Materials can be divided into three groups or families: insulators,
conductors and semiconductors. Insulators are all the materials that don’t let the charge flow through
their structure.
If we have a surplus of electric charges inside an insulator, these charges are
localized, are not free to move through the structure of the insulator.
Typical insulators are, rubber, ceramics, paper, oil, etc... Conductors are materials that, on the contrary, allow the surplus of
charge to flow through their structure, this means that electric charges can
move, can migrate back and forth in the structure of the conductor. For example,
in electric circuits we use copper wires (copper is a good conductor) to
transfer electrons from one point of the circuit to another: we call this flow
of electrons in the wire "electric current".
Another feature of conductors is that atoms forming the conductor can easily
lose the more external electrons, and these electrons can move in the structure
too. To be more concise, conductors always have free electrons inside while
insulators don’t.
Typical conductors are metals, salt solutions, acids etc… Semiconductors are a family of materials which, depending on the
temperature, are both insulators and conductors. At low temperature
semiconductors behave like insulators, at higher temperatures they become
conductors; in general the conductivity of a semiconductor increases with
temperature.
Examples of semiconductors are silicon, carbon, germanium…
Charging a conductor by induction
Objects can be charged:
1) by rubbing them together (in some cases it works well: plastic
and wool, glass and silk…);
2) by touching them with another charged object (for example touching
a metal ball discharged, with a charged plastic rod, we charge the ball
because some of the charges on the rod are distributed onto the ball during
the contact between the two objects).
3) By induction. This third method lets us charge a conductor
without touching it with a charged object. The method works only with a
conductor because it involves the conducting property of metals. Imagine
placing a metal object (for instance a metal ball) on an insulating stand:
to charge it by induction four more steps are needed:
A) Bring a charged rod near the metal ball: the free electrons
(negative particles) of the metal ball are repelled on the side opposite
the rod.
B) Connect a metallic wire from the ball to earth: repelled electrons
flow through the wire toward earth
C) Remove the wire.
D) Remove the rod.
This way the ball is being charged without it touching the charged
rod.
Polarization of insulators.
We know that discharged insulators, are attracted by charged objects. Why
does it happen? (Remember our previous experiment: discharged insulators like
pieces of paper on the desk are attracted by a charged plastic pen). Why does an
insulator with no charge, in which, furthermore, electrons of the atoms aren’t
free to move, react to the presence of a charged body?
Electrons of insulators can’t move in the structure, they are strongly tied to
their atoms, but in any case, within an atom, electrons have some freedom of
movement, and in the presence of charged objects a small distortion of the
charge distribution take place. (see the figure)
Each atom become an ELECTRIC DIPOLE in which the center of the negative
charge differs from the center of the positive charge, the atoms now have
positive and negative poles (hence the term dipole). The net effect is the
production of a slight charge on the surfaces of the insulators. This charge is
responsible for the attraction to the charged object; the phenomenon is called
POLARIZATION of the insulator.
