Transcript 1st Law of Thermodynamics Heat Transfer

1st Law of Thermodynamics
Heat Transfer
Lecture 4
February 15, 2010
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Review from last week
In a cold cloud, all precipitation begins in the form
of snow (ice crystals)
• 5 Main Precipitation Types
1. Rain  drops of liquid water
2. Snow  ice crystals
3. Sleet  frozen rain drops
4. Freezing Rain  rain the freezes on
contact with a cold surface
5. Hail  large pieces of ice
How do we get this variety if the origin of the
precipitation is the same?
Snow
• The surface temperature is
25°F (-4°C) and increases with
height before decreasing.
• However, since the
temperature remains below
freezing at every height, any
precipitation that falls will
remain as snow.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
• Flurries - Light snow falling for short durations.
No accumulation or light dusting
• Showers - Snow falling at varying intensities for
brief periods of time. Some accumulation is
possible.
• Squalls - Brief, intense snow showers
accompanied by strong, gusty winds.
Accumulation may be significant. Snow squalls
are best known in the Great Lakes region.
• Blowing Snow - Wind-driven snow that reduces
visibility and causes significant drifting.
• Blizzard - Winds over 35 mph with snow and
blowing snow reducing visibility to less than ¼
mile for more than 3 hours.
Sleet
• Surface is below freezing
• As snow falls into the layer of air
where the temperature is above
freezing, the snow flakes partially
melt.
• As the precipitation reenters the
air that is below freezing, the
precipitation will re-freeze into ice
pellets that bounce off the ground,
commonly called sleet.
• The most likely place for freezing
rain and sleet is to the north of
warm fronts. The cause of the
wintertime mess is a layer of air
above freezing aloft.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
Freezing Rain
• Freezing rain will occur if the
warm layer in the atmosphere
is deep with only a shallow
layer of below freezing air at
the surface.
• The precipitation can begin as
either rain and/or snow but
becomes all rain in the warm
layer.
• The rain falls back into the air
that is below freezing but
since the depth is shallow, the
rain does not have time to
freeze into sleet.
• Upon hitting the ground or
objects such as bridges and
vehicles, the rain freezes on
contact.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
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 = ½ mv2
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
First Law of Thermodynamics
• Conservation of energy:
q = Δe + w
• The amount of heat (q) added to a
system is equal to the change in
internal energy (Δe) of the system plus
any work (w) done by the system
Heat
• Heat is a form of energy and is the total
internal energy of a substance
• Therefore 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
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
1ºC
• Different objects have different specific
heat values
Substance
Liquid Water
Ice
Wood
Sand
Air
Value (J g−1K−1)
4.183
2.050
0.420
0.835
1.012
• 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
Latent Heat
• Latent heat is the amount of energy
released or absorbed by a substance
during a phase change
FOR WATER:
Lowest energy
334 J/g
2260 J/g
released
released
SOLID
LIQUID
334 J/g
SOLID
absorbed
GAS
2260 J/g
LIQUID
LIQUID
absorbed
GAS
Highest energy
Example 1: Getting out of a swimming pool
• In the summer, upon exiting a swimming pool
you feel cool. Why?
• 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.
Example 2: Citrus farmers
• An orange crop is destroyed if
temperatures drop below freezing for
a few hours.
• To prevent this, farmers spray water
on the orange trees. 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.
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 latent
heat during cloud
formation drives
many atmospheric
processes.
Types of Heat Transfer
• Heat can be transferred by:
–Conduction
–Convection
–Advection
–Radiation
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 from
the flame.
•Molecules at this end of the road will gain
energy and begin to vibrate faster
•As they do, their temperature increases
and 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.
Conduction
• The measure of how well a substance can
conduct heat depends on its molecular
structure.
Substance
Heat
Conductivity
Still air at 20 °C
0.023
Water at 20 °C
0.60
Ice
2.1
Granite
2.7
Iron
80
• 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!
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)
• Convection occurs naturally in the
atmosphere
• On a sunny day, the Earth’s 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.
Convection
• As the warm air rises, the heavier cold air flows
toward 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
• The warm thermals cool as they
rise.
• In fact, the cooling rate as a parcel
rises can be calculated
– If the thermal consists of dry air, it
cools at a rate of ~10°C/km as it
rises. This is called the lapse rate.
• Convection is one process by
which clouds can form.
• As the temperature of the thermal
cools, it may reach a point where
it reaches saturation (the temp.
and dewpoint are the close to the
same)
• Thermals condense and form a
cloud.
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 air
toward a region of colder air
– Cold air advection (CAA): wind blows cold air
toward a region of warmer air
“Cold Air
Advection”
“Warm Air
Advection”
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
• The 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
• 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 wavelengths are red.
Radiation
Emitted 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 Albedo
represents 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.
Examples of Heat Transfer
• http://www.wisconline.com/objects/ViewObject.aspx?ID=SCE3
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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
• Thus 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
Solar Radiation Budget
Earth-Atmosphere Energy Balance