Kinetic
energy is energy of
motion. Some examples (both
large and microscopic scale) are mentioned and
sketched above. This is a relatively easy to
visualize and understand form of energy.
Radiant energy This is probably
the most important form of energy that we'll be dealing
with. Electromagnetic radiation is another
name for radiant energy. Sunlight is an example of
radiant energy. We'll start using the term
electromagnetic radiation in class on Thursday.
Radiant energy is something that we can see and feel (you
feel warm when you stand in sunlight). Something that
is not quite so obvious is that everyone in the classroom is
emitting radiant energy. This is infrared light, an
invisible form of radiant energy. And actually it's
not just the people; the walls, ceiling, floor and even the
air in the classroom are also emitting infrared light.
We can't see it and, because it's there all the time, I'm
not sure whether we can feel it or not.
2. We'll get the topic of energy
units out of the way
Joules
are the units of energy that you would probably
encounter in a physics class. Your
electric bill shows the amount of energy
that you have used in a month's time, the
units are kilowatt-hours.
We'll usually be using calories as units of
energy. 1 calorie is the energy need to warm 1
gram of water 1 C (there are about 5 grams of water
in a teaspoon).
3.
Temperature provides a measure of
the average kinetic
energy of the atoms or molecules in a
material.
The atoms or
molecules inside the warmer object will be moving
more rapidly (they'll be moving freely in a gas,
just "jiggling" around while still bonded to each
other in a solid). Since kinetic
energy is energy of motion, temperature gives you
an idea of the average speed of the moving atoms
or molecules in a material.
You need to be careful what temperature
scale you use when using temperature as a measure
of average kinetic energy. You must use the
Kelvin temperature scale because it does not go
below zero (0 K is known as absolute zero). The
smallest kinetic energy you can have is zero
kinetic energy. There is no such thing as
negative kinetic energy.
There
are three temperature scales that we might
have occasion to use in this class.
They're shown below. There are two
temperatures that you should try to remember
for each scale.
The boiling and freezing points of
water on both the Celsius and the Fahrenheit scales (the
freezing point of water and the melting point of ice are
the same). Remember that the Kelvin scale doesn't go
below zero. 0 K is referred to as absolute zero,
it's as cold as you can get. A nice round number of
the average temperature of the earth is 300 K, that's the
last temperature value to remember.
Here's some additional temperature data that I'm
including just in case you're interested.
You certainly don't need to try to
remember all these numbers. The world high
temperature record value of 136 F above was measured in
Libya at a location that was only about 35 miles from
the Mediterranean coast. Water, as we will see,
moderates climate, it reduces the extremes, so it seems
odd that such a high temperature would have been
recorded there. The World Meteorological
Organization recently decided the 136 F reading was
invalid and the new world record is the 134 F
measurement made in Death Valley. There is also
some question about the 134 F Death Valley value (see this
article in Wikipedia). There seems to be
some agreement that 129 F is the highest reliable
measurement of temperature. Temperatures that hot
have been measured at multiple locations.
The continental US cold temperature record of -70 F
was set in Montana and the -80 F value in Alaska.
The world record -129 F was measured at Vostok station
in Antarctica. This unusually cold reading was the
result of three factors: high latitude, high altitude,
and location in the middle of land rather than being
near or surrounded by ocean (again water moderates
climate, both hot and cold).
Liquid nitrogen is very cold but it is still quite a
bit warmer than absolute zero. Liquid helium gets
within a few degrees of absolute zero, but it's
expensive and there's only a limited amount of helium
available. So I would feel guilty bringing some to
class; plus I don't think it would look any different
than liquid nitrogen.
4. Energy, temperature, and specific
heat
When you add energy to an object, the object will usually
warm up (or if you take energy from an object the object
will cool). It is relatively easy to come up with an
equation that allows you to figure out what the temperature
change will be (this is another equation I'll try to
remember to write on the board before the next
quiz. Try to understand it, you don't have to memorize
it.
The temperature change, ΔT, will first depend on how
much energy was added, ΔE. This
is a direct proportionality, so ΔE is
in the numerator of the equation (ΔE
and ΔT are both positive when energy
is added, negative when energy is removed)
When you add equal amounts of energy to large and small
pans of water, the water in the small pan will get
hotter. The temperature change, ΔT,
will depend on the amount of water, the mass. A small
mass will mean a large ΔT, so mass
should go in the denominator of the equation.
Specific heat is what we use to account for the fact that
different materials react differently when energy is added
to them. A material with a large specific heat will
warm more slowly than a material with a small specific
heat. Specific heat has the same kind of effect on ΔT
as mass. Specific heat is sometimes called "thermal
mass" or "thermal capacity."
Here's an important example that will show the effect of
specific heat (see p. 45b in the ClassNotes).
Equal amounts of energy (500 calories) are
added to equal masses (100 grams) of water and soil.
We use water and soil in the example because most of the
earth's surface is either ocean or land. Before we do the
calculation, try to guess which material will warm up the
most. Everything is the same except for the specific
heats. Will water, with its 4 times larger specific
heat, warm up more or less than the soil?
The details of the calculation are shown below.
With its higher specific heat, the
water doesn't heat up nearly as much as the soil.
If we had been removing energy the water wouldn't cool
off as much as the soil would.
5. Water moderates climate
These different rates of warming of water and soil have
important effects on regional climate.
Oceans moderate the climate. Cities near a large
body of water won't warm as much in the summer and won't
cool as much during the winter compared to a city that is
surrounded by land. Water's ΔT
is smaller than land's because water has a higher
specific heat.
The yearly high and low monthly average temperatures are
shown at two locations above. The city on the coast
has a 30o F annual range of temperature
(range is the difference between the summer and winter
temperatures). The city further inland (assumed to be
at the same latitude and altitude) has an annual range
of 60o F. Note that both cities have the
same 60o F annual average
temperature.
Water moderates climates - it reduces the difference
between summertime high and wintertime low temperatures.
Growing tomatoes in the desert
- practical application
Here's another situation where you can take
advantage of water's high specific heat to moderate climate
on a smaller scale (it fits better in the Spring semester
edition of the class than the Fall semester).
You need to start tomatoes
early in Tucson (mid February), so that they can produce
fruit before it gets too hot. I usually start mine
in February and you need to protect the plants from frost.
Here's one way of doing that. You
moderate the climate and surround each plant with a "wall o
water" - a teepee like arrangement that
surrounds each plant. The cylinders are filled with
water and they take advantage of the high specific heat of
water and won't cool as much as the air or soil would during
a cold night. The walls of water produce a warm moist
micro climate that the tomato seedlings love. The
plastic is transparent so plenty of sunlight can get
through. Note the brocolli growing in the background,
it isn't nearly as sensitive to the cold and doesn't require
protection.
6.
Energy transport processes
By far the most important process is at
the bottom of the list above. Energy transport in the
form of electromagnetic radiation (sunlight for example) is
the only process that can carry energy through empty
space. Electromagnetic radiation travels both to the
earth (from the sun) and away from the earth back into
space. Electromagnetic radiation is also responsible
for about 80% of the energy transported between
the ground and atmosphere.
You might be surprised to learn that latent heat is the
second most important transport process. The term
latent heat can refer to both a type of energy and an energy
transport process (the energy is hidden in the water vapor,
the water vapor can move around and carry that energy with
it).
Rising parcels of warm air and sinking parcels of cold
air are examples of free convection. Because of
convection you feel colder or a cold windy day than on a
cold calm day (the wind chill effect). Ocean
currents are also an example of convection.
Convection is also one of the ways of rising air motions
in the atmosphere (convergence into centers of low pressure
and fronts are two other ways we've encountered so
far). Caution: convection is 1 of 4 energy transport
processes and 1 of 4 processes that cause rising air
motions.
Conduction is the least important energy transport at
least in the atmosphere. Air is such a poor conductor
of energy that it makes a very good insulator.
7. Energy transport by
conduction
Conduction is the first of four
energy transport processes that we will cover (and the
least important transport process in the
atmosphere). The figure below illustrates this
process. Imagine heating the end of a piece of
something just so you can visualize a hot object.
If you held the object in air it would slowly lose
energy by conduction and cool off.
How does that happen? In the
top picture some of the atoms or molecules near the
hot object are colliding with the object are picking
up energy.
In the middle picture the initial layer of
molecules are colored orange. They are moving
faster than they were. They begin to collide
with their outer neighbors and start to share energy
with them.
In the third picture molecules further out (yellow)
have now gained some energy. The random motions
and collisions between molecules is carrying energy
from the hot object out into the colder surrounding
air.
Conduction transports energy from hot to
cold. The rate of energy transport depends first
on the temperature gradient or temperature difference
between the hot object and the cooler
surroundings. If the object in the picture had
been warm rather than hot, less energy would flow and
energy would flow at a slower into the surrounding
air. If there were no temperature difference
there wouldn't be any energy transport at all.
The rate of energy transport also depends on the
material transporting energy (air in the example
above). Thermal conductivities of some common
materials are listed. Air is a very poor
conductor of energy and is generally regarded as an
insulator.
Water is a little bit better conductor.
Metals are generally very good conductors (cooking
pans are often made of stainless steel but have
aluminum or copper bottoms to evenly spread out heat
when placed on a stove). Diamond has a very high
thermal conductivity (apparently the highest of all
known solids). Diamonds are sometimes called
"ice." They feel cold when you touch them.
The cold feeling is due to the fact that they conduct
energy very quickly away from your warm fingers when
you touch them.
I brought a propane torch (2 of them actually, one to
serve as a backup) to class to demonstrate the behavior
of materials with different thermal conductivities.
Here's what I wanted to illustrate
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|
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Copper is a good
conductor. You must move your fingers
several inches away from the end to keep from
getting burned.
|
Glass has much lower
thermal conductivity. You can hold onto
the glass just a couple of inches from the flame
and not feel any heat. Because energy is
not being carried away from the end of the piece
of glass, the glass can get hot enough to begin
to glow red.
|
You can put your
finger alongside the flame with just 1/2 inch or
so of separation. Air is a very poor
conductor. Don't put your finger above the
flame though.
|
Because air has such a low thermal conductivity it is
often used as an insulator.
It is important, however, to keep the air trapped in small
pockets or small volumes so that it isn't able to move and
transport energy by convection (we'll look at convection
momentarily). Here are some examples of insulators
that use air:
Foam is often used
as an insulator. Foam is filled with
lots of small air bubbles, that's what
provides the insulation.
You
can safely hold onto a foam cup filled with liquid nitrogen (-320 F)
because the foam does such a good job
insulating your fingers from the cold liquid
inside.
Thin insulating layer of air in a double pane
window. I
don't have double pane windows in my
house. As a matter of fact I leave a
window open so my cats can get in and out of
the house (that's not particularly energy
efficient). It also means there are
lots of mosquitoes in the house in the
summer.
We really
haven't needed winter coats yet in Tucson
this semester.
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| Down
feathers are often used in coats
and sleeping bags. Packing together a
bunch of the "clusters" produces very good
insulation provided the feathers stay
"fluffed up" and trap air. source
of this image |
Synthetic fibers (Primaloft -
Synergy are shown above in a microphotograph) have
some advantages over down. There is
still some insulation when wet and the
material is hypoallergenic. source of
this image |
 |
|
A
photograph of aerogel (image source),
sometimes known as solid air. It's
an excellent insulator because it is mostly
air. The very small
particles in the aerogel are scattering light in
the same way air molecules do. That's why
it has this sky blue color.
|
A
scanning electron microscope photograph
of asbestos which was once widely used
as insulation. Asbestos
fibers can cause lung cancer and other
damage to your lungs when inhaled.
The white bar at the top left edge of
the image is 50 um across.
You can find this
image and read more about asbestos here.
|
8. Energy transport by convection
I used the torch again to heat up the broken glass
graduated cylinder. The glass gets so hot that you can
see it starting to glow red.
How would you cool off a hot object like this? You
could just hold onto it and it would eventually cool by
conduction. If you were in a little bit more of
a hurry you could blow on it. That's forced
convection, the energy transport process we will be
covering next. Or you could stick the hot end of the
cylinder into some water (you'd hear a short hissing sound
and the glass would probably shatter). The hissing
would mean the hot piece of glass had evaporated some
water. That would be an example of latent heat energy
transport which we'll be discussing later in the period.
Rather than moving about randomly, the atoms or molecules
move together as a group (organized motion). Convection
works in liquids and gases but not solids (the atoms or
molecules in a solid can't move freely).
In the middle of the
picture above a thin layer of air surrounding a hot object
has been heated by conduction. Then a person is blowing
the blob of warm air off to the right. The warm air
molecules are moving away from the hot object together as
a group (that's the organized part of the motion).
Cooler air moves in and surrounds the hot object and the
whole process repeats itself.
And actually you don't need to force convection, it
will often happen on its own.
A thin layer of air in the
figure above (lower left) is heated by conduction.
Then because hot air is also low density air, it actually
isn't necessary to blow on the hot object, the warm air
will rise by itself.. Energy is being transported
away from the hot object into the cooler surrounding
air. This is called free convection. Cooler
air moves in to take the place of the rising air and the
cycle repeats itself.
The example at upper right is also free convection.
Room temperature air in contact with a cold object loses
energy and becomes cold high density air. The sinking
air motions that would be found around a cold object have
the effect of transporting energy from the room temperature
surroundings to the colder object.
In both examples of free convection, energy is being
transported from hot toward cold.
I could put my finger alongside the flame from the
propane torch without any problem. There's very little
energy transported sideways through air by
conduction.
Be careful if you put
your finger or hand above the torch. That's
because there's a lot of very hot
air rising from the torch. This is energy
transport by free convection and its something you
can sometimes see.
Up at the front of the classroom you
might have been able to
see (barely) the shimmering of hot rising air
when I held the torch in front of the projector
screen. There is a technique, called
Schlieren photography, that can better catch
these barely visible air motions (it is able to
see and photograph the differences in air
density). The photo at right is an example
and shows the hot rising air above a candle.
The photo was taken by Gary Settles from
Penn State University and can be found at this
site.
This was as far as we were
able to go in class today. We'll
finish the section below at the start of
class on Thursday
9.
Real world examples of energy transport by
conduction and convection (see p. 48a in
the ClassNotes)
Now some surprisingly practical applications, I think,
of what we have learned about conductive and convective
energy transport. Energy transport really does
show up in a lot more everyday real life situations than
you might expect.
Note first of all there is a temperature difference
between your hand and a room temperature (70 F)
object. Energy will flow from your warm hand to
the colder object. Metals are better conductors
than wood. If you touch a piece of 70 F metal it
will feel much colder than a piece of 70 F wood, even
though they both have the same temperature. A
piece of 70 F diamond would feel even colder because it
is an even better conductor than metal. I brought
a piece of aluminum and a piece of wood (oak) to class
so that you could check this out for yourself.
Something that feels cold may not be as cold as it
seems.
Our perception of cold
is more an indication of how quickly
our body or hand is losing energy
than a reliable measurement of
temperature.
Here's another
example
It's pleasant standing outside on a
nice day in 70 F air, it doesn't feel warm or
cold. But if you jump into 70 F pool water you
will feel cold, at least until you "get used to" the
water temperature (your body might reduce blood flow
to your extremities and skin to try to reduce energy
loss).
Air is a poor conductor. If you go out in 40
F weather you will feel cold largely because there is
a larger temperature difference between you and your
surroundings (and temperature difference is one of the
factors that affect rate of energy transport by
conduction).
If you
stick your hand into a bucket of 40 F water, it will
feel very cold (your hand will actually soon begin to
hurt). Keep some warm water nearby to warm up
your hand.
Water is a much better conductor than air.
Energy flows much more rapidly from your hand into the
cold water. I mentioned in class that I thought
this might be good for you. The reason is that
successive application of hot and then cold is
sometimes used to treat arthritis
joint pain (it used to work wonders on my Dad's
knee).
You can safely stick your hand into
liquid nitrogen for a fraction of a second.
There is an enormous temperature difference between
your hand and the liquid nitrogen which would
ordinarily cause energy to leave your hand at a
dangerously high rate (which could cause your hand to
freeze solid). It doesn't feel particularly cold
though and doesn't feel wet. The reason is that
some of the liquid nitrogen evaporates and quickly
surrounds your hand with a layer of nitrogen
gas. Just like air, nitrogen is a poor conductor
(air is mostly nitrogen). The nitrogen gas
insulates your hand from the cold for a very short
time (the gas is a poor conductor but a conductor
nonetheless) If you leave your hand in the
liquid nitrogen for even a few seconds it would
freeze. That would cause irreparable damage.
A question came up in class a few semesters ago
about sticking you hand (or maybe just the tip of one
finger) into molten lead. I've never seen it
done and certainly haven't tried it myself. But
I suspected that you would first need to wet your
hand. Then once you stick it into the lead the
water would vaporize and surround your hand with a
thin layer of gas, water vapor. The water vapor
is a poor conductor just like the nitrogen and oxygen
in air, and that protects your hand, for a short time,
from the intense heat. Here's a
video (and water does play a critical role)
Wind chill
Wind chill is a really good example of energy transport by
convection. As a matter of fact I'm hoping that
whenever you hear of energy transport by convection you'll
first think of wind chill. Wind chill is also a
reminder that our perception of cold is an indication of how
quickly our body is losing energy rather than an accurate
measurement of temperature.
Your body works hard to keep its
core temperature around 98.6 F. If
you go outside on a 40 F day (calm winds) you will
feel cool; your body is losing energy to the colder
surroundings (by conduction mainly). Your body
will be able to keep you warm for a little while
(perhaps indefinitely, I don't know). The 5
arrows represent the rate at which your body is losing
energy.
A thermometer behaves differently,
it is supposed to cool to the temperature of the
surroundings. Once it reaches 40 F and has the
same temperature as the air around it the energy loss
will stop. If your body cools to 40 F you will
die.
If you go outside on a 40 F day with 30 MPH winds your
body will lose energy at a more rapid rate (because
convection together with conduction are transporting energy
away from your body). Note the additional arrows drawn
on the figures above indicating the greater heat loss.
This higher rate of energy loss will make it feel colder
than a 40 F day with calm winds.
Actually, in terms of the rate at which your
body loses energy, the windy 40 F day would feel the
same as a 28 F day without any wind. Your body is
losing energy at the same rate in both cases (9 arrows
in both cases). The combination 40 F and 30
MPH winds results in a wind chill temperature of 28 F.
You would feel colder on a 40 F day with 30 MPH winds but
the actual temperature is still 40 F. The thermometer
will again cool to the temperature of its surroundings, it
will just cool more quickly on a windy day. Once the
thermometer reaches 40 F there won't be any additional
energy flow or further cooling. The thermometer would
measure 40 F on both the calm and the windy day.
Standing outside on a 40 F day is not an
immediate life threatening situation. Falling into
40 F water is, you might last 30 minutes (though you
might lose consciousness before that and die by
drowning).
Energy will be conducted away from your body
more quickly than your body can replace it. Your
core body temperature will drop and bring on hypothermia.
Be sure not to confuse hypothermia with hyperthermia
which can bring on heatstroke and is a serious outdoors
risk in S. Arizona in the summer.
Talk of how long you would last in 40 F water
reminds me of a page from National Geographic Magazine
that lists some of the limits
of human survival. I can't just scan the
original and add it to the notes without violating
copyright laws. But if you click on the link above
you'll find all of the same information online in the
form of a quiz.
Here are scanned images of the
lecture outline or summary sheet with some of the
main details filled in (this is from the Fall 2016
class but it's very nearly the same as what we came
up with in class today).
