Amanda- that's a nice question, but our answer will be a little indirect.
For some forms of energy, it's very convenient to classify them as
kinetic or potential. If you're watching large objects move around,
there's obvious kinetic energy associated with the motion. That kinetic
energy is reduced if, for example, a ball happens to fly upward. We say
that when it's up it has gravitational potential energy. That gets
converted back to kinetic on the way down. Likewise, if you compress a
spring, we say it has potential energy. Similarly, pulling two
oppositely charged particles apart results in potential energy, which
you can say is stored in the electric field.
Taking these categories which are useful for large-scale mechanical
properties and imposing them on all phenomena can be misleading. Take
light, for example. You can think of it as consisting of photons
whizzing around, and call its energy entirely kinetic. Or you can say
that it is made up of electrical and magnetic fields, and we just said
that we usually call the field energy potential. Which is right? There
is no answer, because we're just talking about name choices, not about
how things actually behave.
Now on to some of your other specific energy forms, trying to give conventional answers:
Heat: The thermal energy in a material partly consists of particles
jiggling around, so that's kinetic. In a gas, that's the main piece. In
a solid, as they jiggle the particles squeeze and pull on each other,
like little springs being compressed and stretched. So that part is
potential. In a typical solid, the thermal energy is half kinetic and
half potential.
Sound: In a solid, this is half kinetic and half potential, like
the thermal energy. In fact, the thermal energy is just a bunch of
sound waves moving around randomly. Something similar holds for a
liquid. In a gas, the sound energy is kinetic, if you look at it on the
microscopic scale of moving molecules.
So these classifications are useful but not to be taken too
seriously because nature doesn't care what names we call it. What does
count is which equations it obeys.
Mike W.
(published on 10/22/2007)
