We quickly reviewed the material on
convection, the second energy transport process that we have
discussed. Those notes were stuck onto the end of the Tuesday Oct. 6 online notes.
Now some practical applications 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 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.
Something that feels cold may not be as cold as it seems. Our perception of cold is more an indication of how quickly our hand is losing energy than a reliable measurement of temperature.
Here's a similar situation.
It's pleasant standing outside on a nice day in 70 F air. 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 extremeties and skin to try to reduce energy loss).
Air is a poor conductor. If you go out in 40 F weather you will feel colder 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 (I would suggest you give it a try
sometime), it will feel very cold (your hand will
actually soon begin to hurt). Water is a much better conductor
than air. Energy flows much more rapidly from your hand into the
cold water.
Ice
feels cold even though it is not a particularly good
conductor. This is because of the large temperature difference
between your hand and the water.
What about liquid nitrogen? It has a temperature
of
-320F! You can safely stick your
hand in liquid nitrogen for a fraction of a second. It doesn't
feel particularly cold and doesn't feel wet. Some of the liquid
nitrogen quickly evaporates and surrounds your hand with a layer of
nitrogen
gas. This gas is a poor conductor and insulates your
hand from the cold for a short time.
We're now in a perfect position to understand the concept of wind chill temperature.
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 works hard to keep its core temperature around
98.6 F. A thermometer
behaves differently, it is supposed to cool to the temperature of the
surroundings. Once it reaches 40 F it won't lose any additional
energy.
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). 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. The combination 40 F and 30 MPH winds results in a wind chill temperature of 28 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. 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.
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 which is also a serious outdoors risk in S. Arizona.
At this point I showed a picture from the March 2005 issue of National Geographic. A Buddhist monk was standing in a frigid waterfall. The caption for the photograph read "To focus the mind and increase awareness of self, Shingon Buddhists like Souei Sakamoto practice takigyo,chanting for hours while standing in frigid waterfalls at the Oiwasan Nissekiji Temple in Toyama, Japan." (I can't really scan the photograph and include it in the classnotes because of copyright laws)
A second photograph from the December 2005 issue showed a monk hanging from a tree by his feet. The caption there read "To see life as it truly is - that's the goal of a student in China who strengthens mind and body under the rigorous tutelage of a Shaolin kung fu master."
Perhaps the most amazing example of a physical and mental task is the 1000-day challenge undertaken by the "marathon monks" of Mount Hiei, Japan.
I hope you don't mind an occasional digression like this. I spend a lot of time riding my bicycle uphills. It's not really painful but can definitely be uncomfortable. I've noticed that you can sometimes be distracted by a thought and ride a mile or so and completely blank out the discomfort. With some "Buddhist monk like" training I wonder if maybe I couldn't ride uphill more or less indefinitely and not feel any discomfort at all. This time of the year it is often a little cool in the morning. In another month or so it will be cold. With some mental training I hope be to be able to blank the feeling of cold fingers and toes. I'm not there yet but will continue to work on it.
Latent heat energy transport was the next topic of the day. Now that I am putting these notes online, I am realizing we covered a lot of material in class on Thursday.
Energy transport in the form of latent heat is the second most important energy transport process (second only to electromagnetic radiation).
If you had an object that you wanted to cool off quickly you could blow on it. Or you could stick it into some water, that would cool it off pretty quickly. You'd here a brief sizzling sound, the sound of boiling water. A lot of energy would be taken quickly from the hot object and used to boil a small amount of water.
Latent heat energy transport is sometimes a little hard to visualize or understand because the energy is "hidden" in water vapor or water.
When your body starts to lose energy, it feels cold.
A 240 pound man or woman running at 20 MPH has just
enough
kinetic energy (if you could capture it) to
be able to melt an ordinary ice cube. It would take 8 people
running at 20 MPH to
evaporate the resulting water.
You can consciously remove energy from water vapor to make it condense or from water to cause it to free (you could put water in a freezer; energy would flow from the relatively warm water to the colder surroundings). Or if one of these phase changes occurs energy will be released into the surroundings (causing the surroundings to warm). Note the orange energy arrows have turned around and are pointing from the material toward the surroundings.
A can of cold drink will warm more quickly in warm moist surroundings than in warm dry surroundings. Heat will flow from the warm air into the cold cans in both cases. Condensation of water vapor is an additional source of energy and will warm that can more rapidly. The condensation may actually be the dominant process.
You feel cold when you step out of a shower and water on your body evaporates. The opposite situation, stepping outdoors on a humid day and actually having water vapor condense onto your body (it can happen to your sunglasses but not to you, your body is too warm). If it did happen it would warm you up.
This figure shows how energy can be transported from one location to another in the form of latent heat. The story starts at left in the tropics where there is often an abundance or surplus of sunlight energy. Some of the incoming sunlight evaporates ocean water. The resulting water vapor moves somewhere else and carries hidden latent heat energy with it. This hidden energy reappears when something (air running into a mountain and rising, expanding, and cooling) causes the water vapor to condense. The condensation releases energy into the surrounding atmosphere.
Energy arriving in sunlight in the tropics has effectively been transported to the atmosphere in Tucson.
The following figure wasn't shown in class.
The formation of a cloud means that
latent heat is being released into the air. Two energy transport
processes are at work in this picture: convection and latent heat
(conduction is also present, that is how is energy is transported from
the hot ground into the thin layer of air in contact with the
ground. In this case energy is being transported vertically in
the form of latent heat.
We'll spend the next two or three class periods on electromagnetic (EM) radiation. It is the most important energy transport process because it can travel through empty space. The notes that follow are a little more detailed that was done in class.
To really understand EM radiation you need to understand electric fields. To understand electric fields we need to quickly review a basic rule concerning static electricity.
Static electricity was demonstrated (one of the poorer demonstrations of the semester) using a sweater (a gift from my Aunt Ethel and Uncle Nelson made of acrylic fiber and wool) and two balloons.
Each balloon was rubbed with the sweater. The balloons (and the sweater) became electrically charged (the balloons had one polarity of charge, the sweater had the other). We didn't know what charge the balloons carried just that they both had the same charge.
If you bring the balloons close to each other they are
pushed apart by
a repulsive electrical force.
The sweater and the balloon carry opposite
charges. IF
they are
brought together they experience an attractive electrical force.
Our next step in understanding EM is to learn something about electric field arrows. Imagine placing a + charge at the three positions shown in the figure below.
1. What two phase changes are occurring in the picture? (you might not be able to see them, also the cloud you see is not carbon dioxide gas) The dry ice is first of all sublimating (turning from solid to gas). The sublimation is invisible. The cloud that you do see is composed of water droplets or ice crystals. Water vapor coming into contact with the cold dry ice condenses to form water droplets (or perhaps ice crystals).
Is energy being transported from the surroundings INTO the dry ice or AWAY from the dry ice and into the surrounding air? Energy flows from the warmer surrounding air into the much colder dry ice. That is what cools the air enough for the cloud to form and become visible. The condensation that occurs as the cloud forms also releases hidden latent heat energy which goes into the dry ice.
2. The person is trying to cool the hot steaming bowl of soup by blowing on it.
What two energy transport processes are at work
here?
(conduction is not one of the answers I'm looking for)
Blowing on the bowl of soup is forced convection. The hot soup is also evaporating (the soup is steaming hot). The energy needed for water in the soup to evaporate is taken from the soup and cools the soup (just like the water that evaporates off your wet body when you step out of a shower takes energy from your body and makes you feel cold).
3. A person is standing outside on a cold windy day in A, has fallen into cold water in B, and is perspiring heavily in C.
Match each energy transport process below with the most appropriate situation in the drawing.
Conduction___B___
Convection ___A___
Latent
heat ___C____
4. Would the formation of a cloud WARM or COOL the surrounding air? Water vapor condenses to form the cloud. Latent heat energy is released into the surrounding air when water vapor condenses.
Does the formation of frost WARM or COOL the air? This is a similar situation. Latent heat energy is released into the surroundings as water vapor changes directly to ice (deposition).
Now some practical applications 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 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.
Something that feels cold may not be as cold as it seems. Our perception of cold is more an indication of how quickly our hand is losing energy than a reliable measurement of temperature.
Here's a similar situation.
It's pleasant standing outside on a nice day in 70 F air. 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 extremeties and skin to try to reduce energy loss).
Air is a poor conductor. If you go out in 40 F weather you will feel colder 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).
We're now in a perfect position to understand the concept of wind chill temperature.
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. The combination 40 F and 30 MPH winds results in a wind chill temperature of 28 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. 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.
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 which is also a serious outdoors risk in S. Arizona.
At this point I showed a picture from the March 2005 issue of National Geographic. A Buddhist monk was standing in a frigid waterfall. The caption for the photograph read "To focus the mind and increase awareness of self, Shingon Buddhists like Souei Sakamoto practice takigyo,chanting for hours while standing in frigid waterfalls at the Oiwasan Nissekiji Temple in Toyama, Japan." (I can't really scan the photograph and include it in the classnotes because of copyright laws)
A second photograph from the December 2005 issue showed a monk hanging from a tree by his feet. The caption there read "To see life as it truly is - that's the goal of a student in China who strengthens mind and body under the rigorous tutelage of a Shaolin kung fu master."
Perhaps the most amazing example of a physical and mental task is the 1000-day challenge undertaken by the "marathon monks" of Mount Hiei, Japan.
I hope you don't mind an occasional digression like this. I spend a lot of time riding my bicycle uphills. It's not really painful but can definitely be uncomfortable. I've noticed that you can sometimes be distracted by a thought and ride a mile or so and completely blank out the discomfort. With some "Buddhist monk like" training I wonder if maybe I couldn't ride uphill more or less indefinitely and not feel any discomfort at all. This time of the year it is often a little cool in the morning. In another month or so it will be cold. With some mental training I hope be to be able to blank the feeling of cold fingers and toes. I'm not there yet but will continue to work on it.
Latent heat energy transport was the next topic of the day. Now that I am putting these notes online, I am realizing we covered a lot of material in class on Thursday.
Energy transport in the form of latent heat is the second most important energy transport process (second only to electromagnetic radiation).
If you had an object that you wanted to cool off quickly you could blow on it. Or you could stick it into some water, that would cool it off pretty quickly. You'd here a brief sizzling sound, the sound of boiling water. A lot of energy would be taken quickly from the hot object and used to boil a small amount of water.
Latent heat energy transport is sometimes a little hard to visualize or understand because the energy is "hidden" in water vapor or water.
Latent heat energy transport is
associated with changes of
phase (solid to liquid, water to water vapor, that sort of thing) A
solid to liquid phase change is melting, liquid to gas is
evaporation, and sublimation is a solid to gas phase change (dry ice
sublimates when placed in a warm room, it turns directly from solid
carbon dioxide to gaseous carbon dioxide).
In each case energy must be added to the material changing phase. You can consciously add or supply the energy (such as when you put water in a pan and put the pan on a hot stove) or the needed energy will be taken from the surroundings (from your body when you step out of a shower in the morning).
In each case energy must be added to the material changing phase. You can consciously add or supply the energy (such as when you put water in a pan and put the pan on a hot stove) or the needed energy will be taken from the surroundings (from your body when you step out of a shower in the morning).
When your body starts to lose energy, it feels cold.
You can consciously remove energy from water vapor to make it condense or from water to cause it to free (you could put water in a freezer; energy would flow from the relatively warm water to the colder surroundings). Or if one of these phase changes occurs energy will be released into the surroundings (causing the surroundings to warm). Note the orange energy arrows have turned around and are pointing from the material toward the surroundings.
A can of cold drink will warm more quickly in warm moist surroundings than in warm dry surroundings. Heat will flow from the warm air into the cold cans in both cases. Condensation of water vapor is an additional source of energy and will warm that can more rapidly. The condensation may actually be the dominant process.
You feel cold when you step out of a shower and water on your body evaporates. The opposite situation, stepping outdoors on a humid day and actually having water vapor condense onto your body (it can happen to your sunglasses but not to you, your body is too warm). If it did happen it would warm you up.
This figure shows how energy can be transported from one location to another in the form of latent heat. The story starts at left in the tropics where there is often an abundance or surplus of sunlight energy. Some of the incoming sunlight evaporates ocean water. The resulting water vapor moves somewhere else and carries hidden latent heat energy with it. This hidden energy reappears when something (air running into a mountain and rising, expanding, and cooling) causes the water vapor to condense. The condensation releases energy into the surrounding atmosphere.
Energy arriving in sunlight in the tropics has effectively been transported to the atmosphere in Tucson.
The following figure wasn't shown in class.
We'll spend the next two or three class periods on electromagnetic (EM) radiation. It is the most important energy transport process because it can travel through empty space. The notes that follow are a little more detailed that was done in class.
To really understand EM radiation you need to understand electric fields. To understand electric fields we need to quickly review a basic rule concerning static electricity.
Static electricity was demonstrated (one of the poorer demonstrations of the semester) using a sweater (a gift from my Aunt Ethel and Uncle Nelson made of acrylic fiber and wool) and two balloons.
Each balloon was rubbed with the sweater. The balloons (and the sweater) became electrically charged (the balloons had one polarity of charge, the sweater had the other). We didn't know what charge the balloons carried just that they both had the same charge.
Our next step in understanding EM is to learn something about electric field arrows. Imagine placing a + charge at the three positions shown in the figure below.
Then choose one of the three arrows
at the bottom of the picture to show both the direction and the force
that would be exerted on each charge.
Here's the answer. The closer
the charge is to the center, the greater the strength of the outward
force. With just a little thought you can see that if you were to
place + charges at other
positions you would quickly end up with a
figure that looks like the pattern at the bottom of p. 59 in the
photocopied ClassNotes.
The electric field arrows in this
picture show the direction and give an idea of the strength that would
be exerted on a positive placed at any position in the figure.
You'll find the following on p. 60 in the photocopied ClassNotes.
We imagine turning on a source of EM radiation and then
a
short time
later we take a snapshot. The EM radiation is a wavy pattern of
electric and magnetic field arrows. We'll ignore the magnetic
field lines. The E field lines sometimes point up, sometimes
down. The pattern of electric field arrows repeats itself.
Note the + charge near the right side of the picture. At the time this picture was taken the EM radiation exerts a fairly strong upward force on the + charge.
Textbooks often represent EM radiation with a wavy line like shown
above. But what does that represent?
This picture was taken a short time after the first snapshot when
the radiation
had
traveled a little further to the right. The EM radiation now
exerts a somewhat weaker downward force on the + charge.
The + charge is now being
pushed upward again. A
movie
of
the +
charge, rather than just a series of snapshots, would show the
charge
bobbing up and down much like a swimmer in the
ocean would do as waves passed by.
The wavy pattern used to depict EM radiation can be described spatially in terms of its wavelength, the distance between identical points on the pattern. By spatially we mean you look at different parts of the radiation at one particular instant frozen in time.
Or you can describe the radiation temporally using the frequency of oscillation (number of up and down cycles completed by an oscillating charge per second). By temporally we mean you at one particular point for a certain period of time.
I'm in a little bit of a rush today putting today the online notes before the weekend. So many of the remaining figures have been taken from the Fall 2008 notes and may differ slightly from today's class (mostly just the colors are different)..
EM radiation can be created when you cause a charge to move up and down. If you move a charge up and down slowly (upper left in the figure above) you would produce long wavelength radiation that would propagate out to the right at the speed of light. If you move the charge up and down more rapidly you produce short wavelength radiation that propagates at the same speed.
Once the EM radiation encounters the charges at the right side of the figure above the EM radiation causes those charges to oscillate up and down. In the case of the long wavelength radiation the charge at right oscillates slowly. This is low frequency and low energy motion. The short wavelength causes the charge at right to oscillate more rapidly - high frequency and high energy.
These three characteristics: long wavelength / low frequency / low energy go together. So do short wavelength / high frequency / high energy. Note that the two different types of radiation both propagate at the same speed.
The following figure wasn't shown in class. It summarizes and reinforces how energy can be transported from one place to another (even through empty space) in the form of electromagnetic (EM) radiation.
Here are the answers (in red) to the series of questions shown in class.
You'll find the following on p. 60 in the photocopied ClassNotes.
Note the + charge near the right side of the picture. At the time this picture was taken the EM radiation exerts a fairly strong upward force on the + charge.
The wavy line just connects the
tips of a bunch of electric
field
arrows.
The wavy pattern used to depict EM radiation can be described spatially in terms of its wavelength, the distance between identical points on the pattern. By spatially we mean you look at different parts of the radiation at one particular instant frozen in time.
Or you can describe the radiation temporally using the frequency of oscillation (number of up and down cycles completed by an oscillating charge per second). By temporally we mean you at one particular point for a certain period of time.
I'm in a little bit of a rush today putting today the online notes before the weekend. So many of the remaining figures have been taken from the Fall 2008 notes and may differ slightly from today's class (mostly just the colors are different)..
EM radiation can be created when you cause a charge to move up and down. If you move a charge up and down slowly (upper left in the figure above) you would produce long wavelength radiation that would propagate out to the right at the speed of light. If you move the charge up and down more rapidly you produce short wavelength radiation that propagates at the same speed.
Once the EM radiation encounters the charges at the right side of the figure above the EM radiation causes those charges to oscillate up and down. In the case of the long wavelength radiation the charge at right oscillates slowly. This is low frequency and low energy motion. The short wavelength causes the charge at right to oscillate more rapidly - high frequency and high energy.
These three characteristics: long wavelength / low frequency / low energy go together. So do short wavelength / high frequency / high energy. Note that the two different types of radiation both propagate at the same speed.
The following figure wasn't shown in class. It summarizes and reinforces how energy can be transported from one place to another (even through empty space) in the form of electromagnetic (EM) radiation.
You add energy when you cause an
electrical charge to move up and down
and create the EM radiation (top left).
In the middle
figure, the EM
radiation then travels out
to the
right (it could be through empty space or through something like the
atmosphere).
Once
the EM radiation encounters an electrical charge at another location
(bottom right),
the energy reappears as the radiation causes the charge to move.
Energy
has been transported from left to right.
Here are the answers (in red) to the series of questions shown in class.
1. What two phase changes are occurring in the picture? (you might not be able to see them, also the cloud you see is not carbon dioxide gas) The dry ice is first of all sublimating (turning from solid to gas). The sublimation is invisible. The cloud that you do see is composed of water droplets or ice crystals. Water vapor coming into contact with the cold dry ice condenses to form water droplets (or perhaps ice crystals).
Is energy being transported from the surroundings INTO the dry ice or AWAY from the dry ice and into the surrounding air? Energy flows from the warmer surrounding air into the much colder dry ice. That is what cools the air enough for the cloud to form and become visible. The condensation that occurs as the cloud forms also releases hidden latent heat energy which goes into the dry ice.
2. The person is trying to cool the hot steaming bowl of soup by blowing on it.
Blowing on the bowl of soup is forced convection. The hot soup is also evaporating (the soup is steaming hot). The energy needed for water in the soup to evaporate is taken from the soup and cools the soup (just like the water that evaporates off your wet body when you step out of a shower takes energy from your body and makes you feel cold).
3. A person is standing outside on a cold windy day in A, has fallen into cold water in B, and is perspiring heavily in C.
Match each energy transport process below with the most appropriate situation in the drawing.
4. Would the formation of a cloud WARM or COOL the surrounding air? Water vapor condenses to form the cloud. Latent heat energy is released into the surrounding air when water vapor condenses.
Does the formation of frost WARM or COOL the air? This is a similar situation. Latent heat energy is released into the surroundings as water vapor changes directly to ice (deposition).