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Thermodynamics → Temperature
for that matter, what is heat? (It's not the same thing as
temperature). I will answer these questions. I promise. But in order to
do so, I must first deal with a slightly "different" topic. Energy.
is Energy, then?
may have previously learned that there are different types of
in the world. For example, when an obsolete petrol (gasoline) driven
drives down a road, chemical energy stored in the fuel gets converted
to thermal energy (heat), and sound energy; when the fuel is ignited.
The thermal energy turns into kinetic energy (motion) when it causes a
piston to rise and fall; and this motion drives the car down the road.
learned this then, I'm afraid, it's not entirely
true. There's only one type of energy. Kinetic energy. All the rest are
just kinetic energy in disguise.
already said that kinetic energy is motion. In fact, kinetic
a moving object is defined as follows:
½ × mv2,
where Ekinetic ≡
energy of object,
≡ ||mass of object,
≡ ||total speed
(magnitude of velocity) of the object. |
So, kinetic energy is just mass in motion. It's proportional to mass.
(That's fair enough, our moving object is made of atoms, which are
themselves made of smaller particles, each of which is moving at around
the same average speed as the rest of the object, and thus has
kinetic energy. Multiplying by the object's mass is just like adding up
all the kinetic energies of the individual particles.)
it comes to energy, there's a very important rule - probably
the single most important law of physics in the universe. This is the
law of conservation of energy:
CANNOT BE CREATED OR DESTROYED;
IT CAN ONLY BE TRANSFERRED FROM ONE BODY TO ANOTHER.
you have a set of billiard balls on a table. When one ball
strikes another, it slows down because the energy gets transferred into
the other ball, which in turn speeds up. When a ball rebounds off the
edge of the table, some of the molecules that make up the ball strike
some the particles that make up the table. These particles in turn
rebound off other particles in the table, so just like tightly packed
billiard balls; the shock vibrates through the wood of the table. It
then keeps moving around through the wood until at the outer edge of
the table, the wood particles repeatedly strike air molecules; again,
just like billiard balls, causing air vibrations, which we hear as
sound. The energy never disappears totally - it just keeps getting
spread out in this fashion until it ends up as thermal energy, widely
lowest form of energy.
Thermal energy is also kinetic energy.
a room full of air. The air is made of individual particles
(molecules, to be precise). These particles don't stand still - they
all have their own share of kinetic energy accumulated from the odd
billiard table, as well as sources like the Sun. Remember, they can't
lose their kinetic energy unless they transfer it elsewhere, and since
this residual energy is already all around, that's kind of difficult to
do. So, the gas particles are all doomed to travel at their own speed
around the room, bouncing off the walls (and each other) like billiard
balls. Now if you look at all the gas particles in the room together,
and take the average energy of each particle; there's a name for that:
it's called temperature.
Well, not quite. You actually have to multiply the average kinetic
energy by a conversion factor to convert it into units of kelvins, but
that's all you have to do.
where ||T ≡
| || ||E
energy per particle, |
||k is a conversion
factor, known as the Boltzmann Constant
The Boltzmann constant, which is simply a conversion factor between
temperature and energy units, has the following values, depending on
whether you deal with energy in units of joules (J) or electron-volts
1.3806505×10−23 J/K |
| ||= 8.617
339×10−5 eV/K |
This also explains why it's impossible to get colder to a temperature
colder than 0K (absolute zero). How, after all, can a particle have
energy lower than zero? The very concept of temperatures lower than
absolute zero simply doesn't make sense!
is the transfer of the kinetic energy of particles from one
body to another. A hotter object placed next to a cooler object will
always transfer heat from itself into the cooler object, until both
objects are of equal temperature.
example, if you place a cup of water, at room temperature; into a
room, at the same temperature; then for every time an air particle
rebounds off and transfers energy to a water particle, a water particle
will transfer an equivalent amount of energy into the air - so that on
average, the temperature of the water will remain unchanged.
If, on the
other hand, the air and water are of different relative
temperatures, then the amount of energy transferred between the two
bodies will be unbalanced, hence the cooler body heats up and
hotter body cools down.
remember that temperature is only the average
kinetic energy per molecule. So, even though a cup of water may only be
a few degrees above freezing (nowhere near boiling point), the
interactions between molecules mean that some molecules will be moving
much faster than others. In fact, some of the molecules in water move
so fast that they fly right out of the cup and into the atmosphere.
you know why water evaporates!
burns! It Burns!
I'm sure it does. Speaking of which, have you ever wondered why
the effect of skin burns is exactly the same as that of frostbite? It
may seem hard to believe, but frostbite is actually a burn injury,
just like any other burn, is caused by an excess of heat.
environment, your skin is constantly under assault from
collisions by gas molecules. But this is balanced by molecular
vibrations due to the temperature inside
your body, so the impacts on your skin are canceled out - the particles
that make up your skin hardly move at all on average, and no
your skin comes into contact with something which is either much
much colder than your internal body temperature, then your skin, which
forms the interface between a low and high temperature zone,
takes a rather one-sided battering, which is how burns occur.
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