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Energy and Heat Transfer
AOS 101 Discussion Sections 302 and 303
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Energy Energy is the capacity to do
work Work is done on something
when it is either pushed, pulled, or lifted over some distance
Kinds of energy Kinetic energy
KE = ½mv2
Potential energy PE = mgh
Mechanical energy Chemical energy Thermal energy Radiant energy
Laws of Thermodynamics 1st Law of Thermodynamics
Energy cannot be created or destroyed Energy lost during one process must equal the
energy gained during another 2nd Law of Thermodynamics
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
Amount of heat added
Internalenergy
Work done
Heat Heat is a form of energy and is the total
internal energy of a substance Revisiting the 1st law
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: Conduction Convection Advection Radiation
Heat Capacity and Specific Heat 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 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 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 climateSubstance Specific Heat (J/gK)
Water (liquid) 4.183
Ice 2.050
Wood 0.420
Sand 0.835
Air 1.012
Latent Heat Latent heat is the amount of energy released or
absorbed by a substance during a phase change
LIQUID
2260 J/g
released
334 J/g
released
2260 J/g
absorbed
334 J/g
absorbed
Lowest energy Highest energy
SOLID
SOLID
LIQUID
LIQUID
GAS
GAS
FOR WATER:
Orange Example Farmers spray their
oranges with water when a frost event is about to occur 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
Why do you feel cool when you get out of the pool? Drops of liquid water are still on your skin after getting out These drops evaporate into water vapor
This liquid to gas phase change causes energy to be absorbed from your skin
Cumulus Cloud Example Formation of clouds
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 Conduction is the transfer of heat from
molecule to molecule within a substance Molecules must be in direct contact with each
other
Thermal Conductivity Thermal conductivity is the measure of how
well a substance can conduct heat Depends on its molecular structure
If it is a bad conductor, it is a good insulator
Substance Thermal Conductivity (W/Km)
Air 0.024
Soil 0.2
Asphalt 0.75
Glass 1.05
Stainless Steel 16
Copper 401
Silver 429
Convection 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 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 Advection is the transfer of heat in the
horizontal direction The wind transfers heat by advection Occurs frequently Why is advection important?
Important for the formation of precipitation and fog
arcticstudies.pbworks.com
Types of Advection Two types
Warm air advection (WAA) Wind blows warm air toward a region of colder air
Cold air advection (CAA) Wind blows cold air toward a region of warmer air
www.aviationweather.ws
Radiation Radiation is the travel of energetic particles or
waves traveling through space or another kind of medium to heat it up For Example:
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 A perfect absorber and emitter of radiation
Radiation, Convection, and Conduction
Solar Radiation 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 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 Equinoxes
Where day and night are of equal length Vernal Equinox – March 20 Autumnal Equinox – September 23
Solstices Summer Solstice – June 21
Longest day of the year in the Northern Hemisphere Where the sun is at it’s northernmost point from the
equator Winter Solstice – December 21
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 All things with a temperature above absolute
zero emit radiation Radiation allows heat to be transferred
through wave energy These waves are called electromagnetic waves
Wavelengths of the radiation emitted by an object depends on the temperature of that object i.e., the sun mainly emits radiative energy in the
visible spectrum, and the earth emits radiative energy in the infrared spectrum
Shorter wavelengths carry more energy than longer wavelengths
Radiation
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 Energy can be:
Absorbed Increasing the internal energy of the gas molecules
Reflected Albedo is the percentage of the light reflected off an
object Scattered
Light deflected in all directions: forward, backward or sideways
Also called diffused light Transmitted
Absorb
e
dRefl
ecte
dTran
smitt
ed
Scattere
d
Kirchoff’s Law Good absorbers of a particular wavelength are
good emitters at that wavelength and vice versa
Our atmosphere has many selective absorbers Carbon dioxide, water vapor, etc… These gases are good at absorbing IR radiation
but not solar radiation These gases are called greenhouse gases
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 Energy