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Chapter 10
Heat transfer
&
Change of Phase
3 methods of heat
transfer
1. Conduction
2. Convection
3. Radiation
10.1- Conduction
Conduction is the
transfer of heat energy
in a substance caused
by the collision of the
particles in the
substance.
Good and bad
conductors
Solids whose atoms or
molecules have loosely
held electrons are GOOD
conductors of heat.
Silver is the BEST
conductor of heat.
Wood, wool, paper, cork,
and plastic foam are poor
conductors of heat, since
their electrons are firmly
attached to their molecules.
Materials like these are
called insulators.
fun fact
A “firewalker” can walk across hot wooden
coals, because wood is such a poor
conductor of heat.
air keeps us warm!
Air is a very poor conductor. Many items
that are good insulators (and poor
conductors) are this way because they
have many air spaces.
snow and heat?
Snow is a poor conductor of heat.
Many animals use this to their advantage when
burrowing in the snow to help reduce the loss of their
own body heat.
insulation
in
homes
Insulation in homes slows the rate at which heat
flows.
It keeps warm air in the house longer in the
winter, and keeps heat from the outside out of the
inside during the summer.
10.2- convection
Liquids and gases transfer heat mainly by
convection, which is the transfer by motion
of a fluid in currents.
convection in fluids
Convection occurs in all fluids.
As the fluid is heated, the molecules close to
the heat source begin moving faster and
spread out, becoming less dense. Due to the
decreased density, the warmer fluid rises up,
while cooler, denser fluid sinks to the bottom.
•
Convection currents in the air produce
winds.
10.3- Radiation
Radiation is the
transfer of thermal
energy by mean of
electromagnetic
waves.
An example of
radiation is the sun
warming the Earth.
Energy transferred
by radiation is called
radiant energy.
the electromagnetic
spectrum
EM waves include radio waves, infrared
waves, and visible light waves.
The differences between them are the
wavelengths.
The wavelength of radiation is related to the frequency of
radiation.
Frequency is the rate of vibration of a wave source.
The lower the frequency, the longer the wave.
All substances
above absolute zero
emit radiant energy.
The average
frequency of the
radiant energy is
directly proportional
to the temperature of
the emitter. F ~ T
The sun is hot, therefore, it emits a higher
frequency energy.
The Earth is cool, therefore, it emits a lower
frequency (infrared) energy, called terrestrial
radiation.
All objects continually
emit radiant energy in a
mixture of frequencies.
In addition to emitting
energy, all objects
absorb energy.
Good emitters are also
good absorbers.
An object that absorbs radiant
energy looks dark. If it absorbs all
the radiant energy on it, it looks
black.
A black container full of hot water
will cool faster than a white or
shiny one.
painting your house a light
color is good because:
Lightly colored buildings reflect much of
the incoming radiant energy. This helps
them stay cooler in the summer.
Lightly colored objects are also poor
emitters, so they retain much of their
internal energy during the winter, and stay
warmer.
Phase Changes
phases of matter
1. Solid
2. Liquid
3. Gas
4. Plasma: occurs when molecules from a
gas break into ions and electrons.
(Illuminating gas found in fluorescent and
other vapor lamps)
Phases of Matter
Phase
Definite Volume?
Definite Shape?
Solid
Yes
Yes
Liquid
Yes
No (takes the shape of its container)
Gas
No
No (takes the shape of its container)
10.5- Evaporation and
Sublimation
Evaporation:
Process by
which a liquid
changes into a
gas.
Sublimation: A
solid evaporates
directly to a gas.
(Reason why ice
cubes left in a
freezer for a long
time get smaller).
Melting and Freezing
•
Melting: When an object goes from
a solid to a liquid
•
Freezing: When an object goes
from a liquid to a solid
10.6- Condensation
Opposite of evaporation.
Condensation is the changing of
a gas to a liquid.
Deposition
•
Deposition: Changing from a gas directly to a
solid
•
Ex: Frost forming on a leaf
10.7 & 10.8
Boiling, Freezing, and Melting Points
Boiling Point: The temperature at which a
substance boils
Melting Point: The temperature at which a
substance melts.
Freezing Point: The temperature at which a
substance freezes.
10.9
Heat of Vaporization & Fusion
Heat of fusion: the amount of energy needed to
change any substance from solid to liquid (and
vice versa).
For water: 335 joules per gram
Heat of vaporization: the amount of energy
needed to change any substance from liquid to
gas (and vice versa).
For water: 2255 joules per gram
Phase Change
Triangle
Gains Heat Energy
Loses Heat Energy
Solid
Gas
Liquid