Thermal Energy & Heat
Chapter 6
Sections 1 & 2
Temperature &
Thermal Energy
Section 1
What is Temperature?
When we think of temperature
we think of how hot or cold
something is.
 Temperature – the measure
of the average value of kinetic
energy of molecules in random
motion.
Think of a glass of water…
Its made of molecules that are in
random motion.
Random = different speeds – some
fast, some slow
If they are moving, the molecules
have KE.
Temperature measures the
average value of their
movement.
The more kinetic energy the
molecules have, the higher the
temperature.
High temp = faster molecules
Low temp = slower molecules
Ex. Water molecules move faster in
a cup of hot water than they do in
cold water.
Thermal Expansion
When molecules speed up,
they move farther apart.
The object expands (stretches)
When molecules slow down,
they move close together.
The object contracts (shrinks)
Solids:
In the summertime power lines will sag,
whereas in the winter they will be
pulled more tightly.
Liquids:
You probably been told never to put a
glass bottle of soda in the freezer.
Why? It can explode and you will have
glass all over you freezer.
Gases:
Place a balloon in a very warm room
and it will expand, then place it in a
very cold room and it will shrink.
The amount of expansion or
contraction depends on:
The type of material
Liquids expand more than solids
The degree to which
temperature changes
The greater the temperature
change, the more expansion or
contraction
Measuring Temperature
Thermometer – a device used
to measure temperature
Consists of a liquid such as
mercury or alcohol sealed inside
a narrow tube with markings
symbolizing degrees.
Temperature changes cause the
liquid to rise and fall as the liquid
expands and contracts.
Temperature Scales
There are three common
scales:
Fahrenheit
Celsius
Kelvin
Each is divided into regular
intervals
Fahrenheit Scale
The most common in the U.S.
Divided into degrees
Fahrenheit (oF)
Freezing point of water = 32oF
Boiling point of water = 212oF
Space between boiling and
freezing is 180 equal degrees.
Celsius Scale
Most commonly used in the
rest of the world
Divided into degrees Celsius
(oC)
Freezing point of water = 0oC
Boiling point of water = 100oC
Space between boiling and
freezing is 100 equal degrees
oC are bigger than oF
Kelvin Scale
The SI Unit for temperature
Divided into Kelvins (not degrees!)
Boiling point of water = 373 K
Freezing point of water = 273K
Absolute zero = 0 K (the lowest
possible temperature an object
can have!)
Kelvin and Celsius degrees are
the same size!
Temperature Conversions
Fahrenheit  Celsius
oC = (5/9)(oF – 32)
Celsius  Fahrenheit
oF = (9/5)(oC) + 32
Please Excuse My Dear Aunt
Sally!
Temperature Conversions
Kelvin can only be calculated
if you know the object’s
temperature in oC.
K = oC + 273
oC = K - 273
Thermal Energy
Is the sum of the KE and PE of
all molecules in an object.
All molecules have PE that can
be converted into KE
They exert attractive forces to
each other
PE increases as molecules are
farther apart.
Temperature Vs. Thermal
Energy
How hot or cold something
feels does not necessarily give
you enough information to
determine thermal energy.
Thermal energy is related to
quantity of the molecules!
Examples
A glacier and an ice cube have
the same temperature, but the
glacier has much more thermal
energy due to its size.
Compare a glacier to a boiling pot
of water.
 Which has a higher temperature?
 Which has more thermal energy?
Heat
Section 2
Heat
Is thermal energy that is transferred from
one object to another when the objects
are at different temperatures
The amount of heat that is transferred
when two objects are brought into
contact depends on the difference in
temperature between the two objects.
Heat continues to be transferred until
equilibrium is reached (both objects are
at the same temperature).
Transfer of Heat
Thermal energy always moves from
warmer to cooler objects.
Heat never flows from a cooler
object to a warmer object.
The warmer object loses thermal
energy and becomes cooler while
the cooler object gains thermal
energy and becomes warmer until
their temperatures are equal.
There are three ways heat can
be transferred:
Conduction
Radiation
Convection
Conduction
The transfer of thermal energy by
direct contact
Ex: The bottom of a pot on a stove is
heated by direct contact between it
and the heat source
Occurs when particles in a material
collide with neighboring particles.
Energy is passed from molecule to
molecule during these collisions.
Occurs most easily in solids and
liquids where atoms and
molecules are closer together
As a result, heat is transferred
more rapidly by conduction in
solids and liquids than in gases.
Radiation
The transfer of energy in the form
of electromagnetic waves
Ex: You feel the warmth of the Sun
when standing on the beach.
These waves can carry energy
through empty space as well as
through solids, liquids, and gases.
Convection
The transfer of thermal energy
by the movement of molecules
from one part of a material to
another
Ex: water boiling in a pot on the
stove
Occurs most easily in liquids or
gases where molecules can
move much more easily than
in a solid
Forms convection currents
Convection Current
A continuous cycle of heating
and rising, cooling and sinking
A substance is heated which
makes it less dense and causes it
to rise. When it rises it cools,
which causes it to become more
dense and sink. The process
continually repeats itself.
Natural Convection
Natural convection occurs when a
warmer, less dense fluid is pushed
away by a cooler denser fluid.
Ex: Wind movement near a lake or
ocean is caused when air over
land is heated and becomes less
dense. Denser cool air rushes in ,
pushing the warm air up. The
cooler air is then heated by the
land and the cycle is repeated.
You feel the cooler air rushing in as
wind.
Forced Convection
Occurs when an outside force,
such as a fan, pushes a fluid,
such as air or water, to make it
move and transfer heat
Ex: Computer use fans to keep
electronic components from
getting too hot.
Thermal Conductors
Any material that easily transfers
heat
If a material has some electrons
that are not held tightly by the
nucleus, they are free to move
around and bump into each other,
transferring thermal energy.
The best thermal conductors are
metals, such as gold and copper.
Thermal Insulators
Any material in which heat
does not flow easily
Liquids and gases are usually
better insulators than solids
because their particles are
farther apart and cannot
transfer thermal energy by
conduction as easily as a solid.
Air is a good insulator. Many
insulating materials contain air
spaces.
Good conductors are poor
insulators and vice-a-versa.
Specific Heat
The amount of heat needed to
raise the temperature of 1 kg
of substance by 1 °C
More heat is needed to
change the temperature of a
material with a high specific
heat than one with a low
specific heat.
Ex: Sand heats up and cools
down faster than water.
Hotter than the water during the
day
Cooler than the water at night
Thermal Pollution
The increase in the temperature of a
body of water caused by adding
warmer water
Sources:
Hot water produced by power plants
that is released into a body of water
Rain falling on hot asphalt that runs off
into waterways
Effects:
Contains less oxygen
Can cause some organisms to
become more sensitive to chemical
pollutants, parasites, & disease
Reducing thermal pollution:
Use cooling towers to cool warm
water before it is released into
waterways