Heat Transfer

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Chapter 22

• Heat Transfer

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The Big Idea

• Heat can be transferred by conduction, by
  convection, and by radiation
• Heat transfer from warmer to cooler objects
• Will reach same temperature
• Thermal equilibrium
• Three ways to equalize temperatures
• Conduction
• Convection
• Radiation

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22.1 Conduction

• Conduction transfer of energy with in materials
  and between different materials that are in
  direct contact
• Conductors materials that conduct heat well
• Metals are the best conductors
• Silver is the most conductive
• In conduction, collisions between particles
  transfer thermal energy, without any over
  transfer of matter

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Conductors

• Materials that have loose outer electrons are
  good conductors of heat
• Metals best conductors of heat and electricity
• If materials are in the same vicinity, they will
  have the same temperature (room temperature)

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Insulators

• Liquids and gases make good insulators
• Insulator any material that is a poor conductor
  of heat and that delays the transfer of heat
• Heat is energy and is tangible

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22.2 Convection

• Conduction involves transfer of energy from
  molecule to molecule
• Energy moves but molecules do not
• Convection heat transfer by movement of the
  heated substance itself
• In convection, heat is transferred by movement of
  the hotter substance from one place to another

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Convection Continued

• Convection occurs in all fluids
• Convection works
• Fluid is heated
• Expands of fluid
• Fluid becomes less dense and rises
• Application of Archimedes's principle
• Convection currents have influence on air

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Moving Air

• Convection currents stirring produce winds
• Uneven absorption of heat causes uneven heating
  near the surface and creates convection currents
• During the day, land is warmer than the air and
  produces a breeze. At night, the land is cooler
  than the water so air flows in the opposite
  direction

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Cooling Air

• Rising warm air expands
• Less atmospheric pressure squeezes on higher
  altitudes
• When air expands it cools
• Molecules get speed from other molecules
• With expanding air the average speed of molecules
  decreases and the air cools

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22.3 Radiation

• Suns heat is transmitted by radiation
• Radiation energy transmitted by electromagnetic
  waves
• Radiant energy any energy transmitted by
  radiation

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Radiation Continued

• In radiation, heat is transmitted in the form of
  radiant energy or electromagnetic waves
• Radiant energy
• Radio waves - Visible light
• Micro waves - X-rays
• Radiant energy is listed in wavelengths
• Longest to shortest

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22.4 Emission of Radiant Energy

• All substances continuously emit radiant energy
  in a mixture of wavelengths
• Low temperature objects emit long waves
• High temperature objects emit short waves
• An infrared thermometer measure the infrared
  radiant energy emitted by a body and converts it
  to temperature

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Emission of Radiant Energy Continued

• People emit low frequency of infrared
• Types of light
• Hot enough, visible light
• 500 oC, red light
• Higher then 500 oC, yellow light
• Stellar radiation radiant energy emitted by
  stars
• White hot, blue hot, red hot

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Emission of Radiant Energy Continued

• Suns temperature (5500 oC) emits radiant energy,
  visible on electromagnetic spectrum
• Terrestrial radiation radiant energy that is
  emitted by the Earth
• Infrared waves, not visible to us
• Radiant energy encounters objects, some absorbed
  some reflected
• Absorbed increases internal energy

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22.5 Absorption of Radiant Energy

• Everything emits energy
• Everything absorbs energy from the environment

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Absorption and Emission

• Objects absorb and radiate energy at the same
  rate
• Goes to thermal equilibrium with its environment
• When an object radiates more energy it has a new
  thermal equilibrium
• Good emitters of radiant energy are also good
  absorbers poor emitters are poor absorbers

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Absorption and Emission Continued

• Dark objects remain hotter than their
  surroundings on a hot day and cool faster at
  night
• All objects in thermal contact reach thermal
  equilibrium

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Absorption and Reflection

• Absorption and reflection are opposite processes
• Good absorber reflects little radiant energy
• Appear dark
• Radiant energy that enters an opening has little
  chance of leaving before it is completely absorbed

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Absorption and Reflection Continued

• Good reflectors are poor absorbers
• Light colored objects reflect more light and heat
  and dark colored objects absorb more light and
  energy
• Sun during the day is a net absorber, at night it
  is a net emitter

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22.6 Newtons Law of Cooling

• An object hotter then its surroundings eventually
  cools to match the surrounding temperature
• Rate of cooling how many degrees change per unit
  of time
• The rate of cooling of an object depends on how
  much hotter the object is than the surroundings

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Newtons Law of Cooling Continued

• The colder the objects surroundings, the faster
  the object will be cool
• The temperature difference is small, the rate of
  cooling is low and vice versa
• Newtons law of cooling states the rate of
  cooling is approximately proportional to the
  temperature difference between the object and its
  surroundings

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22.7 Global Warming and the Greenhouse Effect

• Greenhouse effect warming of the planets
  surface due to the trapping of radiation by the
  planets atmosphere

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Causes of Greenhouse Effect

• Two concepts
• All things radiate heat and wavelength depends on
  temperature
• High temperatures have short waves
• Low temperatures have long waves
• Transparency of things depends on the wavelength
  of radiation

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Causes of the Greenhouse Effect Continued

• Same effect in Earths atmosphere
• Surface of Earth absorbs energy
• Reradiates part of the energy in longer
  wavelengths
• Long wavelengths cannot escape the Earths
  atmosphere, warms Earth
• Earths temperature depends on the energy
  balanced between incoming solar radiation and
  outgoing terrestrial radiation

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Consequences of the Greenhouse Effect

• Over years, solar radiation that comes to Earth
  equals terrestrial radiation Earth emits
• Materials such as fossil fuels changes absorption
  and reflection of solar radiation
• Not if energy is solar, wind, water
• Can change Earths temperature

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Consequences of the Greenhouse Effect Continued

• The near unanimous view of climate scientists is
  that human activity is a main driver of global
  warming and climate change
• Water vapor is the main greenhouse gas
• CO2 is the most rapidly increasing

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THE END! ?