Energy and Heat Transfer

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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

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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

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Conservation of Energy
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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

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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

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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 climate

Substance Specific Heat (J/gK)
Water (liquid) 4.183
Ice 2.050
Wood 0.420
Sand 0.835
Air 1.012
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Latent Heat

• Latent heat is the amount of energy released or
  absorbed by a substance during a phase change

FOR WATER
2260 J/g released
334 J/g released
SOLID
LIQUID
GAS
2260 J/g absorbed
334 J/g absorbed
SOLID
LIQUID
GAS
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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.

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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

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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

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Conduction

• Conduction is the transfer of heat from molecule
  to molecule within a substance
• Molecules must be in direct contact with each
  other

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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
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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)

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Convection
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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

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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

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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

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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

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Radiation, Convection, and Conduction
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Solar Radiation

• The suns 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)

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Solar Radiation
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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 its northernmost point from
  the equator
• Winter Solstice December 21
• Shortest day of the year in the Northern
  Hemisphere
• Where the sun is at its southernmost point from
  the equator
• How radiation changes with latitude and date

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Solar Radiation Budget
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Energy Budget
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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

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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

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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

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Kirchoffs 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 Earths surface thus
  keeping us warmer then we would otherwise be

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More Examples

• Energy