Electric Charge

Electric Charge

 

The simplest electrical phenomenon is static electricity, the temporary “charging”
of certain objects when they are rubbed against each other. Run a comb
through your hair when it’s dry, and the hair and comb begin to attract each other,
indicating that they are charged. Other familiar examples include clothes sticking
together in the dryer, and the sudden shock that you sometimes get when shaking
someone’s hand after walking across a carpet with rubber-soled shoes.
What you may not have noticed is that static electricity can result in both
attractive and repulsive forces. The comb attracts the hair and vice-versa, but the
hairs repel each other, and two combs similarly charged will likewise repel each
other. To explain this we say that there are two types of electric charge, called
positive and negative. When objects become charged by rubbing against each other,
one always becomes positive and the other becomes negative. Positively charged
objects (the hair, for instance) attract negatively charged objects (the comb) and
vice-versa, but two positives repel each other, as do two negatives. In summary,
like charges repel, while unlike charges attract.
What’s happening at the atomic level is this: All atoms contain particles called
protons and electrons, which carry intrinsic positive and negative charges, respectively.
Ordinarily, the number of protons in a chunk of matter is almost exactly
equal to the number of electrons, so their static-electricity effects cancel out on large
scales. However, rubbing certain objects together transfers some of the electrons
from one to the other, leaving the first object positively charged (because it now
has has an excess of protons) and the other object negatively charged (because it
now has an excess of electrons).
In the official scientific system of units, the amount of electric charge on an object
is measured in units called coulombs (abbreviated C). The total charge on all theprotons in a gram of matter is typically about 50,000 C, while the electrons in the
same gram of matter would carry a total charge of −50, 000 C. These numbers may
seem inconveniently large, but they’re not very relevant to everyday life because
all we normally measure is the excess of one type of charge over the other. The
amount of excess charge that readily builds up on a person’s hair is less than a
microcoulomb, that is, 0.000001 C. (“Micro” is the metric prefix for a millionth,
0.000001.) The charge of a single proton turns out to be 1.6 × 10−19 C, while the
charge of a single electron is minus the same amount. Thus, the number of excess
electrons on a charged comb is quite enormous, but only a tiny, tiny fraction of all
the electrons in the comb.
How long an object remains electrically charged depends on how easily the excess
electrons can find their way back to the excess protons. Some materials, such as
metals, allow electrons to move through them quite readily, while other materials,
such as paper, plastic, and dry air, offer quite a bit of resistance to the motion
of electrons. Materials in the first class are called conductors, while materials in
the second class are called insulators. The distinction between conductors and
insulators is merely a matter of degree, however; all materials conduct to some extent.
Furthermore, any insulating material will become a good conductor if it is
subjected to electrostatic forces that are strong enough to rip electrons out of the
atoms. The most dramatic example is lightning: the sudden discharge of thunderclouds
through a column of air, which is momentarily made into a conductor by the
enormous static charges. The shock that you get when you shake someone’s hand
is the same phenomenon, on a much smaller scale.