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## Energy and Heat Transfer

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### Energy and Heat Transfer AOS 101 Discussion Sections 302 and 303 www.spirit-alembic.com – PowerPoint PPT presentation

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Title: Energy and Heat Transfer

1
Energy and Heat Transfer
• AOS 101 Discussion Sections 302 and 303

2
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

3
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

4
Conservation of Energy
5
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

6
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

7
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
8
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
9
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.

10
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

11
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

12
Conduction
• Conduction is the transfer of heat from molecule
to molecule within a substance
• Molecules must be in direct contact with each
other

13
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
14
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)

15
Convection
16
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

17
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

18
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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22
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

23
Radiation, Convection, and Conduction
24
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)

25
Solar Radiation
26
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

27
Solar Radiation Budget
28
Energy Budget
29
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

30
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

31
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

32
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

33
More Examples
• Energy
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