128 PART 2^ |^ THE STARS
short-wavelength photons are rare. Similarly, most collisions are
not extremely gentle, so long-wavelength (low-energy) photons
are also rare. Consequently, blackbody radiation is made up of
photons with a distribution of wavelengths with medium wave-
lengths most common. Th e wavelength of maximum intensity
(max) is the wavelength at which the object emits the most
intense radiation and occurs at some intermediate wavelength.
(Make special note that max does not refer to the maximum
wavelength but to the wavelength of maximum.)
■ Figure 7-6 shows the intensity of radiation versus wave-
length for three objects of diff erent temperatures. Th e curves are
high in the middle and low at either end because the objects emit
most intensely at intermediate wavelengths. Th e total area under
each curve is proportional to the total energy emitted, and you
can see that the hotter object emits more total energy than the
cooler objects. Look closely at the curves, and you will see that
the wavelength of maximum intensity depends on temperature.
Th e hotter an object, the shorter will be the wavelength of its
maximum emitted intensity. Th e fi gure shows how temperature
determines the color of a glowing blackbody. Th e hotter object
emits more blue light than red and thus looks blue, and the
cooler object emits more red than blue and consequently looksHot objects emit blackbody radiation, but so do cold
objects. Ice cubes are cold, but their temperature is higher than
absolute zero, so they contain some thermal energy and must
emit some blackbody radiation. Th e coldest gas drifting in space
has a temperature only a few degrees above absolute zero, but it
too emits blackbody radiation.
Two features of blackbody radiation are important. First, the
hotter an object is, the more blackbody radiation it emits. Hot
objects emit more radiation because their agitated particles col-
lide more often and more violently with electrons. Th at’s why a
glowing coal from a fi re emits more total energy than an ice cube
of the same size.
Th e second feature is the relationship between the tempera-
ture of the object and the wavelengths of the photons it emits.
Th e wavelength of the photon emitted when a particle collides
with an electron depends on the violence of the collision. Only a
violent collision can produce a short-wavelength (high-energy)
photon. Th e electrons in an object have a distribution of speeds;
a few travel very fast, and a few travel very slowly, but most travel
at intermediate speeds. Th e hotter the object is, the faster, on
average, the electrons travel. Because high-velocity electrons are
rare, extremely violent collisions don’t occur very often, and
Temperature, Heat, and Thermal Energy
MASS | ENERGY | TEMPERATURE AND HEAT | DENSITY | PRESSURE
What’s the difference between temperature and
heat?O
ne of the most Common
Misconceptions in science
involves temperature. People often
say “temperature” when they really mean
“heat,” and sometimes they say “heat” when
they mean something entirely different. This
is a fundamental idea, so you need to
understand the difference.
Even in an object that is solid, the atoms
and molecules are continuously jiggling
around bumping into each other. When
something is hot, the particles are moving
rapidly. Temperature is a measure of the
average motion of the particles.
(Mathematically, temperature is proportional
to the square of the average velocity.) If you
have your temperature taken, it will probably
be 37.0°C (98.6°F), an indication that the
atoms and molecules in your body are
moving about at a normal pace. If you
measure the temperature of a baby, the
thermometer should register the same
temperature, showing that the atoms and
molecules in the baby’s body are moving at
the same average velocity as the atoms and
molecules in your body.
The total energy of all of the moving
particles in a body is called thermal energy.
You have much more mass than the baby, so
you must contain more thermal energy even
though you have the same temperature. The
thermal energy in your body and in the baby’s
body has the same intensity (temperature) but
different amounts. People often confuse
temperature and thermal energy, so you must
be careful to distinguish between them.
Temperature is like an intensity, and thermal
energy is a total amount.
Many people say “heat” when they should
say “thermal energy.” Heat is the thermal
energy that moves from a hot object to a
cool object. If two objects have the same
temperature, you and the infant for example,
there is no transfer of thermal energy and no
heat.
When you hear someone say “heat,” check
to see if he or she doesn’t really mean thermal
energy. You may have burned yourself oncheese pizza, but you probably haven’t burned
yourself on green beans. At the same
temperature, cheese holds more thermal
energy than green beans. It isn’t the tempera-
ture that burns your tongue, but the fl ow of
thermal energy, and that’s heat.3