CHARGE MOTION IN CONDUCTORS
AND INSULATORS
The electrons in the metallically bonded structure will be repulsed by the electrons on
the rod. As they can move, they will migrate away from the rod leaving the right side
of the sphere electrically positive and the left side electrically negative. The net effect
will be an attraction between the two structures.
Consider an isolated, uncharged, metallically coated ball and an isolated,
charge, insulating rod.
Note: the charge on the ball should be uniformly distributed throughout the ball’s volume.
1.)
3.)
The same thing will happen if the charge on the rod is positive with attraction between
the two happening as shown.
Now bring them close. What happens?
2.)
4.)
When the ball touches the rod, electrons will jump to the rod. Because the rod is an
insulator, they will stay where they land leaving the rod, still, electrically positive. The
pith ball, though, having lost electrons, will have more positive charge than negative.
The negative will still be closer to the rod, but the increase in positive charge on the
ball will effectively govern and the ball will be repulsed.
Bring the two together and an interesting thing happens:
5.)
CHARGE MOTION IN INSULATORS
7.)
The electrons in the ball can’t migrate as they did in the conductor because
the bonding is covalent, but that doesn’t mean the negatively charged rod
won’t affect them.
Now you have an, uncharged, covalently bonded (I.e., an insulator) ball and
an isolated, charge, insulating rod.
6.)
8.)
So what happens when a negatively charged rod comes in from the right? The
electron in the atoms of the ball spend more time on the left side of the atom as they
try to get away from the negative charge on the rod.
To understand, you need to know something about the atom. So let’s take
ONE ATOM on the surface of the ball and expand it up so we can view it
closely. If this atom had only 1 proton and one electron:
In other words, you get a charge
polarization.
nucleus
Electrons spend more time over
here thereby shifting the negative
charge center to the left
p+
p+
e!
9.)
What we’d find would be the proton fixed in the nucleus with the electron orbiting at
somewhere around 14,000 miles per second (remember, the atom is only .
0000000001 meters across--one angstrom).
What’s important to note is that
the electron is at A as much as
often as it is at B, at C as often
as at D, etc. This means the
AVERAGE position of the
electron over time is at the
center where the proton is.
That’s why normal atoms are
electrically neutral. Their
electrons are, on average,
electrically centered on the
positive proton.
11.)
So to what does this all come down? When a charged rod is brought close to the
insulator, the electrons won’t migrate through the insulating structure but they WILL
move as forces dictate INSIDE EACH ATOM. Polarization will come into play, and
the surface of the object on one side will appear to have a different charge than the
surface of the structure on the other side. With that, you will get attraction.
C
A
there will be attraction
nucleus
p+
B
D
10.)
12.)