A comb attracts bits of paper after being rubbed with dry hair, the cracking sound made by nylon or terylene socks when taken off from our body etc., are some of the phenomena observed in our daily life. These phenomena are studied in physics under the topic of electrostatics.
Electrostatics is the study of static electric charges. In
nature, there are two types of electric charges, i.e., positive (+) charges and
negative (-) charges.
Electric Charge
Every substance comprises atoms containing equal numbers
of positive and negative charges. A positively charged body has more positive
charges and a negatively charged body has more negative charges. Electric
charges can exist independently either as positive or negative charges. The SI
unit of electric charge is the coulomb represented by the symbol ‘C’.
Conductors
Substances that allow electricity to pass through them are known as conductors. Ex: Metals, human body, graphite, coal, etc.
Earth is a good conductor of electricity. In conductors, there
are free electrons and they are responsible for the conduction of electricity
in the conductors.
Insulators or
Dielectrics
Substances that do not allow free flow of charge through
them are called insulators. Ex: Plastic, diamond, sealing wax, wood etc.,
In insulators, there are no free electrons. When an insulator
is kept in an electric field, it remains as an insulator only up to a certain
limit of the strength of the electric field. Beyond that limit of the electric
field, it becomes a conductor. The maximum strength of the electric field up to
which an insulator can withstand without becoming a conductor is called
dielectric strength.
Semi-Conductors
Substances that allow the charges partly through them are
called semiconductors. Ex: Germanium, Silicon etc.,
Electrification
Electrification is a process by which a body is electrified.
Methods of
Electrification
A body can be charged by one of the methods such as friction,
contact or conduction and induction.
Charging by friction
If a body is charged by rubbing it against another body, then
the process of charging is called charging by friction. In this process, one
body loses some electrons and the second body gains the same number of
electrons.
Insulators do not contain free electrons that move about from
one atom to another like in metals. However, they can be charged by rubbing
with a suitable material. They generate static electricity due to friction.
Some examples are:
A Perspex (an acrylic plastic
material) rod on the other hand becomes positively charged if rubbed with a
woolen cloth. In this case, the electrons are transferred from Perspex to the
cloth and the cloth becomes negatively charged.
When we rub a glass rod with silk,
then some of the electrons from the glass rod are transferred to the silk.
Since the glass rod has a deficiency of electrons as compared to the protons in
its nucleus, therefore, it gets positively charged. At the same time, the silk
tends to have an excess of electrons as compared to protons and, therefore, gets
negatively charged.
Cellulose Acetate rod when rubbed with wool becomes positively charged.
When we rub an ebonite rod with the
cat’s skin, some of the electrons from the cat’s skin are transferred to the
ebonite rod. As the ebonite rod has an excess of electrons as compared to protons,
therefore, it gets negatively charged. At the same time, the cat’s skin has a deficiency of electrons as compared to the protons, therefore, it gets
positively charged.
Rubber rod when rubbed with hair or
fur becomes negatively charged.
It must be remembered that during positive or negative
electrification. It is the electrons and not the protons that get transferred.
If one body gets positively charged due to rubbing, then the
body that is used for rubbing gets negatively charged at the same time. Thus,
equal and opposite electric charges are produced at the same time.
Read more: Understanding Sound And Its Applications
Conduction
If a body is charged by contact with another charged body,
then the process is called charging by conduction. In this process, the body
acquires the same charge and the total charge is shared by the two bodies.
Charging by induction
If a body is charged in the presence of another charged body
without contact, then the process is called charging by induction. In this
process charge acquired by the body is equal and opposite to the charge on the
charging body. Here there is no sharing of charges but only redistribution of
charges on the body.
Charge can be passed from one object
to another, but it cannot be lost.
Every electron has a charge of -1.6 x
10-19 C, and every proton has a charge of +1.6 x 10-19 C.
The charge can neither be created nor
destroyed, but the charge acquired by and body is in the integral multiples of
ne.
Law of Electric Forces
Like charges repel each other whereas
unlike charges attract each other.
Charge always resides on the outer
surfaces of the body.
Coulomb’s law
The electrostatic force of interaction between two point
electric charges is directly proportional to the product of the charges,
inversely proportional to the square of the distance between them and acts
along the straight line joining the two charges.
Electroscope
An Electroscope is an instrument for detecting the presence
of very weak electrical charges and for determining whether that charge is
positive or negative.
Gold-Leaf Electroscope
A gold-leaf electroscope consists of two gold leaves fixed to
the lower end of a brass rod so that they hang parallel to each other. This
brass rod passes through an insulating cork, say of ebonite, fitted on the
mouth of a glass vessel. At the upper end of the rod, there is a brass disc.
When a gold leaf electroscope is uncharged, the gold leaves remain
collapsed (remains parallel).
In charging by contact, the gold leaves receive the same kind
of charge as the charging body.
The gold leaves, being similarly charged, repel each other
and therefore diverge. They remain diverged even after the charged rod is
removed.
Lightening
Lightening occurs when a charge jumps from one cloud to another
or from clouds to objects on the Earth.
Lightening is a gigantic electric discharge occurring between
a cloud and the Earth. It was the American inventor, Benjamin Franklin
(1706-1790), who showed that lightning was static electricity, formed by the
friction between droplets and particles (ice crystals), in rain clouds.
An electric discharge due to lightning, passing through a
building can cause serious damage to the building. However, buildings can be
protected from such damage if a lightning conductor is installed in the
building.
A lightning rod consists of a thick copper strip fixed to an
outside wall of a building. It reaches above the highest part of the building
terminating in sharp spikes. Its lower end is terminated on a metal plate
buried in the ground.
The lightning rod intercepts the cloud discharges (and flashes to the Earth) before it reaches the threatened building and passes the current harmlessly to the ground.