Download Ch. 12 notes

Chapter 12: Thermal Energy
Honors Physics Teacher Notes - Grothaus
Main Idea:
“Thermal Energy is related to the motion of an object’s particles and can be transferred and
transformed”
Only doing section one. Section two is covered in chemistry – don’t need to do it this year
Thermal Energy:
Look at a balloon. Is anything moving inside?
What is the energy of something moving called? What happens when the movement
increases (velocity)?
1
2
Kinetic energy – energy of movement. Formula: KE  mv
2
What happens as v increases? What happens if v=0?
Kinetic energy of moving molecules increases as the velocity of the particles increases. When
we are talking about particles within an object or in a gas, we can feel the kinetic energy
as thermal energy (not “heat”). More movement, more kinetic energy = more thermal
energy.
However, when we are talking temperature of a substance, it is affected by the average
kinetic energy of the particles!
Why average??
Temperature is directly proportional to the average kinetic energy of a substance.
Two substances that have the same temperature have the same average
kinetic energy.
Bucket of hot water has more KE than a glass of water at a higher
temperature, but the average KE of the water in the glass is higher.
Look at our balloons. If we blow up two the same size, what would happen if we stuck one in
the freezer for 15 minutes while someone holds the other in their hands, or even puts it
over the heat lamp for 15 minutes?
Gases vs. Solids – how are they different?
Atoms aren’t free to move around in a solid, but higher kinetic energy still means more thermal
energy. It gets hotter! Can a solid also get bigger?
Train tracks, concrete slabs, etc.
Demo – soda can on hot plate
“When matter gets warmer, the atoms or molecules in the matter move faster” (take up more
room or less room?)
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Question: What happens when you leave an open soda on the counter?
Soda can demo, balloon on flask demo
Temperature:
“Temperature is the quantity that tells how hot or cold something is compared with a
standard”. Not one of your vocabulary words, but should be.
Temperature depends only on the average KE of the particles in an object.
Not – number of particles
Thermal energy, however depends on both.
Same temperature, but the bigger muffin has 10 times the number of particles, so
it has 10 times the thermal energy.
Small glass of hot water vs. large bucket of cold water?
Another example – total points vs. average grade in different classes
How do you normally measure temperature?
What are the three scales we normally use?
Three different scales:
1. Fahrenheit – United States only
32 degrees - water freezes and 212 degrees - water boils
2. Celsius – makes sense
0 - water freezes and 100 - water boils
Most used in the world
20º is room temperature, body temp is 37º
(1741 – by Anders Celsius - based on the properties of water)
3. Kelvin – scientific temperature scale -- SI unit for temperature
Its degrees are the same size as the Celsius degrees and called Kelvins
0 is the lowest possible temperature – called absolute zero
N
At absolute zero – no movement of atoms and molecules – zero KE
-273 degrees C
Explanation of absolute zero:
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
https://www.youtube.com/watch?v=TNUDBdv3jWI
Which of the above scales has the smallest degree size?
Equilibrium and Thermometers: (begin water boiling)
How do you measure your body heat? How do you measure the temperature in a room?
We spent a lot of time last semester talking about equilibrium. Most things want to be in
equilibrium. Same with thermal energy. – Thermal Equilibrium
In a gas, the molecules spread out – see p. 322, figure 4
In a liquid, same thing – remember that ‘fluids” in physics means both gases and liquids.
In a solid? - how quick depends…..
A thermometer works because the fluid inside (mercury, colored alcohol) reaches thermal
equilibrium with it’s surroundings.
Technically, a thermometer does not measure the temperature of the air or your body, it
measures it’s own temperature as it reaches thermal equilibrium with its surroundings.
Demos
Are they really at different temperatures?
Take a cake out of the oven – pan vs. cake – which is hotter?
Heat
When you touch a hot stove, energy transfers into your hand because the stove is
hotter than your hand. Energy always goes from the warmer substance to
the cooler substance. That energy that transfers from one thing to another
because of a difference in temperature is called heat.
Things do not technically contain heat.
Matter contains energy in several forms, but it does not contain heat.
Heat is energy in transit, moving from something with a higher temperature to
something with a lower temperature.
When two objects are in contact and heat flows from one to another, they are in thermal
contact.
Always: When two substances of different temperatures are in thermal
contact, heat flows from the higher-temperature substance to the
lower temperature substance.
Missed things from last time? :
Explanation of absolute zero:
https://www.youtube.com/watch?v=TNUDBdv3jWI
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Veritasium – misconceptions about heat:
https://www.youtube.com/watch?v=vqDbMEdLiCs
Minute physics:
https://www.youtube.com/watch?v=yXT012us9ng
If time – How hot can things get - https://www.youtube.com/watch?v=4fuHzC9aTik
Why does touching a hot object cause damage in your skin?
It's a matter of energy transfer. Temperature can be thought of as a measure of the average kinetic energy of the molecules in a substance. When a hot object (like a
stove eye) comes into contact with a cooler object (like a human hand), the faster moving molecules in the stove eye bump into (and tranfer some of their kinetic energy
into) the molecules in your hand. This causes the molecules in your hand to move faster, creating a rise in the temperature of your hand. This is called thermal conduction.
If molecules in your hand received enough energy, they can be altered. In the case of proteins and enzymes, certain temperature ranges are required for them to retain
their functional shape. Heat them up too much and they get "bent out of shape" and lose their function. That's obviously not a good thing for your body. This occurs at
relatively lower temperatures than the following effects.
At higher temperatures, the water in your skin may boil. As water boils, it expands greatly in volume. Cells are mostly water, so if the water within them boils, they will
burst.
Even higher up, certain substances within your body (such as sugars, fats, and proteins) will begin to burn. Burning a substance greatly alters its chemical structure. When
you burn an organic substance, you typically get water vapor and carbon dioxide (among other gases) as a by-product. A human being obviously cannot survive in the
form of gas.
So in conclusion, high temperatures cause injury by altering the chemicals in your body, either by changing their shape (like enzymes) changing their phase (like water), or
changing their composition (like carbohydrates and lipids).
It's also possible that certain connective substances in your tissues like collagen might be melted by high temperatures, but I'm less sure of this.
Worksheet on temperature scales?
Three different types of heat transfer:
Heat can be transferred by conduction, by convection, and by radiation.
Conduction
Demo with hot water The heat transfers material by conduction.
Conduction of heat is the transfer of energy within materials and between
materials that are in direct contact (thermal contact).
Materials that conduct heat well are known as heat conductors. Metals are the best
conductors. Silver is the best, then copper, aluminum and iron. (ice melting in
video)
How does it work? Remember that the higher the temperature, the faster the atoms or
molecules move. As they move faster, they start to collide with the atoms or
molecules that are next to them. Faster moving molecules mean higher
temperature, which then causes them to collide with the next molecules, etc.
In conduction, collisions between particles transfer thermal energy, without any
overall transfer of matter.
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Conductors
Metals are good conductors because they have atoms with “loose” outer electrons. They
are good conductors of heat AND electricity. The looser the outer electrons, the
better the conductor.
Wood on the other hand, doesn’t conduct heat very well. Poor conductors are called
insulators. Other examples are wool, straw, paper and cork. (Balloon?)
Question: 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?
Insulators:
Liquids and gases are generally good insulators. An insulator is any material
that is a poor conductor of heat and delays the transfer of heat.
Air is a very good insulator.
Porous materials with small air spaces are good insulators: wool, fur and feathers
Birds stay warm in the cold because they fluff their feathers and cause air
pockets between the feathers and the bird’s skin.
Snowflakes have a lot of air caught in their crystals and act as good insulators.
Like a blanket, warm clothes, insulation in a house, etc., snow does not a
source of heat, it just keeps the heat from escaping too fast.
Question: What adaptations do polar bears have to be able to live up in the arctic?
Blubber, thick hide, black skin, fur is hollow and transparent
They actually tend to over heat – slow movement, swimming
Question: If you are driving around in the north during winter and you see some houses with lots of icicles
hanging from the eaves, and others with very few icicles. Explain what is going on with respect to insulation.
Heat is energy. Cold is not – merely the absence of heat!
No insulation is perfect. Eventually the heat will get out, but just a lot slower!
https://www.youtube.com/watch?v=eDAQPxR9-YM
sleek geeks – physics of firewalking
https://www.youtube.com/watch?v=7dgpsI1MdQI
mythbusters – Adam walks on hot coals
What is fire?:
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
https://www.youtube.com/watch?v=1pfqIcSydgE
Fire in space: https://www.youtube.com/watch?v=9zdD7lfB0Fs
Convection
Conduction involves the transfer of energy from molecule to molecule. Energy moves,
but the molecules do not
Convection is a means of heat transfer by movement of the heated substance
itself, such as by currents in a fluid. (molecules do move)
Only happens in fluids – not solids
You’ve heard that warm air rises. Air above a stove will rise and heat the air above.
In convection, heat is transferred by movement of the hotter substance from one
place to another.
When you heat a pot of water, the water expands, becomes less dense and rises. It rises
in the same way that wood floats on water. The less dense material gets pushed
up by the denser surrounding fluid. Cooler water then moves to the bottom, heats
up and floats up, etc.
Air does the same thing. Picture on page 434 – Different parts of the surface of the Earth
heat at different rates. Water can absorb a lot of heat. Land gets warm faster
during the day, but then cools down faster after the sun goes down. This
difference causes air currents. (Camp fires on shore – which way will the fire
lean?)
Warm air expands and then rises. As it rises, it cools down
This temperature difference on different surfaces of the Earth is the major cause of wind
patterns. Convection in action!
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Atmospheric turbulence is caused by convection of gases in the atmosphere.
Thunderstorms and hurricanes are excellent examples of large-scale atmospheric
convection. Ocean currents that move water and materials over large distances also result
from convection!
Natural Convection in a fish tank:
https://www.youtube.com/watch?v=eAHxhYBaEFo
Demo: Candle flame: hand above vs. fingers to the side (like about to pinch it)
Why does this work?
Convection causes the hot air to rise – not go to the side
Conduction would bring the heat to your fingers, but air is a good insulator!
Important – why do shore breezes patterns change from day to night? See graphic below.
Radiation
How does the sun warm Earth’s surface?
Conduction?
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Why not?
Convection?
Why not?
The sun’s heat is transmitted by radiation.
Radiation is energy transmitted by electromagnetic waves.
In radiation, heat is transmitted in the form of radiant energy or electromagnetic
waves.
Radiant energy includes radio waves, microwaves, infrared radiation (such as heat from
a fireplace), visible light, ultraviolet radiation, X-rays and gamma rays.
See poster on the wall.
Will study the wavelength / frequency relationship in the next unit.
Question: Why is it hotter at the equator than north or south of it?
Flashlight on paper – horizontal vs. at an angle. Which one will get hotter?
Candle flame: hand above vs. fingers to the side (like about to pinch it)
Emission of Radiant Energy
We absorb radiant energy from the sun, right?
What would happen if we absorbed the energy without ever getting rid of some of it?
All substances continuously emit radiant energy in a mixture of wavelengths.
Low temperature objects (cooler) emit long waves. (low energy)
Higher temperature objects (warmer) emit waves with shorter wavelengths (high energy)
Every day temperatures emit long, lazy wavelengths – in between radio and light waves
Shorter-wavelength infrared waves absorbed by our skin produce the sensation of heat.
Heat radiation is infrared radiation.
When we speak about heat radiation, we are speaking of infrared radiation.
Infrared thermometers – like the aural (ear) thermometers!
People with a surface temperature of 310 K, emit light in the low-frequency infrared part
of the spectrum – can’t see each other in the dark
If an object is hot enough, some of the radiant energy it emits is in the range of
visible light. At about 500º C, an object begins to emit red light (the longest
waves that we can see)
Higher temps – yellow light (like sunlight), about 1500ºC, all colors – so
“white hot”
Stars: Blue-hot stars are hotter than white-hot stars and red-hot is less hot
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
“stellar radiation” - see graphic (our sun - 5500ºC)
Below: Star Color Examples in the Fall / Winter
http://mail.colonial.net/~hkaiter/life_cycle_of_a_star.htm
Remember that the hotter the star – the shorter it’s life cycle
Earth – radiant energy emitted by Earth is called terrestrial radiation
Below our threshold of sight.
Radiant energy is partly reflected and partly absorbed – when absorbed, it raises the
thermal energy of the object (heats things up)
Specific Heat:
Some materials are easier to warm up than others.
Beach sand vs. ocean water
When an object is heated, its thermal energy increases and its temperature can increase.
The amount of the increase in temperature depends on the size of the object and its
composition.
The specific heat of a material is the amount of energy that must be added to a unit mass
of the material to raise its temperature by one temperature unit. In SI units,
specific heat (C) is measured in J/(kgK).
Can also be measured in J/(kgC), so watch units if you look at different tables.
Change in temperature: T (degrees C or K)
Heat (amt. of thermal energy transferred to the object): Q (joules)
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Mass: m (kg)
Q  mCT  mC(T f Ti )
Units here are in grams, not kg. Always look at units given any table.
Why is the specific heat of water so high compared to other materials?
“Water has a high specific heat index—it absorbs a lot of heat before it begins to get hot. This is why water is
valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps
regulate the rate at which air changes temperature, which is why the temperature change between seasons is
gradual rather than sudden, especially near the oceans.”
Concept check:
1. Water has an unusually high specific heat capacity. Which one of the following statements logically
follows from this fact?
a. Compared to other substances, hot water causes severe burns because it is a good conductor
of heat.
b. Compared to other substances, water will quickly warm up to high temperatures when heated.
c. Compared to other substances, it takes a considerable amount of heat for a sample of water to
change its temperature by a small amount.
2. Explain why large bodies of water such as Lake Michigan can be quite chilly in early July despite the
outdoor air temperatures being near or above 90°F (32°C).
Textbook problems: p. 326: 1 – 3; p. 342: 42, 43, 44
Bellwork:
In your lab notebook:
p. 329: 9, 11, 13, 16, 17, 18
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Newton’s Law of Cooling
Yes, another law attributed to Newton!
Common sense!
This one states that the rate of cooling of an object – whether by conduction,
convection or radiation – is approximately proportional to the temperature
difference ΔT between the object and its surroundings.
Take a cup of warm water and a cup of boiling water. Which one is going to cool faster?
If you take cookies out of the oven and you want it to cool as fast as possible where
would you put it?
Same holds true for heating. Ice will melt a lot faster in a warm room than a cool room.
Greenhouse Effect
The Greenhouse effect is named after the way people use greenhouses to raise the
temperature to be able to grow plants even in winter – florists, farmers, etc.
Today, the greenhouse effect is mostly referred to when talking about the warming
of a planet’s surface due to the trapping of radiation by the planet’s
atmosphere.
Causes of the Greenhouse Effect - 2 things to look at to understand
1. Remember that higher temperature objects radiate shorter (higher energy)
wavelengths and that cooler objects radiate longer (lower energy)
wavelengths.
2. Glass and air is transparent (lets light through) – but only to certain
wavelengths of light.
Air – transparent to both infrared (longer) waves and visible (shorter)
waves.
However, if there is an excess of carbon dioxide and water vapor, it
then absorbs the infrared waves (doesn’t let it through).
Glass – transparent to visible light waves but always absorbs infrared
waves.
Why does your car get so hot when it’s sunny out, even on a sunny winter day?
The windshield (glass) absorbs the infrared rays, but lets in the shorter
visible light rays (otherwise you couldn’t see out!)
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Some light will be reflected back out, but a lot of the light gets absorbed by
the seats, dashboard, etc. in the car. The inside of the car heats up.
These things then emit radiation back out, but because the heat in the car
isn’t as hot as the sun, this radiation has much shorter wavelengths
and can’t get back out of the glass.
Most of the re-radiated energy stays in the car making the inside even
warmer.
As hot as the interior gets, it won’t radiate waves that will be able to get
through the glass.
https://www.youtube.com/watch?v=ZzCA60WnoMk
NASA video simply explaining greenhouse effect
Same thing happens in the atmosphere
It is transparent to solar radiation – about 30% gets immediately reflected
back out to space, but 70% gets into our atmosphere
What gets in radiates the Earth and everything on it. These things then reradiate (terrestrial radiation) in longer wavelengths.
Atmospheric gases (water vapor, carbon dioxide and methane – and
clouds!) will then absorb and re-emit much of this terrestrial radiation
back down to Earth. If not, Earth would be about 30ºC cooler!
Best slideshow to review: http://slideplayer.com/slide/4617567/
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Honors Physics Teacher Notes - Grothaus
Chapter 12: Thermal Energy
Measuring Specific Heat:
Calorimeter – will be doing experiment with this in Chemistry
Closed system – want temperature changes to only be because of the reaction inside.
Known: mass and temperature of the water (cold); mass and temperature of a hot substance
Measured – after thermal equilibrium occurs, then measure the temperature of the water
If the system is closed, then no loss of energy occurs
That means that the amount of thermal energy that is lost from the hot substance has to equal the amount
of thermal energy that the water gains.
E A  EB
Note: a gain of thermal energy means an increase in temp. (positive)
a loss of thermal energy means a decrease in temp. (negative)
Since it is a closed system and there is no other energy changing (no work being done), then
Q  E  mCT  mC(Tf Ti )
Combining the two equations:
mAC A (Tf TA )  mBC B (Tf TB )
or:
CA 
mBC B (Tf TB )
mA (Tf TA )
How to solve for the final temperature?
Practice problems: on a separate piece of paper: p. 328: 1, 3; p. 342: 45, 46
Answers to concept checks:
Answer: C
A substance with a high specific heat capacity is a substance that requires a relative large quantity of heat to cause a small temperature
change. Because of this, water does not change its temperature as rapidly as other substances that are heated in the same manner; choice B
does not logically follow. Specific heat capacity should not be confused with thermal conductivity. Thermal conductivity is the measure of the
ability of a substance to conduct heat; choice A has to do with thermal conductivity.
Lake Michigan is a body of water with a large m value and a large C value. It would take a lot of solar energy absorption to increase its
temperature from the cold wintry temperatures to the higher summertime temperatures. It may take a couple of months of summer before the
heating of the large mass of water is "complete."
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