Transcript 1st Law of Thermodynamics Heat Transfer

1st Law of Thermodynamics
Heat Transfer
Lecture 6
October 14, 2009
Review
• GOES:
Geostationary Operational Environmental Satellite
– Maintain constant altitude (~36,000 km) over a single
point, always over the equator
– Imagery is obtained approximately every 15 minutes
– Generally has poor spatial resolution but good
temporal resolution
• POES: Polar Operational Environmental Satellites
– circular orbit moving from pole to pole closer to the
Earth (879 km) than GOES
– Sees the entire planet twice in a 24 hour period.
– Good Spatial Resolution: Lower altitude results in
higher resolution images
– Poor Temporal Resolution: Over any point on Earth,
the satellite only captures two images per day.
Review
• Visible
– Measures visible light (solar radiation, 0.6 m) which is reflected
back to the satellite by cloud tops, land, and sea surfaces.
– Thus, visible images can only be seen during daylight hours!
• Infrared (IR)
– Displays infrared radiation (10 to 12 m) emitted directly by
cloud tops, land, or ocean surfaces.
– Wavelength of IR depends solely on the temperature of the
object emitting the radiation
– Advantage: You can always see the IR satellite image
• Water Vapor (WV)
– Displays infrared radiation emitted by the water vapor (6.5 to
6.7 m) in the atmosphere
– Can determine dry layers from moist layers in the atmosphere
Review
RADAR
 Radar uses electromagnetic radiation to sense
precipitation.
 Sends out a microwave pulse (wavelength of
4-10 cm) and listens for a return echo.
 If the radiation pulse hits precipitation
particles, the energy is scattered in all
directions
 The intensity of precipitation is measured by
the strength of the echo, in units of decibels
 Doppler Radar: can determine velocity as well
as reflectivity
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.
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