Sample
Chapter One
ELECTRIC CHARGES
AND FIELDS
1.2 ELECTRIC CHARGE
Historically the credit of discovery of the fact that amber rubbed with
wool or silk cloth attracts light objects goes to Thales of Miletus, Greece,
around 600 BC. The name electricity is coined from the Greek word
elektron meaning amber. Many such pairs of materials were known which
on rubbing could attract light objects
like straw, pith balls and bits of papers.
You can perform the following activity
at home to experience such an effect.
Cut out long thin strips of white paper
and lightly iron them. Take them near a
TV screen or computer monitor. You will
see that the strips get attracted to the
screen. In fact they remain stuck to the
screen for a while.
It was observed that if two glass rods
rubbed with wool or silk cloth are
brought close to each other, they repel
each other [Fig. 1.1(a)]. The two strands
of wool or two pieces of silk cloth, with
which the rods were rubbed, also repel
each other. However, the glass rod and
wool attracted each other. Similarly, two plastic rods rubbed with cat’s
fur repelled each other [Fig. 1.1(b)] but attracted the fur. On the other
hand, the plastic rod attracts the glass rod [Fig. 1.1(c)] and repel the silk
or wool with which the glass rod is rubbed. The glass rod repels the fur.
If a plastic rod rubbed with fur is made to touch two small pith balls
(now-a-days we can use polystyrene balls) suspended by silk or nylon
thread, then the balls repel each other [Fig. 1.1(d)] and are also repelled
by the rod. A similar effect is found if the pith balls are touched with a
glass rod rubbed with silk [Fig. 1.1(e)]. A dramatic observation is that a
pith ball touched with glass rod attracts another pith ball touched with
plastic rod [Fig. 1.1(f )].
These seemingly simple facts were established from years of efforts
and careful experiments and their analyses. It was concluded, after many
careful studies by different scientists, that there were only two kinds of
an entity which is called the electric charge. We say that the bodies like
glass or plastic rods, silk, fur and pith balls are electrified. They acquire
an electric charge on rubbing. The experiments on pith balls suggested
that there are two kinds of electrification and we find that (i) like charges
repel and (ii) unlike charges attract each other. The experiments also
demonstrated that the charges are transferred from the rods to the pith
balls on contact. It is said that the pith balls are electrified or are charged
by contact. The property which differentiates the two kinds of charges is
called the polarity of charge.
When a glass rod is rubbed with silk, the rod acquires one kind of
charge and the silk acquires the second kind of charge. This is true for
any pair of objects that are rubbed to be electrified. Now if the electrified
glass rod is brought in contact with silk, with which it was rubbed, they
no longer attract each other. They also do not attract or repel other light
objects as they did on being electrified.
Thus, the charges acquired after rubbing are lost when the charged
bodies are brought in contact. What can you conclude from these
observations? It just tells us that unlike charges acquired by the objects
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