Thermal Physics

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The transfer of heat

• Convection
• Conduction
• Radiation

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Important terms

• Conduction
• Thermal conduction
• Thermal insulator
• Thermal conductivity
• Convection
• Refrigerator

• Radiation
• Infrared radiation
• Blackbody
• Absorption
• Emission
• vacuum flask

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Examples

• Heat is defined as energy in transit.
• Heat moves from place to place by one of the
  three mechanisms
• Convection conduction radiation
• Examples
• You can keep a pot of hot coffee on a hot stove.
• Heating unit, air conditioner,
• You feel hot if you stand near an open fire.

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Convection

• Convection is the process in which heat is
  carried from place to place by bulk movement of a
  fluid.
• Natural convection forced convection
• Examples
• Home heating system.
• The smoke from a campfire rises.
• When part of a fluid is warmed, the surrounding
  cooler and denser fluid exerts a buoyant force on
  the warm fluid and pushes it up. The warm fluid
  is pushed up and replaced by the surrounding
  cooler fluid. This cooler fluid, in turn, is
  warmed and pushed upward.

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Convection

• Refrigerator
• Air cooled by the cooling source sinks to the
  bottom of the fridge.
• What if the cold source is put in the bottom?

• Heating system,
• Heated air near the heat source rises up and then
  sinks when it cools again.
• What happens when the heat source is placed on
  the top?

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Heat conduction

• Conduction is the process whereby heat is
  transferred by molecules in a material.
• Fast moving molecules at a higher T collide with,
  and transfer some energy to, less energetic
  molecules.
• Any bulk motion of the material playing no role
  in this kind of heat transfer.
• Examples
• The metal handle becomes hot when the pan is
  heated, with the heat transferred from the burner
  to the handle.

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Thermal conduction

• The internal energy of an object is the sum of
  the potential energies due to the electrostatic
  bonding within the lattice, and the vibrational
  kinetic energies.
• The hotter atoms and molecules vibrate with
  greater energy than cooler ones. By means of
  collisions, the more energetic molecules pass on
  some of their energy to their less energetic
  neighbors.

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Conductors and insulators

• Thermal conductors Materials that conduct heat
  well are called thermal conductors, such as
  metals which have many free electrons.
• Metals, aluminum, copper, gold silver,
• Why do some cooking pots have copper coating on
  the bottoms?
• Thermal insulators Substances that conduct heat
  poorly are called thermal insulators such as
  nonmetals which have less free electrons.
• Wood, glass, and most plastics,
• Air, body tissue

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The applications of insulators

• Air
• Insulation in homes and in material for warm
  clothing use air.
• Double-pane windows use air trapped between two
  glass panes to reduce conductive heat loss.
• Body tissue
• The inner core of the body can be kept warm in a
  cold environment, as shown in the next slide,

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Isotherms in the body

• P292

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Heat radiation

• The process of transferring energy via
  electromagnetic waves is called radiation. It
  does not need a transport medium like in
  convection conduction.
• All bodies continuously radiate energy in the
  form of electromagnetic waves.
• In the transfer of energy by radiation, the
  absorption of electromagnetic waves is just as
  important as the emission.

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Examples in Radiation

• For ordinary temperatures, the radiation is in
  the infrared region.
• Generally, an object does not emit much visible
  light until the temperature of the object exceeds
  about 1000K.
• Absorptions
• Anyone who is standing by a roaring fire has
  experienced the radiated heat.
• The sunbathers feel hot, because their bodies
  absorb energy from the suns electromagnetic
  waves.

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Absorbers and emitters

• Blackbody is used when referring to an object
  that absorbs all the electromagnetic waves
  falling on it.
• A material that is a good (poor) absorber, is
  also a good (poor) emitter.
• Black surface, a good absorber, uncomfortable to
  wear in summer,
• Light color, a poor absorber, cooler to wear in
  summer.

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Radiation energy

• The Stefan-Boltzmann law of radiation
• The radiant energy Q, emitted by an object from a
  surface area A, at the temperature T, with an
  emissivity e is given by
• Sigma
  Stefans constant
• The emissivity changes with the wavelength.
• Black surface, e1
• Shining surface, smaller e,

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Problems

• Why clear nights are cooler than cloudy nights?
• Clear night
• The radiation from the earth, T300K,
• The radiation from space, little (T0K),
• Cloudy night
• The radiation from the earth, absorbed
• The radiation from the cloud, trapped

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Greenhouse effect

• The sun has a surface T of 6000K, its radiation
  is most intense at ?max 4.84E-7m and easily
  passes through the glass.
• In the greenhouse at T of 300K, the greatest
  radiation intensity is at ?max 96.6E-7m
  (infrared) and will not easily pass through
  glass.
• The incident radiation enters the greenhouse and
  the radiation from the objects within is trapped
  inside, the greenhouse warms up.

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What is the function of Ozone?

• Ozone is created in the upper atmosphere by solar
  radiation acting on oxygen.
• It absorbs all the incoming radiation with
  wavelength less than 0.3um, which is lethal to
  life.
• Any decrease in ozone could lead to increased
  incidence of skin cancer and eye cataracts (???),
  and to crop damages.

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A vacuum flask (thermos bottle)

• The flask is able to effectively block heat
  transfer from the outside either through the
  walls or through the top.
• A vacuum flask restricts all three kinds of heat
  transfer processes.

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• The vacuum eliminates conduction and convection
  between the glass walls.
• The glass walls and the cork or plastic stopper
  conduct little heat through the top of the flask.
  
• The silvered layer on the inner glass wall is a
  poor emitter of radiation while any radiation is
  likely reflected back by the silvered lining.

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Comfort temperature for people?

• What is the temperature for a lightly-dressed
  person to feel comfortable?
• Our body are constantly generating thermal energy
  at a rate of about 100W when sitting at rest.
• Most of this thermal energy is generated
  relatively deep inside the body. It must be
  transferred to the surrounding in a number of
  ways.

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