09-05-2013, 04:36 PM
ELECTRIC CHARGES AND FIELDS
ELECTRIC CHARGES.pdf (Size: 1.18 MB / Downloads: 60)
INTRODUCTION
All of us have the experience of seeing a spark or hearing a crackle when
we take off our synthetic clothes or sweater, particularly in dry weather.
This is almost inevitable with ladies garments like a polyester saree. Have
you ever tried to find any explanation for this phenomenon? Another
common example of electric discharge is the lightning that we see in the
sky during thunderstorms. We also experience a sensation of an electric
shock either while opening the door of a car or holding the iron bar of a
bus after sliding from our seat. The reason for these experiences is
discharge of electric charges through our body, which were accumulated
due to rubbing of insulating surfaces. You might have also heard that
this is due to generation of static electricity. This is precisely the topic we
are going to discuss in this and the next chapter. Static means anything
that does not move or change with time. Electrostatics deals with the
study of forces, fields and potentials arising from static charges.
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.
UNIFICATION OF ELECTRICITY AND MAGNETISM
In olden days, electricity and magnetism were treated as separate subjects. Electricity
dealt with charges on glass rods, cat’s fur, batteries, lightning, etc., while magnetism
described interactions of magnets, iron filings, compass needles, etc. In 1820 Danish
scientist Oersted found that a compass needle is deflected by passing an electric current
through a wire placed near the needle. Ampere and Faraday supported this observation
by saying that electric charges in motion produce magnetic fields and moving magnets
generate electricity. The unification was achieved when the Scottish physicist Maxwell
and the Dutch physicist Lorentz put forward a theory where they showed the
interdependence of these two subjects. This field is called electromagnetism. Most of the
phenomena occurring around us can be described under electromagnetism. Virtually
every force that we can think of like friction, chemical force between atoms holding the
matter together, and even the forces describing processes occurring in cells of living
organisms, have its origin in electromagnetic force. Electromagnetic force is one of the
fundamental forces of nature.
Maxwell put forth four equations that play the same role in classical electromagnetism
as Newton’s equations of motion and gravitation law play in mechanics. He also argued
that light is electromagnetic in nature and its speed can be found by making purely
electric and magnetic measurements. He claimed that the science of optics is intimately
related to that of electricity and magnetism.
CONDUCTORS AND INSULATORS
A metal rod held in hand and rubbed with wool will not show any sign of
being charged. However, if a metal rod with a wooden or plastic handle is
rubbed without touching its metal part, it shows signs of charging.
Suppose we connect one end of a copper wire to a neutral pith ball and
the other end to a negatively charged plastic rod. We will find that the
pith ball acquires a negative charge. If a similar experiment is repeated
with a nylon thread or a rubber band, no transfer of charge will take
place from the plastic rod to the pith ball. Why does the transfer of charge
not take place from the rod to the ball?
Some substances readily allow passage of electricity through them,
others do not. Those which allow electricity to pass through them easily
are called conductors. They have electric charges (electrons) that are
comparatively free to move inside the material. Metals, human and animal
bodies and earth are conductors. Most of the non-metals like glass,
porcelain, plastic, nylon, wood offer high resistance to the passage of
electricity through them. They are called insulators. Most substances
fall into one of the two classes stated above*.
When some charge is transferred to a conductor, it readily gets
distributed over the entire surface of the conductor. In contrast, if some
charge is put on an insulator, it stays at the same place. You will learn
why this happens in the next chapter.
This property of the materials tells you why a nylon or plastic comb
gets electrified on combing dry hair or on rubbing, but a metal article
like spoon does not. The charges on metal leak through our body to the
ground as both are conductors of electricity.