Lecture 1- Thermodynamics
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Transcript of Lecture 1- Thermodynamics
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1st Law of Thermodynamics
Heat Transfer
Lecture 1
August 25,2011
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Energy
Energy is the ability or capacity to do work
on some form of matter
Work is done on matter when matter is
either pushed, pulled, or lifted over some
distance
Potential energy how much work that
an object is capable of doing
PE = mgh
Kinetic energy the energy an object
possesses as a result of its motion
KE = mv2
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Laws of Thermodynamics
1st Law of Thermodynamics Energycannot be created or destroyed.
Energy lost during one process must equalthe energy gained during another
2nd Law of Thermodynamics Heat canspontaneously flow from a hotter object toa cooler object, but not the other way
around. The amount of heat lost by the warm
object is equivalent to the heat gained bythe cooler object
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First Law of Thermodynamics
Conservation of energy:q = e + w
The amount of heat (q) added to asystem is equal to the change in
internal energy (e) of the system plus
any work (w) done by the system
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Heat
Heat is a form of energy and is the totalinternal energy of a substance
Therefore the 1st law states that heat is reallyenergy in the process of being transferredfrom 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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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)
The heat capacity of a substance per unit
mass is called specific heat.
Can be thought of a measure of the heat
energy needed to heat 1 g of an object by
1C
Different objects have different specific
heat values
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1 g of water must absorb about 4 times asmuch heat as the same quantity of air to raiseits temperature by 1 C
This is why the water temperature of a lake orocean 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 Value (J g1K1)
Liquid Water 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 energyreleased 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
FOR WATER:
Lowest energy
Highest energy
SOLID
SOLID
LIQUID
LIQUID
GAS
GAS
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Example 1: Getting out of a swimming pool
In the summer, upon exiting a swimming pool
you feel cool. Why?
Drops ofliquid 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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Example 2: Citrus farmers
An orange crop is destroyed iftemperatures drop below freezing for
a few hours. To prevent this, farmers spray water
on the orange trees. Why?
When the temperature drops below32oF, liquid water freezes into ice.
This liquid to solid phase changecauses energy to be released to thefruit.
Thus, the temperature of the orangeremains warm enough to preventruin.
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Example 3: Cumulus 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.
The release of latentheat during cloudformation drivesmany atmospheric
processes.
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Types of Heat Transfer
Heat can be transferred by:
ConductionConvection
AdvectionRadiation
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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
If you put one end of a metal rod over a
fire, that end will absorb the energy fromthe flame.
Molecules at this end of the road will gain
energy and begin to vibrate faster
As they do, their temperature increasesand they begin to bump into the molecules
next to them.
The heat is being transferred from the
warmer end to the colder end, and
eventually to your finger.
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Conduction
The measure of how well a substance can
conduct heat depends on its molecularstructure.
Air does not conduct heat very well
This is why, in calm weather, the hot ground
only warms the air near the surface a few
centimeters thick by conduction!
SubstanceHeat
Conductivity
Still air at 20 C 0.023
Water at 20 C 0.60
Ice 2.1
Granite 2.7
Iron 80
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Convection Convection is the transfer of heat by the
mass movement of a fluid (such as water andair) in the vertical direction (up and down)
Convection occurs naturally in the
atmosphere On a sunny day, the Earths surface is heated
by radiation from the Sun.
The warmed air expands and becomes less
dense than the surrounding cold air.
Because the warmed air is less dense
(weighs less) than cold air, the heated air
rises.
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As the warm air rises, the heavier cold air flowstoward the surface to replace the rising air.
This cooler air becomes heated in turn and rises.
The cycle is repeated.
This vertical exchange of heat is called convection
and the rising air parcels are known as thermals
Convection
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Convection The warm thermals cool as they
rise.
In fact, the cooling rate as a parcelrises can be calculated If the thermal consists of dry air, it
cools at a rate of ~10C/km as itrises. This is called the lapse rate.
Convection is one process bywhich clouds can form.
As the temperature of the thermalcools, it may reach a point where
it reaches saturation (the temp.and dewpoint are the close to thesame)
Thermals 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
Happens frequently on Earth
Two types:
Warm air advection (WAA): wind blows warm airtoward a region of colder air
Cold air advection (CAA): wind blows cold airtoward a region of warmer air
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Cold AirAdvection Warm Air
Advection
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All things with a temperature above
absolute zero emit radiation Radiation allows heat to be transferred
through wave energy
These waves are called electromagneticwaves The wavelengths of the radiation emitted
by an object depends on the temperatureof that object (i.e., the sun mainly emitsradiative energy in the visible spectrum,and the earth emits radiative energy in theinfrared spectrum).
Shorter wavelengths carry more energy
than longer wavelengths
Radiation
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A photon of ultra-violet radiation carries moreenergy than a photon of infrared radiation.
The shortest wavelengths in the visible spectrumare purple, and the longest wavelengths are red.
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RadiationEmitted radiation can be:
Absorbed
Increasing the internal energy of the gas molecules.
Reflected
Radiation is not absorbed or emitted from an object but it
reaches the object and is reflected back. The Albedorepresents the reflectivity of an object and describes the
percentage of light that is sent back.
Scattered
Scattered light is deflected in all directions, forward,backward, sideways. It is also called diffused light.
Transmitted
Radiation not absorbed, reflected, or scattered by a gas. The
radiation passes through the gas unchanged.
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Examples of Heat Transfer
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Kirchoffs Law
Good absorbers of a particular wavelength aregood emitters at that wavelength and vice versa
Our atmosphere has many selective absorbersCarbon Dioxide, Water Vapor, etc
These gases are good at absorbing IR radiationbut not solar radiation
Thus these gases are called greenhouse gases
due to the fact they help to absorb and reemit IRradiation back toward the Earths surface thuskeeping us warmer then we would otherwise be
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Solar Radiation Budget
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Earth-Atmosphere Energy Balance
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