Heat and Thermal Energy
Dear Sir or Madam:
What exactly is "heat", scientifically? How does it differ from thermal
energy? Infrared rays seem to be radiant, electromagnetic energy. Is
heat produced by rubbing two rocks together electromagnetic energy too?
Heat vs Temperature: A Distinction with a Difference
Heat is just another expression for thermal energy. When you rub two rocks
together, you're converting metabolic (chemical) energy in your muscles into
mechanical energy as long as the the rubbing activity is in progress, and
finally converting the mechanical energy into heat via friction between the
rock surfaces. Energy thus put into the rocks by the frictional activity is
ultimately returned to the surroundings in the form of infrared (heat)
Here's a perspective that is probably more than you wanted to know ... and,
as is so often the case, less than enough.
Because heat is a manifestation of atomic and molecular motion, the kinetic
energy (the energy of motion of any particle of matter) is expressed as KE =
1/2 mv2, where m represents the mass of the moving "particle" and v2
represents the velocity of the particle squared. If atoms and molecules are
cooled and thereby caused to slow down until they cease their vibrational,
rotational, and/or translational motions, they lose their kinetic energy and
thus have no energy to surrender to a temperature sensor. This is why it is
so difficult to measure temperatures very near "absolute zero."
In such very low-temperature realms, the sensor (thermometer) actually alters
the temperature of the system that is to be measured. This occurs because the
sensor must receive energy from the system and translate it into a
temperature reading. If it is to receive energy, it must be at a lower energy
than the system itself. In practical terms, this means that in order to make
temperature measurements in a super-cold realms, the sensor must be cooler
than the system whose temperature is to be measured. Indeed, a thermometer
that's warmer than the system will surrender heat to it and make the
temperature reading meaningless.
The Kelvin temperature scale was conceived to better reflect the relationship
between various possible atomic/molecular motions and energies associated
with those motions.
Consider the classical physics relationship that describes the kinetic energy
of a moving object:
KE = 1/2 mv2 where "m" represents the object's mass and "v," its velocity.
Another form of that equation relates the kinetic energy of a moving molecule
to temperature. For a monatomic gas:
KE = 3/2 kBT(abs) where kB is the Boltzmann constant (1.381 x 10-23 J/K) and
T(abs) is the temperature in degrees Kelvin.
When the two relationships, both equal to KE, are equated we get:
1/2 mv2 = 3/2 kBT(abs)
Notice that most of the components in the relationship are constants: 1/2,
m, 3/2, and kB Note as well that mass, m, doesn't change when a molecule is
If, for purposes of simplification, we remove all the constants (or collect
them into a single, 'universal' constant), we see that the velocity of a
moving molecule is directly proportional to absolute temperature, T(abs), and
atomic and molecular velocities are proportional to T(abs)
T(abs) is proportional to atomic and molecular velocities.
Because the Kelvin temperature scale begins at an atomic/molecular kinetic
energy of zero, increases in Kelvin temperature are directly reflective of
corresponding increases in the energy of the system whose temperature is
being measured. Put another way: Doubling (or halving) the Kelvin temperature
of a body doubles (or halves) the kinetic energy of its atoms and molecules.
This relationship is NOT true for the Celsius or Fahrenheit temperature
scales. For this reason, even though the Celsius temperature scale is still
in widespread use, calculations involving temperatures are almost always
better served when the Kelvin scale is used.
It is easy to convert a Celsius temperature to its corresponding Kelvin
K = C + 273
The key idea to remember is, the addition is an algebraic addition.
For example: 27 C is 27 + 273 = 300 K
Likewise: -100 C = -100 + 273 = 173 K
An Important distinction:
Finally, it is important to recognize that heat and temperature are NOT the
same thing. When you measure the temperature of boiling water with a
thermometer, all you're really getting is an idea of the local energy content
of the molecules in contact with the thermometer bulb. Moving it about in the
hot water tells you the same story, 100C (212 F). Knowing only the
temperature of the water tells you nothing about the total amount of heat in
the quantity of boiling liquid.
Consider this odd scenario: Imagine a large tank of boiling water. Would you,
for a thousand dollars, allow someone to place a single drop of the boiling
water on your hand? I would. Of course it would burn and might even leave a
blister; but for a thousand dollars, I'd take the pain. Now consider an
alternative: Would you, for the same thousand dollars, be willing to jump
into the tank? Not on your life! Somehow, you have the common sense to
recognize that water's capacity to burn you has more to do with its heat
capacity (the total amount of heat a given mass of water can surrender) than
its mere temperature.
To better illustrate the idea of heat capacity, consider this scenario: Your
pizza has just been taken from the oven and you're hungry. The crust is not
too hot to handle when you pick it up. You're confirmed in your belief that
it's at the perfect temperature when you touch the crust to your tongue. It
feels warm, but not uncomfortably hot. So chomp! and Oww! Your mouth is
burned by the pizza sauce. How can this be? Obviously, both the crust and the
sauce are at the same temperature ... after all, they were heated together in
the same oven.
Even though they were both at the same temperature, the sauce (because it
contains more water) contains more thermal energy. Remember, compared to
other substances, water is about the best substance there is for absorbing a
lot of energy while changing temperature only a little. Because of this, more
thermal energy is required to raise the sauce to the same temperature as the
crust. When you put the pizza in your mouth, both the sauce and crust lose
heat until they reach the same temperature as your mouth. The (water
containing) sauce has much more heat to surrender and that's why it burns so
"Things get curiouser and curiouser."
Lewis Carroll: "Alice in Wonderland"
Have you ever seen someone put out a match or candle by wetting his/her
fingers and then quickly pinching out the flame? Perhaps you've done it
yourself. Isn't it odd that you are able to quench a flame that's many
hundreds of degrees "hot" with your bare skin, yet you'd be foolish to stick
your finger in boiling water that's only 100 C? As before, it's all a matter
of heat capacity. Indeed the gas molecules in the flame are energetic.
However, there are not many molecules present, and the gas (compared to
water) has a very low heat capacity. Hence, the little bit of water on your
wetted fingers and the water in the tissue of your skin are easily able to
absorb the heat energy of the flame without resulting in a burn. If you were
to hold your finger in the flame, you would be giving the heat source much
more time to deliver energy to your skin ... thus, a nasty burn.
Here's a surprisingly accurate (unscientific?) way to determine Fahrenheit
Count the chirps made by a cricket in one minute, then compute the
temperature using the formula below. Remember, the temperatures obtained are
those representative of where the "Jiminy Thermometer" is located ...
probably cooler where he is than where you're doing the counting.
Fahrenheit temperature = (chirps per minute / 4 ) + 40
You've asked one of those questions that does not have a really simple
answer [at least I don't think so]. Heat is the energy produced when
atoms/molecules are in some way made to vibrate more. The more their
vibration the larger the amount of heat. Heat and thermal energy are the
same thing. Because a change in energy manifests itself as radiation
according to the equation E = h*nu where E is the change in energy, h is
Planck's constant and "nu" is the frequency of the radiation. From the
photoelectric effect, Einstein showed that electromagnetic radiation can
also behave as though it were a particle, we call a "photon". The heat
produced by rubbing two rocks together, the warmth we feel from a fire
place, are all electromagnetic radiation in the infrared region of the
electromagnetic spectrum. It's all the same whether we are rubbing rocks or
under the covers with our electric blanket. Whether it is more convenient to
describe it as a wave or as a photon particle is our choice -- the object
of our description is what it is. It doesn't change; it's our description
that changes depending upon what is more meaningful to us -- a wave or a
Heat IS thermal energy. It is the energy associated with molecular motion,
including translation, vibration, and rotation.
Infrared rays are a segment of the electromagnetic spectrum. Infrared
radiation is not the same thing as heat! Yes, hot things emit infrared,
but that is simply "black body radiation": make them hotter still, and they
will emit electromagnetic radiation in the visible, UV, and even X-ray
Rubbing two rocks together makes them hotter. In thermodynamic terms, heat
is being added to the rocks. This is NOT electromagnetic radiation, though
the rocks will emit electromagnetic radiation characteristic of their new
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Thermal energy is the sum of the energy of the molecules making up a
substance - kinetic and potential. When this thermal energy is transferred
from one place to another, it is called heat. Radiation is one of three ways
it can be transferred (along with conduction and convection). We call the
radiation that transfers thermal energy - infrared. It is just below the
visible light portion of the EM spectrum and the human eye is not sensitive
Rubbing two rocks together converts mechanical energy (the motion of the
rubbing) to thermal energy. It radiates some of the energy in the form of
infrared electromagnetic radiation.
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Update: June 2012