Thermal Energy and Heat Transfer
Thermal Energy - Kinetic Molecular Theory
Heat is the thermal energy of a substance. Thermal
energy is really the kinetic energy of the individual
molecules as they vibrate, move from one place to another
(not in a solid) or rotate within a substance.

All matter is made up of small particles (atoms and
molecules) that are in constant motion.

Atoms and molecules exert forces on one another to
keep them a certain distance apart. Moving closer
causes for repulsion, moving farther apart causes
attraction. But the distances between molecules and
the strength of the force between them is responsible
for the three physical states of matter: solid, liquid,
and gas.

If an object heats up the particles have more kinetic
energy and therefore move faster.

Energy transfers from one particle to another by
collisions. (See diagram below)
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Thermal Energy and Temperature
Temperature: a system of measuring the average kinetic
energy of all of the particles in a quantity of matter.
Thermometer: a device for measuring temperature that
was originally based on the thermal expansion (increase in
the volume of a substance when it is heated) of
substances. Thermometers must be calibrated with two
reference points.
Fahrenheit: a temperature scale
used in the old imperial system
of measure. (freezing point of
water 32°F, boiling point 212°F)
Celsius: the metric measurement
for temperature. (freezing point
of water 0°C, boiling point
100°C.
Kelvin: the scale designed with
the lowest possible temperature
being 0°K (absolute zero) where
it is believed that all molecular
motion stops.
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Thermometers may also be engineered in different ways.
Heat vs. Temperature
Which has a greater
temperature, the cup or
the kettle?
Which has the greatest
heat?
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Thermal Energy Transfer
There are three ways that heat (thermal energy) is
transferred.
(A) Conduction, (B) Convection, (C) Radiation
(A) Conduction
Conductor is a material that allows heat to transfers
easily through it. Metals are good conductors.
A Heat insulator does not allow easy heat transfer.
Heat Conduction is the process of transferring heat by
particle collision. (See the diagram and table below)
Here heat is conducted from the hot
end to the cold end by molecular
collisions.
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To Conductivity is a relative scale that
describes how well heat can be
conducted in a substance. The
greater the conductivity the easier
heat is conducted.
Copper is almost twice the
conductivity of aluminum which is why
it is used on the bottom of many
cooking pots.
(B) Convection
Convection is the process of transferring heat by a
circulating path of fluid. The circulating path of fluid is
called of convection current. Any part of the fluid that
heats up becomes less dense, leaving cooler more
dense fluid to take its place.
Baseboard heaters
use convection
currents to heat a
room.
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Sea Breezes and land
breezes are really
caused by convection
currents.
(C) Radiation
Radiation transfers heat energy through a wave form of
electromagnetic radiant energy. Wave length is called
infrared radiation. Warm substances radiate infrared
radiation where the energy is either absorbed, reflected
from, or transmitted through the material that receives
this energy. Infrared radiation (heat radiation) is part of
the electromagnetic spectrum.
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Controlling Heat Transfer:
To control heat transfer all three forms of heat transfer must be
considered. The idea is to reduce heat transfer by all three forms.
This is done by:
For Conduction a better insulator is used between the hot and cold
areas. (Example: Use of styrofoam)
For Convection all fluid (water or air) is removed from the area by
either covering or surrounding the hot substance. (Example: Double
glazed windows with air removed between them)
For Radiation a reflector is used to re-reflect radiate heat energy back
towards the hot substance. (Example: The use of aluminum foil in
cooking to reflect radiant heat)
All three examples can be shown in one product,
the thermos bottle.
Diagram of Thermos Bottle
A thermos is a glass envelope holding a vacuum.
Inside a thermos is glass, and around the glass is a
vacuum. The glass envelope is fragile, so it is
encased in a plastic or metal case.
A thermos then goes one step further. The glass is
silvered (like a mirror) to reduce infrared radiation
loss by re-reflecting it inside. The combination of a
vacuum and the silvering greatly reduces heat transfer by convection,
conduction and radiation.
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Activity: Look at pages 258 – 259. Make point form notes (with
diagrams) to describe how heat transfer is effected in windows,
mittens/gloves and with the Greenhouse effect.
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Heat Exchange and Changes of State
When heat is added to a substance (like a pot of water on the stove)
the temperature will rise. However there is a difference in the way two
different substances (like cooking oil or water) will respond to heat
being added.
The heat transfer depends on the temperature difference, the mass
of the substance being heated and the type of substance.
How does each one of these three things effect the amount of heat
transferred?
The greater the temperature difference the more/less heat is
transferred.
The greater the mass of the substance being heated the more/less
heat is transferred.
But what about the type of substance? This has to do with what is
called the heat capacity of a substance. In general, the greater the
heat capacity (whatever that is)…
…the faster/slower a substance will heat up or cool down.
Specific Heat Capacity
When a substance is being heated there is no direct way that we can
measure the amount of heat being transferred.
Instead, heat transfer is measured in an indirect way:
specific heat capacity (c) =
c=
energy transferred
(mass)(temperature change)
E
mT
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The units for c, the specific heat capacity, are:
J
(kg)( O C)
So the specific heat capacity is: the amount of heat transferred
when the temperature of 1.0 kg of a substance changes by 1.0 OC. In
general this means that substances with high specific heat capacities
take a long time to heat up or cool down and take a lot of heat transfer
to cause a temperature change.
Some sample specific heat capacities are listed below:
Substance
Specific Heat Capacity
[J/(kgOC)]
Water
4200
Concrete
3000
Ethyl Alcohol
2500
Ethylene Glycol (antifreeze)
2200
Vegetable oil
2000
Air
995
Aluminum
920
Glass
840
Sand
790
Iron
450
Copper
390
Brass
380
Silver
240
Lead
130
Heat can be calculated then by using the specific heat capacity
formula that has been rearranged for E, energy.
E = mcT
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Principle of Heat Transfer/Heat Exchange:
When two different substances at different
temperatures are mixed, the heat released by the
hotter substance equals the heat gained by the
colder substance.
E H lost = E H gained
or
mh ch t h =  (mc cc t c )
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Changes of State and Latent Heat
When heat is transferred into or out of a substance the result could be
one of two things:
(A)
A temperature change (E=mct)
(B) A change in state
There are 5 changes of state
1.
2.
3.
4.
5.
Fusion or melting (solid to liquid)
Vaporization liquid to gas...boiling is a fast form of this.
Condensation or Liquification (gas to liquid)
Solidification or Freezing (solid to liquid)
Sublimation (either solid to gas or gas to solid directly)
The following diagram shows the changes of state.
The diagram is an ideal heating/cooling curve for a pure substance such
as water. Heat is constantly being added to the system yet at certain
points there is no rise in temperature. The heat flow into the system is
not causing a temperature change but rather a change in state.
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Latent Heats of Fusion and Vaporization
The Latent heat of fusion (LF) is the amount of energy needed to melt
1.0 kg of ice. The word, latent, means to lie hidden. This refers to the
fact that during melting the added heat does not cause a rise in
temperature.
EH
LF =
m
Q
LF =
m
Where E is the heat in Joules and m is the mass in kilograms. Latent
heat is measured in joules per kilogram (J/kg)
The Latent heat of vapourization (Lv) is the heat required to vaporize a
liquid.
EH
LV =
m
LV =
Q
m
like LF, LV is measured in joules per kilogram (J/kg).
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Latent Heats of Fusion and Vaporization
Substance
LF (J/kg)
LV (J/kg)
Aluminum
9.0 x 104
1.1 x 107
Ethyl Alcohol
1.1 x 105
8.6 x 105
Iron
2.5 x 105
6.3 x 106
Lead
2.3 x 104
8.7 x 105
Silver
1.1 x 105
2.3 x 106
Water
3.3 x 105
2.3 x 106
If the latent heats for a substance are known (tabulated) then they can be
used to calculate the heat needed to melt or boil away a certain mass of
the substance.
EH = LF m
EH = LV m
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