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Energy and Heat Transfer
AOS 101 Discussion Sections 302 and 303
Energy
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Energy is the capacity to do work
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Work is done on something when it
is either pushed, pulled, or lifted over
some distance
Kinds of energy
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Kinetic energy
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Potential energy
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KE = ½mv2
PE = mgh
Mechanical energy
Chemical energy
Thermal energy
Radiant energy
Laws of Thermodynamics
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1st Law of Thermodynamics
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2nd Law of Thermodynamics
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Energy cannot be created or destroyed
Energy lost during one process must equal the energy gained
during another
Heat can spontaneously flow from a hotter object to a cooler
object, but not the other way around
The amount of heat lost by the warm object is equivalent
to the heat gained by the cooler object
Conservation of Energy
Internal
energy
Amount of heat
added
Work done
Heat
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Heat is a form of energy and is the total internal energy
of a substance
Revisiting the 1st law
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States that heat is really energy in the process of being
transferred from a high temperature object to a lower
temperature object.
Heat transfer changes the internal energy of both systems
involved
Heat can be transferred by:
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Conduction
Convection
Advection
Radiation
Heat Capacity and Specific Heat
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Heat capacity of a substance is the ratio of heat absorbed
(or released) by that substance to the corresponding
temperature rise (or fall)
Specific heat
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The heat capacity of a substance per unit mass
Can be thought of a measure of the heat energy needed to
heat 1 g of an object by 1ºC
Different objects have different specific heat values
Specific Heat
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1 g of water must absorb about 4 times as much heat as
the same quantity of air to raise its temperature by 1º C
This is why the water temperature of a lake or ocean
stays fairly constant during the day, while the temperature
air might change more
Because of this, water has a strong effect on weather and
climate
Substance
Specific Heat (J/gK)
Water (liquid)
4.183
Ice
2.050
Wood
0.420
Sand
0.835
Air
1.012
Latent Heat
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Latent heat is the amount of energy released or
absorbed by a substance during a phase change
FOR WATER:
SOLID
334 J/g
2260 J/g
released
released
LIQUID
334 J/g
SOLID
Lowest energy
absorbed
GAS
2260 J/g
LIQUID
LIQUID
absorbed
GAS
Highest energy
Orange Example
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Farmers spray their oranges
with water when a frost
event is about to occur
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Why?
When the temperature
drops below 32oF, liquid
water freezes into ice.
This liquid to solid phase
change causes energy to be
released to the fruit.
Thus, the temperature of the
orange remains warm
enough to prevent ruin.
Swimming Pool Example
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Why do you feel cool when you get out of the pool?
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Drops of liquid water are still on your skin after getting out
These drops evaporate into water vapor
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This liquid to gas phase change causes energy to be absorbed from
your skin
Cumulus Cloud Example
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Formation of clouds
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Clouds form when water
vapor condenses into tiny
liquid water drops
This gas to liquid phase
change causes energy to be
released to the atmosphere
Release of latent heat during
cloud formation drives many
atmospheric processes
Conduction
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Conduction is the transfer of heat from molecule to
molecule within a substance
Molecules must be in direct contact with each other
Thermal Conductivity
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Thermal conductivity is the measure of how well a
substance can conduct heat
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Depends on its molecular structure
Substance
Thermal Conductivity (W/Km)
Air
0.024
Soil
0.2
Asphalt
0.75
Glass
1.05
Stainless Steel
16
Copper
401
Silver
429
If it is a bad conductor, it is a good insulator
Convection
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Convection is the transfer of heat by the mass movement
of a fluid (such as water and air) in the vertical direction
(up and down)
physics.arizona.edu
Convection
Moist Convection
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As the temperature of an air parcel cools, it may reach a
point where it reaches saturation (the air temperature
and dew point are the close to the same)
Air parcels condense and form a cloud
Advection
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Advection is the transfer of heat in the horizontal direction
The wind transfers heat by advection
Occurs frequently
Why is advection important?
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Important for the formation of precipitation and fog
arcticstudies.pbworks.com
Types of Advection
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Two types
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Warm air advection (WAA)
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Wind blows warm air toward a region of colder air
Cold air advection (CAA)
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Wind blows cold air toward a region of warmer air
www.aviationweather.ws
Radiation
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Radiation is the travel of energetic particles or waves
traveling through space or another kind of medium to
heat it up
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For Example:
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The suns rays traveling through space and reaching the Earth
The warmth from a fire pit
Radiation back into space from a warm Earth
Black Body
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A perfect absorber and emitter of radiation
Radiation, Convection, and Conduction
Solar Radiation
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The sun’s rays do not hit all areas of the Earth the same
Factors that determine the amount of solar radiation
hitting the Earth
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Position on Earth (latitude, longitude, and elevation)
Time of day (shown below in UTC)
Composition of the atmosphere
Amount and thickness of clouds, if any
Position of Earth in orbit around the sun (i.e. time of year)
Solar Radiation
Solar Radiation
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Equinoxes
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Where day and night are of equal length
Vernal Equinox – March 20
Autumnal Equinox – September 23
Solstices
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Summer Solstice – June 21
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Winter Solstice – December 21
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Longest day of the year in the Northern Hemisphere
Where the sun is at it’s northernmost point from the equator
Shortest day of the year in the Northern Hemisphere
Where the sun is at it’s southernmost point from the equator
How radiation changes with latitude and date
Solar Radiation Budget
Energy Budget
Radiation
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All things with a temperature above absolute zero emit
radiation
Radiation allows heat to be transferred through wave
energy
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These waves are called electromagnetic waves
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i.e., the sun mainly emits radiative energy in the visible
spectrum, and the earth emits radiative energy in the infrared
spectrum
Wavelengths of the radiation emitted by an object
depends on the temperature of that object
Shorter wavelengths carry more energy than longer
wavelengths
Radiation
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A photon of ultra-violet radiation carries more energy than a
photon of infrared radiation
The shortest wavelengths in the visible spectrum are purple, and the
longest are red
Types of Radiation
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Energy can be:
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Absorbed
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Reflected
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Albedo is the percentage of the light reflected off an object
Scattered
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Increasing the internal energy of the gas molecules
Light deflected in all directions: forward, backward or sideways
Also called diffused light
Transmitted
Kirchoff’s Law
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Good absorbers of a particular wavelength are good
emitters at that wavelength and vice versa
Our atmosphere has many selective absorbers
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Carbon dioxide, water vapor, etc…
These gases are good at absorbing IR radiation but not solar
radiation
These gases are called greenhouse gases
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Due to the fact they help to absorb and reemit IR radiation back
toward the Earth’s surface thus keeping us warmer then we would
otherwise be
More Examples
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Energy