Chapter 21 – Temperature, Heat, and Expansion
21.1
Tempera
ture
21.2
Heat
We sense the movement of molecules as warmth. When molecules move
faster, the matter gets warmer.
Temperature is how hot or cold something is compared to a known
value or scale. Most thermometers contain liquids that expand with
heat.
The Celsius scale sets zero at the temperature water freezes and 100°
as the boiling point of water (at sea level) with the increments in
between called degrees.
With the Fahrenheit scale the freezing point of water is 32° and the
boiling point is 212°. Only the U.S. still uses this scale.
The Kelvin scale sets 0°K as absolute zero (where molecules cease
moving) and the freezing point of water as 273°K and the boiling point of
water at 373°K. Science uses the Kelvin scale because temps can be
multiplied easily.
Temperature is
related to the
average kinetic
energy or movement
of the molecules in
a substance.
”The higher
temperature of a substance, the faster is the motion of its molecules.”
The total kinetic energy of a substance is not directly related to
temperature.
Thermal energy
always moves from
warmer objects or
areas to cooler ones.
Heat is the name
given to the energy
transfer based on a
difference in
temperature. Heat
is moving energy
going from one body to another. Thermal contact is when two objects
have a heat flow between them.
”When two substances of different temperatures are in thermal
contact, heat flows from the higher temperature substances into the
lower-temperature substance.” Heat flows according to temperature
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differences not total internal energy differences. **this is somewhat
like a longitudinal wave or the flow of electricity.
21.3
Thermal
Equilibri
um*
This is
directly
from the
state
standards
& likely to
be tested.
17.4
Evidenc
e for
Atoms
21.4
Internal
Energy
21.5
Measure
ment of
Heat
Thermal equilibrium is when objects in thermal
contact reach the same temperature. There is
no heat flow if the objects are in thermal
equilibrium.
”When a thermometer is in contact with a
substances, heat flows between them until they
have the same temperature.”
A thermometer reaches equilibrium with an
object in thermal contact and then reads its own
temperature.
The idea of atoms goes back to Democritus and the ancient Greeks.
John Dalton advocated that matter was made of atoms even though he
had no direct evidence for their existence. In 1827 Robert Brown saw
pollen grains floating on water through a microscope. Brown noticed
that the grains were moving just a bit. The jiggling of small particle just
large enough to be seen is called Brownian motion. Brownian motion is
evidence not only for atoms, but also that atoms and molecules are in
perpetual motion. Brown observed that the higher the temperature of
the water, the more the particles moved. The kinetic energy of
molecules displayed in Brownian motion proportional to temperature.
This was evidence that temperature is related to the motion of
molecules.
http://3.bp.blogspot.com/_D8P4hByt4sc/TRriJx_WUuI/AAAAAAAAAc
0/p6EQnqIakPg/s1600/brownianmotion.gif
Internal energy is the sum of all the different energies in a substance.
Objects really have internal energy rather than heat.
”When a substance takes in or gives off heat, its internal energy
changes.”
”The amount of heat transferred can be determined by measuring the
temperature change of a known mass of a substance that absorbs the
heat.”
When energy is transferred in heat, the temperature change between
the objects depends on the kind of substances and how much of each
substance is involved in the thermal contact. Heat is commonly
measured in calories. One calorie is the amount of heat needed to raise
one gram of water one degree Celsius. The Calories that we use to rate
the energy in food are actually kilocalories. One calorie is equal to
2
21.6
Specific
Heat
Capacity
4.186J. Calorie ratings of food are determined by burning the foods and
measuring how much energy is released.
Different substances can hold more or
less heat. ”The capacity of a
substance to store heat depends on its
chemical composition.”
The amount of heat required to increase
the temperature of a substance 1°C is
called its specific heat capacity.
Specific heat capacity is a substance’s
resistance to changing temperature – like a thermal version of inertia.
Not all internal energy is temperature or kinetic energy of translational
motion. Energy that is absorbed could be in rotational motion of the
molecules, internal vibrations, or potential energy as chemical bonds.
Water molecules absorb energy in these other forms in addition to
temperature (kinetic
energy) increases.
Because of this, water
‘holds’ more energy.
21.7 The Small amounts of water can store lots of energy with only small changes
High
in temperature. Water is used to cool engines because of its high
Specific specific heat capacity.
Heat
Capacity
of Water
”The property of water to resist changes in temperature improves the
climate in many places.” Places near the ocean or with predominantly
on-shore winds have more moderate climates. Northern Europe is
considerably warmer because of the Gulf Stream. On the west coast of
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North America the temperature is warmer in the winter and cooler in
the summer. East coast regions of America have a wider range of
temperatures. Areas in the center of the continent have extreme lows
in the winter and extreme highs in summer.
21.8
With increased temperatures and molecular movement objects expand.
Thermal ”Most forms of matter—solids, liquids, and gases—expand when they
Expansio are heated and contract when they are cooled.”
n
Typically, gases expand the most, then liquids then solids the least.
Expansion rates of various materials must be considered in engineering
and construction. Expansion joints are seen in
sidewalks and bridges.
Since different materials expand at different
rates, this can be used to cause bimetallic strips to
arc. Bimetallic strips can be used in thermostats or
pointer thermometers. When the temperature
causes a bimetallic strip to bend a set amount, it
may close a circuit and cause a furnace to turn on.
Borosilicate glass does not expand much when heated and is used in
test tubes and cookware designed for heating. When heated a metal lid
can expand making it easier to open.
21.9
Liquid water at 0°C will contract until it reaches 4°C – its densest
Expansio point. From 4°C upward, water expands as heated. Ice is water
n of
arranged open crystal structures so the density is 0.9167 g/cm3 whereas
Water
water at 4°C is 1.0 g/cm3.
”At 0°C, ice is less dense than water, and so floats on water.”
The fact that liquid water is more dense than ice is important for life
on earth (and ice fishing). In order for a body of water to freeze, it
must all reach 4°C.
 Mr. Collom sanitized an 8-gallon bucket by filling it to the top
with boiling water.
He
left it in his garage
overnight and found
the
water 2 cm below the
rim in
the morning. What
happened?
 What is wrong with
goblet on the left?
can ice sink below
the
How
water?
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