make a guess
Then Calculate:
 density of air = 1.2 kg/m3
 If all the air the room was condensed into a liquid it
would form a layer about 5mm deep
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Measuring Density: Solids
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Measure the mass
Measure the volume
 Using a ruler (if regular shape)
 Using Eureka Jars (by displacement of water)
 In a measuring cylinder (by displacement of
water)
Use ρ = m
V
Archimedes
287 – 212 BC
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Measuring Density: Liquids
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Measure the mass
Measure the volume using a measuring cylinder
use ρ = m
V
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Measuring Density
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Measure the density of the materials available
Don’t forget water
Enter your results into a table and calculate the density
Material
Mass
Volume
Density
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Density
recap
Aims:
• To experimentally find the density of different materials
• To calculate the density of different materials
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Brownian Motion
08/04/2015
Aims:
• To describe the molecular structure of solids, liquids
and gases
• To explain brownian motion
Starter:
• draw the structure of a solid, liquid and gas in the back of
your books
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Solid
• The atoms/molecules vibrate
• atoms/molecules cannot move past each other
• atoms/molecules are touching
Melt
freeze
Liquid
• The molecules move around randomly
• Molecules are touching
Boil/
evaporate
sublime
deposit
condense
Gas
• The molecules move around randomly
• There is a lot of space between the particles
Handout
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Brownian Motion
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First observed in 1785
by Jan Ingenhauz
If a small visible particle is observed in a fluid it
will slowly move without any visible force being
applied.
Examples: Smoke particles in air
Pollen in water
Molecules in the air collide with the smoke
particles randomly and it is moving due to the
continually altering resultant force.
Robert Brown
(1773–1858)
Movie Brownian Motion
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Demo Brownian Motion
Brownian Motion (Video Clip 1 2)
Albert Einstein
(1879–1955)
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Demo Diffusion
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Bromine in Air
Coloured Water in Gas Jars
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Diffusion
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Diffusion
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When particles are released in an enclosed area (i.e. smoke) they
slowly disperse throughout the fluid (i.e. air).
This is due to collisions between molecules moving the particles
of the smoke in a random direction.
Movie Diffusion
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What is Pressure?
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What is Pressure?
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What is Pressure?
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Pressure
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Definition:
 molecules in a gas have a random motion
 They hit a surface
 When they hit they exert a force.
 This force causes the pressure on a surface
 Can increase the pressure by
 more particles (more impacts, therefore more force)
 Faster particles (harder impacts, therefore more force)
 Smaller area
Symbol: p
Equation: p = F or
F=pA
F
A
Mnemonic: Put Foot on Ant or Fluffy Pink Animals
Units:
 Pascals, Pa
p
A
 N/m2
Note: Pressure is the same at all points in a fluid Stopwatch Graph Home
Questions
1) Calculate the pressure exerted by a 1000N elephant when standing
on the floor if his feet have a total area of 1m2.
2) A brick is rested on a surface. The brick has an area of 2cm2. Its
weight is 10N. Calculate the pressure.
3) 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?
4) Joe sits on a balloon. Joe weighs 500 N and the area of the
balloon being sat on is 20 cm2. The balloon will pop if the pressure
is greater than 20 N/cm2. Does it pop?
5) Jill weighs 600N, the femur bone in her thigh has an area of 4
cm2. What is the pressure in her thigh bone? She jumps from the
first floor window effectively doubling her weight. Her thigh
bones can each stand a pressure of 200 N/cm2. Will her bones
withstand the impact?
6) Why is a sharp knife easier to use than a blunt knife
7) Why would a woman with stilettos damage a floor more than an
elephant would?
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Pressure Difference
Separate Science
Demo:
 What will happen if the bottle of water is filled with water?
 Why?
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Pressure and Depth
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Separate Science
Pressure increases with depth
Pressure is the same in all directions at a certain depth
pressure difference = height × density × g
p=h×ρ×g
So the pressure difference in water (ρ=1000 kg/m3 is a lot greater
than the pressure difference in the atmosphere ρ=1.2kg/m3)
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Brownian Motion
recap
Aims:
• To describe the molecular structure of solids, liquids
and gases
• To explain brownian motion
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Pressure Temperature and Volume
08/04/2015
Aims:
• To explain relationships between temperature, volume and pressure
of gases
Starter:
Copy and complete the table
Symbol
Units
Unit Symbol
Temperature
T
Celsius
oC
Temperature
T
Kelvin
K
kilogram
m
A
Volume
Density
newton
p
J
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Heating Gases
•
What happens to the kinetic energy of the particles when a gas is heated?
•
What happens to the pressure?
gas gets
hotter
more kinetic
energy
more collisions at
greater speed
more pressure
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Heating Gases
If a gas is heated:
1.
Molecules have more kinetic energy (KE)
2.
Move faster
3.
More collisions with the side at greater speed
4.
More Pressure
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Heating Gases
What sort of graph would you get if you plotted ‘temperature (TC)’
against ‘the average kinetic energy (KEave) of the particles’.
KEave
-273
TC
Movie Absolute Zero
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Heating Gases
If you increase the temperature (in Kelvin) what happens to the
kinetic energy?
KE
Tk
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Absolute Zero
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If a gas is cooled the opposite happens
If it is cooled enough the particles stop moving
This happens at Absolute Zero (-273 °C)
The Kelvin Scale of Temperature starts at absolute zero
1 K ≡ °C
Kelvin
0
Centigrade
-
273
180
-93
273
0
373
100
1300
1027
Movie Thermometers
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Pressure Law
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Separate Science
Sometimes called the GayLussac Law, but normally the
pressure law as he has another
one named after him.
Gay Lussac published both the
pressure and Charles’ Law.
Also established the method of
measuring alcohol in drinks
(ABV)
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Pressure Law: Experiment
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Separate Science
Place a flask of air into a water
bath.
Measure the pressure of the
flask using a pressure sensor or
Bourdon (pressure) Gauge
Change the temperature of the
water bath in 5 K intervals.
Volume constant
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Pressure Law
Separate Science
• Definition:
• For a fixed mass of gas at a constant volume, the pressure is
directly proportional to its absolute temperature
• Equation:
• p/T = constant
• Where:
•p is pressure in Pa
•T is temperature in K
• Notes:
• Only works if temperature is in kelvin
• Pa ≡ Nm-2
p (Pa)
• p1/T1 = p2/T2
T (K)
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Worked Examples
Separate Science
• We normally use the pressure law to compare pressures before and
after.
• As p/T = Constant then
p/T before = p/T after
• Hence it is often written as:
• p1/T1 = p2/T2
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Question
Separate Science
• A solid sphere containing helium is at a pressure of 1 x 105 Pa and is
then cooled from 400 K to 300 K.
• What is the final pressure?
p1
= 1 x 105 Pa
T1
= 400 K
T2
= 300 K
p1/T1
= p2/T2
1 x 105 = p2
400
300
p2
= 1 x 105.300
400
= 7.5 x 104 Pa
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Questions
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4.
Separate Science
An airplane contains air at 9 x 104 Pa. During flight the temperature
increases from 260 K to 290K. Assuming the volume of air does not
alter, what is the change in pressure?
A gas cylinder containing explosive hydrogen gas has a pressure of 5 x
106 Pa at a temperature of 300 K. The cylinder can withstand a
pressure of 6 x 106 Pa before it bursts, causing a building-flattening
explosion. What is the maximum temperature the cylinder can
withstand before bursting?
High pressure steam is used in a power station. The steam’s initial
pressure is 5 x 105 Pa. After going through the turbine its pressure is
1 x 105 Pa. The steam’s final temperature is 150 C. What was its
starting temperature assuming the volume of air is constant?
A deep sea diver holds her breath as she surfaces. The pressure in
her lungs is 4 x 105 Pa 30 m down and the temperature is 35 C.
Atmospheric Pressure = 1 x 105 Pa. If she managed to keep the volume
of air constant what temperature would the air in her lungs be when
she surfaced?
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Robert Boyle
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Studied the cold and air
Modern scientist he relied on
experiments to prove his
theories
A Chemist and Physicist not an
alchemist and a magician
Robert Boyle
1627 - 1691
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Boyles Law
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A glass tube is filled with
oil except for air at the
top.
The pressure of the oil is
increased shrinking the air
column.
The gas is left to cool.
The pressure is dropped in
stages and the volume
measured once it has
cooled down.
NB Temperature constant
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Boyle’s Law
• Definition:
• For a fixed mass of gas at a constant temperature, the
volume is inversely proportional to the pressure.
• Equation:
• p1V1 = p2V2
V (m3)
• pV = constant
• Where:
p (Pa)
•V is volume in m3
•p is pressure in Pa
• Notes:
• Mass must be the same (i.e. no particles escape)
• T must be constant
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Questions
1.
2.
3.
4.
In the upper atmosphere a weather balloon has a volume of
50 m3. At ground level (1 x 105 Pa pressure) the volume is
20 m3. What is the pressure in the upper atmosphere
A child takes a 0.01 m3 balloon from atmospheric pressure
1 x 105 Pa into an airplane containing air at 9 x 104 Pa.
Assuming the temperature does not change what is the
volume of her balloon?
A climber carries a polystyrene flask up a mountain. The
volume of the flask is 0.0002 m3 at sea level. The flask
increases in size to 0.0003 m3. Assuming the temperature
does not change. What is the pressure at the top of the
mountain.
A diver is 30 m below the surface where the pressure is 4
x 105 Pa. The volume of their lungs is 0.005 m3. What is
the volume of air they could breathe out at atmospheric
pressure? (assuming the temperature is constant).
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Exam Questions
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Explain the Following
1)
2)
3)
4)
How do hot air balloons work?
How can a small barbeque tank hold enough propane to cook with
all summer long?
Why do helium balloons float
What will effect the time it takes to smell the perfume of a
person who walks past you?
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