Download Chapter 22 - Heat Transfer p.430 Discover activity If several objects

Chapter 22 - Heat Transfer
p.430
Discover
activity
If several objects of different temperatures are brought near to one
another, eventually, they will all have the same temperature. There
are three ways that heat is transferred: conduction, convection, and
22.1
Conductio
n
Conduction is the movement of energy within one substance or
between materials that are touching. Conductors are simply
materials that conduct or move heat easily – like metals.
”In conduction, collisions between particles transfer thermal
energy without any overall transfer of matter.” When a metal is
heated the atoms move more rapidly and collide more frequently with
nearby atoms causing them to gain energy. Because metals have free
electrons they conduct heat and electricity easily.
radiation.
Touching a piece of metal will feel colder than touching a piece of
wood that is the same temperature. Heat flows easily from your hand
to the metal, but not the wood. This is because the electrons in the
wood are tightly bound to the atoms, making it a poor conductor – or
an insulator.
An insulator is simply a material that conducts heat poorly or delays
the transfer of heat. Air and materials containing air spaces (such as
down) make good insulators.
While heat is energy, cold is not a real thing but the absence of heat –
like dark being the absence of light. Insulation in houses slows down
the flow of heat from the house.
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22.2
Conduction moves energy
Convection without moving matter.
Convection is the movement of
test tube
energy by moving the heated
with ice on matter, for example, currents
bottom
in a fluid.
”In convection, heat is
transferred by movement of
the hotter substance from one
place to another.”
Convection or the movement
of energy through fluid
materials depends largely on
buoyancy and gravity. For this
reason matches or candles on
the space station will quickly
extinguish for lack of oxygen.
Warmer fluids are less dense and are buoyed upward while cooler,
denser fluids fill in the space below them.
Convection currents are the source of wind patterns near the shore.
The land heats up more in the day and cools more in the night while
the air over water changes less. So winds go from the cool sea to the
warm land in the day and from the cool land to the warmer sea in the
night.
22.3
Radiation
polar
During the daytime warm air rises and expands. As air expands, it
cools. There is lower pressure near the surface of the land and high
pressure aloft. (We can experience the cooling of air as it expands by
blowing through pursed lips or releasing air from a bicycle tire.)
Heat from the sun cannot reach Earth through either conduction or
convection because there is no matter between us. Energy
transferred or transmitted by electromagnetic waves is called
radiation. The energy transmitted through radiation is called radiant
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regions are
cooler
because of
angle of
sun. Demo
angled
paper with
‘rain’ or
horizontal
energy.
”In radiation, heat is transmitted in the form of radiant energy or
electromagnetic waves.”
22.4
Emission of
Radiant
Energy
”All substances continuously emit radiant energy in a mixture of
wavelengths.”
Objects that are cooler are moving less and make longer waves, such
as microwaves or radio. Object with higher temperatures emit waves
that have higher frequencies and smaller wavelengths. Think about
an electric stove heating up. It begins to emit infrared waves and
then glows as it increases in temperature.
The frequency of radiant energy is directly proportional to
temperature.
Stellar radiation is the radiant energy emitted by stars. The color of
the star can tell us its temperature. Red stars are much cooler than
blue stars. We know this because blue is produced by more energetic
movement of atoms producing higher frequency light.
The earth emits radiation, too, but this is at a much lower frequency
than the radiation from the sun. This is called terrestrial radiation.
Most of this terrestrial radiation is in the infrared part of the
spectrum that is invisible to our eyes.
22.5
Absorption
of Radiant
Radiation from the sun is produced by thermonuclear fusion. This is
hydrogen being fused into helium atoms. The Earth produces
radiation from radioactive decay in its interior (fission). Both the sun
and the Earth produce radiant energy, but Earth’s radiation is at low
infrared frequencies. Some radiant energy is absorbed and some is
reflected.
All objects both absorb and radiate or emit energy. When the rate of
absorption and the rate of emission are the same, the object is in
thermal equilibrium with its environment. If the objects absorbs
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Energy
Collom’s
white roof
Burning
paper
w/glass
22.6
Newton’s
Law of
Cooling
more than it emits, the
temperature increases. If
absorption decreases relative
to radiation, then temperature
will be lower.
”Good emitters of radiant
energy are also good absorbers;
poor emitter are poor
absorbers.”
Radiant energy can reflect repeatedly until the energy is absorbed
and nothing reflects. A good reflector typically absorbs little radiant
energy. Light colored objects or clothes reflect energy without
absorbing much and are cooler. The surface of Earth absorbs energy
in the day and emits it at night. Clear nights will be colder because
clouds reflect energy back to the surface.
 In New Mexico it is much cooler in the shade than in the sun.
Not so in Florida. Why is this?
Rate of cooling is how many temperature degrees of change happen
in a unit of time. The rate of cooling depends on how much
difference there is between objects.
“The colder an object’s surroundings, the faster the object will
cool.”
Newton’s law of cooling says that the rate of cooling is proportional
to the temperature difference.
Chilling wort from a boil to 80°F shows a rapid temperature change at first
because the difference is great. This slows as the wort approaches the cool
water temperature.
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22.7 Global
Warming
and the
Greenhous
e Effect
try colored
films to
show
selective
transparen
cy
When radiation is trapped by the atmosphere and causes the planet’s
surface to warm, this is the greenhouse effect. (To an extent
understanding the greenhouse effect depends on understanding the
idea of
transparency
which we will
discuss in depth
in chapter 27.)
The sun being
hot (high energy
& atomic
movement)
produces short wavelength radiation that passes through the
atmosphere and is absorbed by Earth. The Earth re-emits this energy
but at a longer wavelength—because it is cooler. Our atmosphere is
transparent to
the short wave
radiation from
the sun but more
opaque to the
longer
wavelength
energy being
emitted from the
Earth. Certain
molecules such as
methane and CO2
increase
temperatures by
holding more reemitted
terrestrial radiation. Over long averages, the amount of heat in the
form of terrestrial radiation leaving the earth balances the solar
radiation coming in, so the temperature remains stable and supports
life. ”The near unanimous view of climate scientists is that human
activity is a main driver of global warming and climate change.”
Increased levels of CO2 also leads to more water vapor in the air. As
more energy is trapped patterns in storms and climate such as rainfall.
Remember these are long term patterns or cycles, not short-term
weather. The Rocky Mountains experienced flooding in the fall of
2013 even though our continuing pattern is a long-term drought.
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