Thermodynamics:
the study of thermal energy
Heat (energy)
Transfer
Potential Energy
- Stored in the vibrations
of the molecules
- Limited by the “degrees
of freedom” available to
the molecule
Kinetic Energy
- Motion of atoms and
molecules
- Reflected in the
TEMPERATURE of
the substance
Faster = higher temperature
Potential If the molecules have many “degrees of freedom”,
Energy they can store more potential energy, with less
change in the kinetic energy of the molecules.
Therefore, the temperature will change slowly.
Kinetic
Energy
If the molecules have few “degrees of freedom”,
they can store little potential energy, with more
change in the kinetic energy of the molecules.
Therefore, the temperature will change more
rapidly.
Kinetic-Molecular Theory
As a substance gets hotter, its molecules move faster!
Faster molecules have higher kinetic energy.
A higher kinetic energy is reflected by a higher
temperature!
You may not be able to SEE molecules moving fast
without a microscope, but you can see an
increase in temperature on a thermometer.
Microscopic:
Cannot be seen
by eyes aloneusually you can’t
“measure” the
kinetic energy of the
molecules
Macroscopic:
Can be seen
with eyes aloneyou can
measure the
temperature!
Kinetic-Molecular Theory
As a substance gets hotter, its
molecules move faster!
Faster molecules have higher kinetic
energy.
A solid: the molecules are tightly
packed together and move more
slowly. When you add heat…
A liquid: the molecules are not
packed as tightly together and
move around. When you add
more heat…
A gas: the molecules are not bound
together and move very fast.
When you add more heat…..
A plasma: the atoms themselves are
ripped apart to become ions.
Heat Transfer
Since the atoms in aluminum can store more
potential energy than the atoms in gold, as heat
flows, the gold atoms will gain more kinetic
energy so that the temperature of gold rises
much faster than the temperature of the same
mass of aluminum.
In one episode of the original TV series,
“Mission Impossible”, the team utilized
that fact that gold heats up so quickly
compared to other materials. They
drilled a hole in the bottom of a vault and
inserted an electric heat rod. The gold in
the vault heated up, melted and flowed
through the hole in the bottom of the
vault before any of the paper money or
other coins got too hot.
Would that really work?? Hmmm,
maybe- but don’t try it!
Specific Heat Capacity, “c”
• The specific heat capacity, “c”, of a substance is the amount
of heat required per kilogram to raise the temperature by
one degree.
• Different substances have different specific heat capacities.
Aluminum has a higher specific heat than gold! More heat
must be transferred into aluminum than into gold for the
same change in temperature
• The higher the heat capacity,
the more heat the substance
can “hold” or “give off” with
minimal temperature change.
Unit: J/kg·K (or J/kg·°C)
Specific Heat Capacity of Water
For example, you put 1 kg piece
of metal on a hot plate for two
minutes. You also put a
container of 1 kg water in on an
identical hot plate.
Would you rather place your
finger on the metal or in the
water?
The metal will be at a much
higher temperature!
Both received the same amount
of heat energy.
But water has a higher specific
heat capacity- it can absorb or
release more heat energy with
little temperature change.
–
–
–
–
Water has one of the highest
specific heats of all substances. It
can absorb and give off great
amounts of heat energy with little
temperature change.
It takes a long time to heat water
and it takes a long time for water to
cool down!
Another example: The filling on a
hot apple pie burns our tongues
and not the crust even though they
are the same temperature because
of the water content in the filling.
The filling can give off a lot of heat
and STILL be hot.
The oceans help
maintain a small
range of temperature
on Earth that is
compatible with life
by absorbing heat in
the day and releasing
it at night with little
change in the ocean’s
temperature.
Weather along the Coastline
• Coastal regions do not experience a large change in
temperature because the water absorbs solar radiation in
the day and releases it at night
• In contrast, in a desert
there's a wide daily
range of temperature
because no water is
available to absorb heat
in the day and release it
at night
Table of
Specific Heat
• The average specific
heat capacity of a
human body is
approximately 3500.
Substance
Specific Heat Capacity
(J.kg-1.K-1)
water
4200
ice
2100
ethanol
2400
copper
390
aluminium
900
glass
840
mercury
140
wood
1700
lead
130
Heat transfer
and temperature change
As heat, Q, flows into or out of a substance,
its temperature change, DT, will depend on
the mass, m, of the substance and its
specific heat capacity, “c”.
Q = mcDT
Example: How much heat, Q, is required
to raise the temperature of a 3 kg pan of
water from 15°C to boiling temperature?
(specific heat of water = 4186 J/kg·K)
Q = mcDT
What is DT? Final – original =
100° – 15° = 85°
Q = 3 (4186) 85 =
Q = 1,067,430 J
Q = mcDT
A 0.2 kg block of metal absorbs 1500 J of
heat when its temperature changes
from 20 to 35 degrees Celsius. What
is the specific heat capacity of the
metal?
c = Q ÷ (mDT)
c = 1500 ÷ (0.2 • 15)
c = 500 J/kg·K
Q = mcDT
What is the change in the temperature of
3.5 kg water if 950,000 J of heat are
added? C = 4186 J/kg·K
DT = Q ÷ (mc)
DT = 950,000 ÷ (3.5 • 4186)
DT = 64.9°