Transcript PHY205 Physics of Everyday Life

PHY205H1F Summer
Physics of Everyday Life
Class 5: Temperature, Heat
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Temperature
Heat
Specific Heat Capacity
Thermal Expansion
Thermal Expansion of
Water and Ice
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Conduction
Convection
Radiation
Newton’s Law of
Cooling
• Global Warming and
Greenhouse Effect
Temperature
• It’s one of the first
things we think
about in the
morning: What’s the
temperature like
outside?
• We can measure temperature with a
• Temperature has something to do with
of the atoms in a material
• A cold cup of coffee has a low amount of
• If you put it in the microwave, the microwave radiation
transfers energy to the coffee, increasing its
, which increases its temperature.
Temperature
Absolute temperature (in degrees
Kelvin) is proportional to the average
translational
per
particle in a substance.
• Gas—how fast the gas particles are bouncing
around
• Liquid—how fast particles slide and jiggle past
one another
• Solid—how fast particles move as they vibrate
and jiggle in place
[animated gif downloaded Jan.25 2013 from http://www.deanza.edu/faculty/mccauley/6a_site_images/Translational_motion-250.gif ]
Temperature
CHECK YOUR NEIGHBOR
There is twice as much molecular kinetic energy in 2 liters
of boiling water as in 1 liter of boiling water. Which will be
the same for both?
A.
B.
C.
D.
Temperature
Thermal energy
Both A and B.
Neither A nor B.
Temperature
CHECK YOUR NEIGHBOR
To say that body A has a higher temperature than body B
is to say that body A has more
A.
B.
C.
D.
internal energy.
mass.
kinetic energy per particle.
potential energy.
Temperature Scales
• To convert TF in Fahrenheit to
TC in Celsius:
• To convert TC in Celsius to TF
in Fahrenheit:
• Though Canada officially switched to Celsius in
1970, Fahrenheit is still sometimes used,
especially on kitchen appliances and in hospitals.
“Absolute” Temperature
• Note that temperature is due to the microscopic vibrations
of the atoms in a material (internal thermal energy).
• The coldest possible temperature would be when this
thermal energy dropped to zero: atoms STOPPED
vibrating.
• For all materials, this happens at −273 °C
• The Kelvin temperature scale is the same as Celsius, but
with a different zero point. Absolute zero has T = 0 K.
• To convert TC in Celsius to T in Kelvin:
• To convert T in Kelvin to TC in Celsius:
Temperature Conversion Example
• The commonly accepted average core body temperature
for a human (taken internally) is 98.6 degrees Fahrenheit.
• What is this in Celsius?
• What is this in Kelvin?
Temperature
CHECK YOUR NEIGHBOR
Is it possible for an object to have a negative Kelvin
temperature?
A.
B.
Yes
No
Temperature scales summary
• Celsius
– 0C for
– 100C for
point of water
point of water
• Fahrenheit
– 32F for
– 212F for
point of water
point of water
• Kelvin
– 273 K for
point of water
– 373 K for
point of water
– 0 at absolute zero, when atoms have
STOPPED vibrating!
– same size degrees as Celsius scale
Heat is a form of
energy
• Heat is when internal energy is transferred from one
thing to another due to a temperature difference
• Heat is internal energy in
.
Flow of internal energy
• Heat flows from a
substance to a
substance until thermal equilibrium is
reached
• Heat never flows unassisted from a low-temperature to a
high-temperature substance
[image downloaded Jan.25 2013 from http://summitjourneytowellness.blogspot.ca/2010/02/journey-to-wellness_5148.html ]
Heat
CHECK YOUR NEIGHBOR
If a red-hot thumbtack is immersed in warm water, the
direction of heat flow will be from the
A.
B.
C.
D.
warm water to the red-hot thumbtack.
red-hot thumbtack to the warm water.
There will be no heat flow.
Not enough information.
Quantity of heat
• Measured in joules or calories
• 4.18 joules of heat are required to change the
temperature of 1 gram of water by 1 Celsius
degree
• 4.18
=1
[photo by Scott Wallace, downloaded Jan.25 2013 from http://www.compadre.org/informal/index.cfm?Issue=11 ]
Quantity of Heat
Energy ratings of foods and fuels are
determined from energy released when they
are burned.
Unit of energy, the Calorie, is common for
foods.
• kilocalorie or 1000 calories called a
Calorie
• heat needed to change the temperature
of 1 kg of water by 1C
Specific heat capacity
• Defined as the quantity of heat required to
change the temperature of 1 kg of the substance
by 1 degree Celsius
• Like thermal
—resistance of a
substance to a change in temperature
[image downloaded Jan.25 2013 from http://video.planetgreen.discovery.com/home-garden/hot-water-bottle-bed.html ]
Quantity of Heat
CHECK YOUR NEIGHBOR
The same quantity of heat is added to different amounts of
water in two equal-size containers. The temperature of the
smaller amount of water
A.
B.
C.
D.
decreases more.
increases more.
does not change.
Not enough information.
Quantity of Heat
CHECK YOUR NEIGHBOR
You heat a half-cup of tea and its temperature rises by 4C.
How much will the temperature rise if you add the same
amount of heat to a full cup of tea?
A.
B.
C.
D.
0C
2C
4C
8C
Specific Heat Capacities of Common Materials
Material
Alcohol
Aluminum
Ammonia
Brass
Brick
Cement
Chloroform
Citron Oil
Diamond
Ether
Freon
Gasoline
Glass
Glycerine
Specific Heat
(kJ/kg °C)
2.3
0.87
1.1
0.38
0.9
1.55
1.05
1.84
0.52
2.21
0.91
2.22
0.84
2.43
Specific Heat
(kJ/kg °C)
Gold
0.13
Hydrochloric acid 3.14
Iodine
2.15
Lava
0.84
Mercury
0.14
Milk
3.93
Olive oil
1.97
Paper
1.34
Plastic (hard)
1.67
Salt
0.88
Steel
0.49
Water
4.18
Wood
2
Material
[solids data from http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html ]
[fluids data from http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html ]
The high specific heat capacity of water
• has higher capacity for storing energy than almost any
other substance.
• involves various ways that energy can be absorbed.
– increases the jiggling motion of molecules, which
raises the temperature
– increases the amount of
or
within the molecules, which becomes
potential energy and doesn’t raise temperature
– water molecules can absorb energy without
increasing translational kinetic energy
Specific Heat Capacity
CHECK YOUR NEIGHBOR
Which has the higher specific heat capacity, water or land?
A.
B.
C.
D.
Water
Land
Both of the above are the same.
None of the above.
• Due to rise in temperature of a substance, molecules
jiggle faster and
.
• Most substances
when heated and
when cooled.
– Railroad tracks laid on
winter days expand and
can buckle in hot summer.
– Warming metal lids on
glass jars under hot water
loosens the lid by more
expansion of the lid than
the jar.
[image downloaded Jan. 25 2013 from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/jarlid.html ]
Thermal expansion
Thermal expansion
Different substances expand at different rates.
Example:
• When the temperature of a bimetallic strip of
and
is increased, greater expansion occurs
for the
strip, which bends to turn a pointer, to
regulate a valve, or to close a switch.
Bimetallic strips are used in heaters, oven thermometers,
refrigerators, and electric toasters.
Thermal Expansion
• The metal in a metal ring expands.
• So the ring will get thicker and longer – more
circumference, so larger.
• All linear dimensions of an object expand if the
temperature
: outer diameter, and
inner diameter of ring.
• Also the inner diameter of a metal jar lid
when it is heated!
Thermal Expansion:
The exception to the rule
Expansion of water to ice
• When water becomes ice, it
! Ice has
open-structured crystals resulting from strong
bonds at certain angles that increase its volume.
This make ice
than water.
Thermal Expansion
CHECK YOUR NEIGHBOR
When a sample of 0C water is heated, it first
A.
B.
C.
D.
expands.
contracts.
remains unchanged.
Not enough information.
Conduction
• Transfer of
by electron and
molecular collisions within a substance,
especially a solid
Conduction
CHECK YOUR NEIGHBOR
If you hold one end of a metal bar against a piece
of ice, the end in your hand will soon become cold.
Does cold flow from the ice to your hand?
A.
B.
C.
D.
Yes
In some cases, yes
No
In some cases, no
Insulation
• Doesn’t prevent the flow of internal energy
•
the rate at which internal energy flows
Example: Rock wool or fiberglass between walls slows
the transfer of internal energy from a warm
house to a cool exterior in winter, and the
reverse in summer.
[image downloaded Feb.1 2013 from http://owenscorning.eu/en/products/residential-insulation/pink044.aspx ]
Liquid Nitrogen
• Molecular Nitrogen, N2,
composes 70% of the air we
breath
• Below -196°C ( Kelvin),
Nitrogen is liquid
• We have a big tank of liquid nitrogen (LN2)
at the North end of this building
• We use it to cool things to study materials
– Condensed Matter Physics research
• It is also good at parties to quickly cool
beer
Convection
• Transfer of
involving
only bulk motion of fluids
Examples:
• Visible shimmer of air above a
hot stove or above asphalt on a
hot day
• Visible shimmers in water due
to temperature difference
Convection
Reason warm air rises
• Warm air expands, becomes
, and is
buoyed upward.
• It rises until its density equals that of the
surrounding air.
Example: Smoke from a fire rises and blends with the
surrounding cool air.
[animation from http://bmsscience8209.edublogs.org/files/2010/10/Convection-1zb8331.gif ]
Convection
CHECK YOUR NEIGHBOR
Although warm air rises, why are mountaintops cold and
snow covered, while the valleys below are relatively warm
and green?
A.
B.
C.
D.
Warm air cools when rising.
There is a thick insulating blanket of air above valleys.
Both A and B.
None of the above.
[image of Mt. Kilamanjaro from http://www.bmycharity.com/beckysclimb ]
Cooling by
• Opposite to the warming that occurs when air is
compressed
Example: The “cloudy” region above
hot steam issuing from the nozzle of a
pressure cooker is cool to the touch (a
combination of air expansion and
mixing with cooler surrounding air).
Careful, the part at the nozzle that you
can’t see is steam—ouch!
Gas Cooling By Expansion
• Molecules in a region of expanding gas collide more often
with receding molecules than with approaching ones
• Their rebound speeds therefore tend to decrease, and, as
a result, the expanding gas cools.
• This phenomenon is used in
, which use
cooled coils to remove heat from a low temperature
environment
Radiation
• Transfer of energy via
such as light or infrared.
Radiation
CHECK YOUR NEIGHBOR
The surface of Earth loses energy to outer space due
mostly to
A.
B.
C.
D.
conduction.
convection.
radiation.
radioactivity.
Radiation
[image from http://www.yorku.ca/eye/spectru.htm ]
• Transferred energy
• Exists as electromagnetic waves ranging from
long (radio waves) to short wavelengths (X-rays)
• In
region, ranges from long waves
(red) to short waves (violet)
Wavelength and Frequency
Radiation
CHECK YOUR NEIGHBOR
Which body glows with electromagnetic waves?
A.
B.
C.
D.
Sun
Earth
Both A and B.
None of the above.
• Every object
radiates.
• From the Sun’s surface comes
light, or solar
radiation, which we can see.
• From the Earth’s surface comes terrestrial radiation
in the form of
waves below our
threshold of sight.
Image in
reflected,
visible light
Image in
emitted,
infrared
radiation
[image downloaded Feb.1 2013 from http://www.enjoyspace.com/en/editorial-cases/herschel-the-infrared-universe ]
Radiation
Blackbody Radiation
• Frequency of radiation is proportional to the
absolute
of the source ( f ~ T ).
Radiation
CHECK YOUR NEIGHBOR
Which is the better statement?
A.
B.
A black object absorbs energy well.
An object that absorbs energy well is black.
Radiation
CHECK YOUR NEIGHBOR
Which melts faster in sunshine—dirty snow or clean snow?
A.
B.
C.
D.
Dirty snow
Clean snow
Both A and B.
None of the above.
Reflection of radiant energy
• Any surface that reflects very little or no radiant
energy looks dark
Examples of dark objects: eye pupils, open ends
of pipes in a stack, open doorways or windows
of distant houses in the daytime
• Good reflectors are
poor
.
• Poor absorbers are
poor
.
• A white container will
radiate heat
than a black
container
Radiation
CHECK YOUR NEIGHBOR
A hot pizza placed in the snow is a net
A.
B.
D.
absorber.
emitter.
neither
Rate of cooling ~ T
• Rate is proportional to the temperature
difference, T, between the object and its
• Also applies to rate of warming
Examples:
• Warmer house leaks
internal energy to the
outside than a house that is less warm.
• Frozen food will warm
in a warm room
than in a cold room.
[ image downloaded Feb. 1 2013 from http://www.guardian.co.uk/lifeandstyle/2009/apr/04/space-solves ]
Newton’s law of cooling:
When to add cream to coffee…
• Psy likes to drink his coffee hot, and he likes cream in
his coffee. He buys it at Starbucks but does not want to
drink it until he gets to his home, which is a 5 minute
walk. To keep the coffee as hot as possible, should he
add the cream at Starbucks or at home?
A. Starbucks, before the 5-minute walk
B. At home, just before drinking
C. It doesn’t matter
When to add cream to coffee…
• My reasoning:
• When you add the cream to the coffee, it is going to
decrease its temperature by some amount; this is about
the same whether you do it at home or at Starbucks.
• When you carry the coffee through the environment for 5
minutes, it will be losing heat, since the coffee
temperature is higher than the environment.
• The rate of heat loss over this 5 minutes is proportional
to
• If you added the cream first,
is less while you carry it,
so it is going to lose
heat en route.
• In the end, this leads to
coffee when you
eventually drink it.
Newton’s Law of Cooling
CHECK YOUR NEIGHBOR
It is commonly thought that a can of beverage will cool
faster in the coldest part of a refrigerator. Knowledge of
Newton’s law of cooling
A.
B.
C.
D.
supports this knowledge.
shows this knowledge is false.
may or may not support this knowledge.
may or may not contradict this knowledge.
Greenhouse effect
• Named for a similar
effect in florists’ greenhouses
Greenhouse Gases
• The Earth’s atmosphere contains mostly
Nitrogen and Oxygen, both of which are
(non-absorbing) of both visible
and infrared radiation
• Certain gases are
for visible
radiation, but
for infrared radiation
• These are called “
”:
–
–
–
–
Carbon Dioxide
Water vapour
Methane
Nitrous oxide
[image from http://en.wikipedia.org/wiki/File:Exhaust_pipe_muffler.JPG ]
Greenhouse Effect on Earth
• Energy absorbed as
light from the Sun
• Part reradiated by Earth as longer-wavelength
radiation
• Terrestrial radiation absorbed by atmospheric
greenhouse gases and re-emitted back to Earth.
• Equilibrium temperature
determined by
of greenhouse gases in
the atmosphere
• More greenhouse gases
means higher
earth
Before Class 6 on Wednesday
• Please read Chapters 19 and 20, or at least watch
the 20-minute pre-class video for class 5
• Pre-class reading quiz on chapters 19 and 20 is due
Wednesday June 5 by 10:00am