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2.
Charging by
conduction and induction
Static electricity can be detected by a gold leaf electroscope. One
can make a model of an electroscope at home also (see box). An electroscope
will be able to detect the presence of static charges, but is unable to give
you any information about the sign (negative or positive) of the charges.
Only a reasonable guess can tell you whether the charge on the electroscope
is positive or negative.
Take
a plastic comb. Rub it on a dry surface like a tissue paper or hair.
Now touch the comb to the electroscope plate.
You will observe that the leaves of the aluminum foil, inside the electroscope, separate. This is because the static
charges developed on comb, travel down through the conducting wire
and reach the two leaves of the aluminum foil. The two leaves
acquire similar charges and repel each other. This phenomenon occurs
for a short time only.
The charges developed on the aluminum leaves dissipate or discharge
away in the air quickly.
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Charging an electroscope by conduction |
The
mechanism of charging a (uncharged) body by bringing another charged body
directly in contact is known as charging
by conduction. In conduction, the charges flow from one body into
another till the two bodies in contact reach equilibrium in terms of
charges. In the example seen,
let us say that the comb acquires a charge +Q on rubbing. When it is touched
to the electroscope, which is initially uncharged, the charge +Q flows or
conducts to the aluminum leaves. If the aluminum leaves get a charge of +q,
then the charge remaining on the comb would be (+Q ñ q).
| Now, instead of touching the charged comb to the electroscope,
hold it close to the plate. The aluminum leaves will again part for a
short while. What actually
happens is this : the charges on the comb polarize the charges in the plate.
Like charges attract. So the
top part of the electroscope plate acquires a charge opposite to the charge
on the comb. The lower part of
the electroscope, that is the aluminum leaves, gets charges same as that of
the comb. If the charge on the comb is positive, the top plate of the
electroscope will become negative, the aluminum leaves will become positive.
(Since both the aluminum leaves become positive, they will repel each
other). |
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Charging an electroscope by induction |
The mechanism of charging a (uncharged) body by bringing a charged
body near it, but not in contact with it, is called charging
by induction. In the process of induction the charges do not flow from
one body to the other. The body, in which the charges are being induced,
does not acquire extra charge. The charges get pulled or polarized by the
presence of the external charging body, in this case a charged plastic comb.
| In the beginning we saw how pieces of paper got attracted to a
plastic comb after you combed your dry hair. The phenomenon is due to
charges induced in the paper. The figure shows this clearly.
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3.
Lightening
One
of the best examples of observing effects of static electricity is
lightening. Moisture bearing clouds
are initially electrically neutral. But
as the mass of clouds increase in size, they start rubbing against each
other. This makes some clouds positively charged and some clouds negatively
charged. When a positively charged cloud comes near a negatively charged
cloud, they attract each other. The charges are neutralized or discharged
with each other. While this is happening, sparks are produced. These sparks
are seen as lightening.
Lightening
can occur between a charged cloud and the earth also.
To prevent damages to houses due to lightening strikes, many houses
have a lightening conductor placed on the roofs. The lightening conductor is
made up of a sharp pointed metal
connected directly to the ground. That is, it is earthed properly. The
lightening conductor is placed higher than the roof so that if a lightening
strikes, it strikes the conductor before it can reach the house. The
conductor quickly makes the charges flow down to the earth and the house is
saved from any damage.
Summary
We have seen in this
chapter the effects of static electrical charges. The electrical charges can
be introduced in a body either directly by conduction or indirectly by
induction. The best example of a natural phenomenon showing the effects of
static electricity is lightening.
The reason for the sharply pointed metal at the tip of the lightening
conductor is a bit beyond the syllabus. But to put it in simple words, the
electric potential near a sharp point is higher. So when the lightening
strikes, the sharp metallic point picks up the charges faster than any other
metallic surface.
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How to make an electroscope?
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You
will need a glass bottle, cork, a small metallic plate, a thin foil
of aluminum, a small copper wire.
Take
a glass bottle with a wide mouth. Remember that glass is an
insulator. Take a piece of cork that will fit into the mouth of the
bottle. Make a tiny whole in the cork. Put the copper wire through
the whole. On one end of the copper wire, attach the small metallic
plate. At the other end of the copper wire make a small loop or
hook. Cut a thin strip of aluminum foil. Place the strip on the hook of the copper wire. Now put the cork in
place and close the mouth of the bottle tightly. Your electroscope
is ready.
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Gold leaf electroscope |
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In a ready
made electroscope, the aluminum foil is replaced by gold foil, hence
it is called a gold leaf electroscope. Gold is used because it is a
very good conductor of electricity. Also ultra-thin foils of gold can
be used, as gold is a very malleable metal.
Thinner the foils of the leaves, much easier it is to charge
them.
The
electroscope will not work when the atmosphere has moisture or high
humidity. This is because moisture contains water and water is a
good conductor of electricity. The static charges built up on the
leaves will quickly be taken away by the moisture and we will not be
able to see the leaves part.
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