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PHY205H1F Summer
Physics of Everyday Life
Class 4: Liquids & Gases
• Pressure
• Buoyancy in a Liquid
• Archimedes’ Principle
• What Makes an Object
Sink or Float
• Pascal’s Principle
• Surface Tension
• Capillarity
• The Atmosphere
• Atmospheric
Pressure
• The Barometer
• Boyle’s Law
• Buoyancy of Air
• Bernoulli’s Principle
• Plasma
Atoms • Atoms are the building blocks of all matter
• They are too small to be seen with visible light
• One gram of water has a volume of 1 cm3 and
contains more than 1023 atoms!
• 1023 =
[image from http://www.physics.utoronto.ca/~ifridman/ ]
• This is a scanning tunneling
microscope image of graphite
taken by Igor Fridman, a
graduate student in U of T
Physics
• The dots are individual
carbon
• Atomic structure is composed of:
• An atomic nucleus, which contains
• Orbiting
• The nucleus is composed of and
, which are in turn made of smaller
quarks
[Image retrieved Jan.10, 2013 from http://www.safetyoffice.uwaterloo.ca/hse/radiation/rad_sealed/matter/atom_structure.htm ]
• Protons have electric
charge:
• Electrons have electric
charge:
• All atoms have
the same number of
protons as electrons
The nucleus of an electrically neutral iron atom
contains 26 protons. How many electrons
are in this iron atom?
A. 52
B. 26
C. 24
D. 28
E. zero
Atoms
Check your neighbour
2
The Elements Atoms
• Refer to particles that make up a substance
Elemental substance
• Composed of only
– Lightest and most abundant is hydrogen.
• To date, about 115 elements are known.
– 90 occur in nature.
– Others produced in laboratory are unstable.
Words and can be used
interchangeably. [Image retrieved Feb.4 2013 from http://parisbreakfasts.blogspot.ca/2011/09/financiers.html ]
The atomic number of an element matches the
number of
A. protons in the nucleus of an atom.
B. electrons in a neutral atom.
C. Both of the above.
D. None of the above.
Atoms
Challenge Question: Do you know it?
Periodic Table of the Elements
© 2010 Pearson Education, Inc.
What is the approximate density of water?
A. 10−5 kg/m3
B. 0.01 kg/m3
C. 0.1 kg/m3
D. 1 kg/m3
E. 1000 kg/m3
Discussion question: What do you think?
3
Density
• Amount of mass per unit volume of a
material.
• Unit of density is kg/m3 or g/cm3.
• Example:
Density of water is kg/m3, or g/cm3.
volume
mass Density =
[Image retrieved Jan. 11, 2013 from http://www.amazon.com/Evian-Water-Liter-Pack/dp/B0041HVMU0 ]
If the volume of an object were to double, with no
change in mass, what would happen to its
density?
A. It would remain unchanged.
B. It would double.
C. It would decrease by a factor of two.
D. None of these.
Atoms
Check your neighbour
• Pressure is the per
unit that one object
exerts on another
Pressure = force area
Pressure
• In equation form:
• Depends on area over which
force is distributed
• Units: N/m2, or ( )
When you stand on one foot instead of two, the
force you exert on the floor is
A. less.
B. the same.
C. more.
Pressure
CHECK YOUR NEIGHBOR
4
When you stand on one foot instead of two, the
pressure you exert on the floor is
A. less.
B. the same.
C. more.
Pressure
CHECK YOUR NEIGHBOR Pressure in a Liquid • Force per unit area that a liquid exerts on an
object
• dependent and not dependent
Example: Swim twice as deep, then twice as much
weight of water above you produces twice as much
pressure on you.
Pressure in a Liquid
Effects of water pressure
• Acts to surfaces of a container
• Liquid spurts at right angles from a hole in the surface. – The greater the , the
greater the exiting speed.
Buoyancy in a Liquid
Buoyancy
• Apparent loss of weight of a submerged object
• Amount equals the weight of
5
• Displacement rule: A completely submerged object always displaces
a volume of liquid equal to .
Example: Place a stone in a container that is
brimful of water, and the amount of water
overflow equals the .
http://www.dairygoodness.ca/butter/butter-tips-tricks/how-to-measure-butter
• When butter is in hard, irregular shaped
pieces, fill a 2-cup liquid measuring cup with
water to the 1 cup mark.
• Add enough butter pieces until the level
reaches 1-1/4 cups.
• Drain off the water and you’re left with
of butter!
Archimedes’
Principle
• Discovered by Greek
scientist Archimedes in 250
BC.
• Relates buoyancy to
displaced liquid.
• States that an immersed body (completely or
partially) is buoyed up by a equal to the
weight of the fluid it displaces.
• Applies to gases and liquids.
[ image retrieved Jan.17 2013 from http://personal.maths.surrey.ac.uk/st/H.Bruin/MMath/archimedes.html ]
Demonstration Prediction
• A steel mass of 0.75 kg hangs from a spring
scale.
• When it is not accelerating, the spring scale
reads 7.5 N.
• If Harlow dips the mass into an open container
of water, then stops the motion and lets the
scale settle, what will be the reading on the
scale?
A. Less than 7.5 N, but not zero
B. More than 7.5 N
C. About 7.5 N
D. zero
6
Flotation
• Principle of flotation: – A floating object displaces a weight of fluid equal to its
own weight.
Example: A solid iron 1-ton block may displace 1/8 ton of water and sink. The same 1 ton of iron in a bowl shape displaces a greater volume of water—the greater buoyant force allows it to .
Ch.13 Problem 8
• Your friend of mass 100 kg can just barely float in fresh-
water. Calculate her approximate volume.
You place an object in a container
that is full to the brim with water on
a scale. The object floats, but
some water spills out. How does
the weight of the object compare
with the weight of the water
displaced?
A. Weight of object is greater than weight of water
displaced.
B. Weight of object is less than weight of water displaced.
C. Weight of object is equal to weight of water displaced.
D. There is not enough information to decide.
Flotation
CHECK YOUR NEIGHBOR
The Falkirk Wheel’s two caisson are brimful of water and
the , regardless of whether there are boats in
them. This makes rotation and lifting almost effortless.
[image from http://en.wikipedia.org/wiki/File:FalkirkWheelSide_2004_SeanMcClean.jpg ]
7
Archimedes’ Principle Denser fluids will exert a greater buoyant force on a
body than less dense fluids of the same volume.
Example: Objects will float in saltwater
(density = 1.03 g/cm3) than in freshwater
(density = 1.00 g/cm3).
[ image retrieved Jan.17, 2013 from http://famoustourisms.com/2011/05/dead-sea-enjoy-the-charm-and-sensation-swim-here/ ]
What Makes an Object Float or Sink?
Whether an object floats or sinks depends upon
the
• Weight of the .
• Weight of the .
The weight of the fluid displaced depends on
.
So what really counts is the weight of the object per
.
This is related to the average of the object.
[ image retrieved Jan.17 2013 from http://en.wikipedia.org/wiki/File:Kylpyankka.jpg ]
What sinks? What floats? 1. An object more dense
than the fluid in which it is
immersed will .
2. An object less dense
than the fluid in which it is
immersed will .
3. An object having a
density equal to the
density of the fluid in
which it is immersed will
.
[ from http://www.flickriver.com/photos/rhosoi/popular-interesting/ ]
[ from http://weeboopiper.wordpress.com/tag/pier-7/ ]
[ from http://www.123rf.com/photo_10758041_bluefin-tuna-thunnus-thynnus-saltwater-fish-underwater-blue-sea.html ]
Two solid blocks of identical size are submerged in
water. One block is lead and the other is aluminum.
Upon which is the buoyant force greater?
A. On the lead block
B. On the aluminum block
C. Same on both blocks
D. There is not enough information to decide.
What Makes an Object Float or Sink?
CHECK YOUR NEIGHBOR
8
Pascal’s principle:
• Discovered by Blaise Pascal, a scientist and theologian in the 17th century
• States that a change in pressure at any point in an enclosed fluid at rest is transmitted to all points in the fluid
• Applies to all —gases and liquids
Pascal’s Principle • Application in hydraulic press
Example:
– applied to the left piston is transmitted
to the right piston.
– A load on small piston (left) lifts a load of
on large piston (right).
Surface Tension
The tendency of the surface of a liquid to
and thus to behave like a stretched elastic
membrane.
Examples:
• Insects can stand on the
surface of water, even if they
are more dense than water.
• In a small droplet of water, the
surface will contract until it
forms the shape with the
smallest surface area for its
volume: a sphere
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[image from http://www.dailymail.co.uk/news/article-1371416/Photographer-Adam-Gormley-captures-ant-trapped-raindrop.html#axzz2K9hLsZGF ]
Surface Tension
• Surface tension is caused by molecular
.
• Beneath the surface, each molecule is
attracted in every direction by neighboring
molecules.
• A molecule on the surface of a liquid is
pulled only by neighbors on each side and
downward from below; there is no pull
.
• These molecular attractions tend to pull the
molecule from the surface the liquid,
causing surface tension.
9
Test on Wednesday during class time
• Location: EX310 (last name A-M), EX320 (last name N-Z)
• Test will begin promptly at 10 minutes after the hour and
will be 1 hour and 50 minutes long.
• Please bring a calculator, and, if you wish, a 8.5x11” aid
sheet upon which you may write anything you wish on
both sides
• Test will cover Hewitt chapters 2-5, 7, 8, 13 and 14, and
will include:
• 6 short-answer problems for which you must show your
reasoning
• 16 multiple choice questions – you fill in a bubble sheet
• Questions will be similar in style and level to the Exercises
and Problems at the end of the chapters in Hewitt
Tips for the 2 hour Test
• No phones / ipods etc allowed. You
will need a regular calculator, and a
watch could be handy as well!
• Time Management:
– Skim over the entire test from front
to back before you begin. Look for
problems that you have confidence
to solve first.
– If you start a problem but can’t
finish it, leave it, make a mark on
the edge of the paper beside it,
and come back to it after you have
solved all the easy problems.
• Bring your T-card or other photo ID,
as we will be collecting signatures
PHY205 Physics of Everyday Life
Chapter 14: Gases
• The Atmosphere
• Atmospheric Pressure
• The Barometer
• Boyle’s Law
• Buoyancy of Air
• Bernoulli’s Principle
• Plasma
[image from http://blog.testfreaks.com/information/thq-and-disney-pixars-up-the-videogame-preview/ ] © 2010 Pearson Education, Inc.
Atmospheric Pressure
The global average
sea-level pressure is
Pa, or 1 atm.
10
Atmospheric Pressure in this room What is the force of air pressure on the top of your
outstretched hand?
20 square inches = pounds!
Why don’t you feel that force pushing your hand
down?
What if all the air below your hand was removed (a
vacuum)?
p.s.i.N
pounds
inch
m
m
N000,100
2
2=
Atmospheric Pressure:
Discussion Question
• A suction cup sticks to a wall. It is
A. pulled to the wall by the vacuum.
B. pushed to the wall by the atmosphere.
C. both of these
D. neither of these
[image from http://www.flippersandfins.net/bettabreedingarticle.htm ]
Pressure and “Suction”
A fluid can only walls or objects; a
fluid cannot pull on a wall.
What we call “suction” is when the fluid on
one side has a higher pressure than the
fluid on the other side.
It is the pressure which creates a pushing
force into the lower pressure area (into the vacuum).
This is how we breath:
1. We expand our lung cavity, lowering the
inside.
2. The higher air pressure outside air into
our lungs.
In drinking soda or water through a straw, we
make use of
A. capillary action.
B. surface tension.
C. atmospheric pressure.
D. Bernoulli's principle.
E. none of these
Atmospheric Pressure
CHECK YOUR NEIGHBOR
11
• Pressure at the of a column of air reaching to the
top of the atmosphere is the same as the pressure at the
of a column of water m high.
• Consequence: The highest the atmosphere can push water
up into a pump is 10.3 m.
• Mechanical pumps that don’t depend on atmospheric
pressure don’t have the 10.3-m limit.
Atmospheric Pressure The Barometer • The barometer is a device to
measure .
• It consists of a tube
upside down in a dish filled with
mercury.
• The height of the mercury
column tells us the atmospheric
pressure.
• Atmospheric pressure
decreases with increasing
altitude, so it also measures
—an altimeter.
Why don’t barometers use water instead of
mercury?
A. Water cannot be used because it does not exert
pressure.
B. Water cannot be used because it sticks to the
glass.
C. Water can be used but the barometer will be too
tall.
D. None of the above.
Barometer
CHECK YOUR NEIGHBOR Boyle’s Law
• The product of pressure and volume of a given
mass of gas will always remain the same.
=
LARGE
Pressure Small
Volume
LARGE
Volume Small
Pressure
12
Discussion Question
• When a party balloon is compressed to
one-third its volume with no change in
temperature, what happens to the gas
pressure in the balloon?
A. It drops by a factor of 3
B. It drops by a factor of 2
C. It remains the same
D. It doubles
E. It triples
Buoyancy in Air
Archimedes’ principle applies to as well
as liquids.
• An object surrounded by is buoyed up by a
force equal to the weight of the displaced
[image downloaded Jan.24 2013 from http://saigon-hobby.com/products/2739-air-swimmer-nemo.aspx ]
Buoyancy Example: If you wish to support a mass of 80 kg
(one person) with a helium-filled balloon, how big should the
balloon be?
• What must change about the water so that
the amount coming in the hose equals the
amount leaving the hose?
A. The velocity must increase.
B. The velocity must decrease.
C. The pressure must increase.
D. The pressure must decrease.
In-class Discussion Question
• You put your thumb over the end
of the a hose, thereby reducing
the area through which water
can exit the hose.
13
• We study the steady flow of water from a
water tap, e.g., in your kitchen sink. The
jet of water
A. broadens as it falls.
B. narrows as it falls.
C. does not change its
cross-sectional shape.
D. slows before hitting the
bottom of the sink.
In-class Discussion Question
Bernoulli’s Law
• Consider an ideal fluid, flowing through a tube which narrows.
• It increases its . This means the kinetic energy per volume of the fluid will .
• How can this be? There must be a which does work on the fluid to speed it up.
• The force must come from a difference.
• Pressure must be in the region of increased fluid velocity.
Bernoulli’s Principle
• Discovered by Daniel Bernoulli, a 15th century
Swiss scientist
• States that where the speed of a fluid increases,
internal pressure in the fluid (and
vice versa)
• Applies to a smooth, steady flow
Bernoulli’s Principle
Streamlines
• Thin lines representing fluid motion
• Closer together, flow speed is and
pressure within the fluid is
• Wider, flow speed is and pressure within
the fluid is
14
Bernoulli’s demo, lift.
• If the air were flowing equally
above and below the beach-ball,
there would be no force on the ball
due to the Bernoulli effect.
• The ball would then fall due to its
own weight until the point where
most of the flow is over the top of
the ball, reducing the pressure
there.
• The ball sits at a point of stable
equilibrium, where these forces
balance.
Applications of Bernoulli’s principle
• Blow on the top surface of a paper and the paper
rises.
Reason: Pressure of the moving air is than the
atmospheric pressure beneath it.
• Wind blowing across a peaked roof can lift the roof off the house.
Reason: Pressure is as wind gains speed as it flows over the roof. The greater pressure inside the house lifts the roof up.
• When wind speeds up as it blows over the
top of a hill, what happens to the
atmospheric pressure over the hill?
A. It decreases
B. It increases
C. It stays the same
In-class Discussion Question
Plasma
• Plasma is the
state of matter
(after solids, liquids
and gases).
• A plasma is an . The atoms
that make it up are ionized, stripped of one
or more electrons, with a corresponding
number of free electrons.
15
• Which of the following is in a plasma
state?
A. Dry ice
B. A torch flame
C. Molten lava
D. Liquid hydrogen
E. Helium gas
In-class Discussion Question
Before Class 5 next Monday
• Please read Chapters 15 and 16, or at least watch
the 20-minute pre-class video for class 5
• Pre-class reading quiz on chapters 15 and 16 is due
Monday June 3 by 10:00am
• NOTE
• Midterm Test: Wednesday 1-3 in TBA
• Test will begin promptly at 1:10 and will be 1 hour and 50
minutes long.
• Please bring a calculator, and, if you wish, a 3x5 notecard
upon which you may write anything you wish on both sides
• Test will cover Hewitt chapters 2-5, 7, 8, 13 and 14, and will
include some multiple choice and some short-answer