1st Law of Thermodynamics Heat Transfer

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1st Law of ThermodynamicsHeat 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?

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Snow

• The surface temperature is 25F (-4C) 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
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• 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. 

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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
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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
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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 ½ mv2

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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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First Law of Thermodynamics

• Conservation of energy
• q ?e w
• The amount of heat (q) added to a system 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 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

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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 1ºC
• Different objects have different specific heat
  values

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• 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

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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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• 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.

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• 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.

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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.

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Types of Heat Transfer

• Heat can be transferred by
• Conduction
• Convection
• Advection
• Radiation

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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 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.

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Conduction

• The measure of how well a substance can conduct
  heat depends on its molecular structure.
• 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!

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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)
• 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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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

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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 10C/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.

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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 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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Cold Air Advection
Warm Air Advection
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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
• 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

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• 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.

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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.

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Examples of Heat Transfer

• http//www.wisc-online.com/objects/ViewObject.aspx
  ?IDSCE304

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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
• Thus 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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Solar Radiation Budget
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Earth-Atmosphere Energy Balance
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