|MadSci Network: Physics|
There are two mental models for resistive heating: the
old "orbiting classical particles" model, and the new
"Quantum Atoms" model. I don't know enough about the
second one to talk about it, so my explanation will
focus on orbiting particles, not "Fermi surfaces"
Your teacher is right, the electrons never touch
the nucleus. They are exactly like the electrons
surrounding any atom: they "orbit" without falling
in. In fact, an electric current in a metal wire
is made of electrons supplied by the metal atoms. The
flowing electrons aren't really injected into the
wire from outside. Instead, electric fields produced
by the power supply cause the wandering electrons
(always found within the metal) to begin flowing.
This might be hard to understand unless you
realize that metals aren't made of separate atoms.
In metals, the outer electrons of each metal atom
leave the individual atoms, and they "orbit" among
all the atoms as a whole. A metal object is like
a jar of water. Metals contain an electron
sea or "electric fluid." A metal object is like one
giant atom with many nucleii but only one electron cloud.
During an electric current, it is the shared "electron
fluid" which moves along. And in an electric
circuit, the power supply is not the source of
the electrons. The power supply is simply
an electron pump.
So, why does the metal get hot during electric
current? Because the flowing electrons "collide"
with other electrons! Metals are different than
other materials in that they store heat energy
as wiggling electrons. (In non-metals, the heat
energy takes the form of wiggling atoms.) If
you try to force the metal's electrons to flow
along, those electrons will speed up at first.
But then they will be deflected by other electrons
which had been trying to orbit around the non-moving
atoms. Your flowing electrons get slowed by
the non-flowing electrons, but the non-flowing
electrons get sped up... but those non-flowing
electrons weren't moving in the direction of the
electric current. They fly in all directions with
no average drift and no overall electric current.
Speeding them up doesn't push them in the direction
of the current. Speeding them up makes the crowd
wiggle faster. Speeding them up is the same thing
as heating the metal.
To repeat: the electrical force
from the battery keeps trying to make electrons
flow along, but the "collisions" with other electrons
keep deflecting them and slowing down the overall flow.
The overall flow gets turned into random motion, like that of
the whizzing atoms in a hot gas. But there will always
be SOME overall flow as long as the electric
force is there, and a stronger force makes a faster
flow. That's where Ohm's Law comes from.
Ohm's Law says that electric current is directly
proportional to the voltage difference, but there's
a simpler way to say it: the harder you push, the
faster it flows.
> Also when the electrical energy is transfered to
> heat energy do the electrons just disappear and if not
> do they carry on and still produce a current.
They don't disappear. Are you thinking that electrons are particles of energy, and that they have to change into heat enery? Nope. Electrons aren't particles of energy, they are particles of matter. An electric current is not a flow of energy, it is a flow of matter. A flow of electrical energy is called "electric power," not "electric current." Here's an analogy: when you use a circular drive belt to send energy from an engine to a spinning fan, is the energy made of rubber molecules? Nope. The rubber belt moves slowly in a circle while the mechanical energy flows almost instantly along the belt, going from engine to fan. Electric circuits are the same: the electrons fill the whole circle, and they flow slowly like a belt while the electrical energy flows rapidly from one component to another. More about this stuff is here:
Note that electrons can "collide" without touching. They repel each other because of alike charges, and this means that they can push against each other. If you throw one electron at another, the first one pushes the second one away, while the second one pushes back upon the first and slows it. But they never touch, and the "pushing" is done by the electrical repulsion forces. It's just like a real collision, but the particles never actually make contact.
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