Chapter 22
Heat Transfer
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
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
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)
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
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
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
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
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
22.3 Radiation
• Sun’s heat is transmitted by radiation
• Radiation: energy transmitted by
electromagnetic waves
• Radiant energy: any energy transmitted by
radiation
Radiation Continued
• In radiation, heat is transmitted in the form of
radiant energy or electromagnetic waves
• Radiant energy
– Radio waves
– Micro waves
- Visible light
- X-rays
• Radiant energy is listed in wavelengths
– Longest to shortest
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
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
Emission of Radiant Energy Continued
• Sun’s 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
22.5 Absorption of Radiant Energy
• Everything emits energy
• Everything absorbs energy from the
environment
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
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
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
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
22.6 Newton’s 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
Newton’s Law of Cooling
Continued
• The colder the object’s surroundings, the faster
the object will be cool
• The temperature difference is small, the rate of
cooling is low and vice versa
• Newton’s law of cooling: states the rate of
cooling is approximately proportional to the
temperature difference between the object and
its surroundings
22.7 Global Warming and the
Greenhouse Effect
• Greenhouse effect: warming of the planet’s
surface due to the trapping of radiation by the
planet’s atmosphere
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
Causes of the Greenhouse Effect
Continued
• Same effect in Earth’s atmosphere
– Surface of Earth absorbs energy
– Reradiates part of the energy in longer
wavelengths
– Long wavelengths cannot escape the Earth’s
atmosphere, warms Earth
• Earth’s temperature depends on the energy
balanced between incoming solar radiation
and outgoing terrestrial radiation
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 Earth’s temperature
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
THE END! 