ELECTROSTATICS (PART 1 )
The SI unit of charge is Coulomb.
Properties of Charge
1. Like charges repel each other and unlike charges
attract each other.
2. Charge is a scalar and can be of two types positive or negative.
3. Charge is quantized. The quantum of charge is e.
The charge on any body will be some integral multiples of e, i.e.
Charge on any body can never be (1/3e), 1.5e
etc.
4. During any process, the net electric charge of an
isolated system remains constant or we can say that charge is conserved.
5. A charge particle at rest produces electric field.
A charge particle in an unaccelerated motion produces both electric and
magnetic field but does not radiate energy. But an accelerated charged particle
not only produces an electric and magnetic field but also radiates energy in
the form of electromagnetic waves.
Conductors and Insulators
In conductors electric charges are free to move from
one place to another whereas in insulators they are tightly bound to their
respective atoms.
In an uncharged body there are equal number of
positive and negative charges.
Semiconductors are a third class of materials and
their electrical properties are somewhere between those of insulators and
conductors.
E.g. Silicon, Germanium
Charging of a Body
(i) Charging by Rubbing
All material bodies contain large number of electrons
and equal number of protons in their normal state. When rubbed against each
other, some of the electrons from one body pass onto the other body. The body
that donates the electrons becomes positively charged while that which receives
the electrons become negatively charged.
Eg: - When glass rod is rubbed with silk cloth, glass
rod becomes positively charged because it donates the electrons while the silk
cloth becomes negatively charged because it receives electrons.
Electricity so obtained by rubbing two objects is
known as frictional electricity.
(ii) Charging by Contact
When a negatively charged ebonite rod is rubbed on a
metal object, such as a sphere, some of the excess electrons from the rod are
transferred to the sphere. Once the electrons are on the metal sphere, where they
can move easily, they repel one another and spread out over the sphere’s
surface. The insulated stand prevents them from flowing to the earth. When the
rod is removed the sphere is left with a negative charge distributed over its
surface.
è The
process of giving one object a net electric charge by placing it in contact with
another object that is already charged is known as charging by contact.
(iii) Charging by Induction
It is also possible to charge a conductor in a way
that doesn’t involve contact.
In figure(a) a negatively charged rod is brought
close to (but not touched) a metal sphere. In the sphere, the free electrons
close to the rod move to the other side (by repulsion). As a result, the part
of the sphere nearer to the rod becomes positively charged and the part
farthest from the rod and the part farthest from the rod negatively charged.
This phenomenon is called induction. When a metal wire is attached between the
sphere and the ground as in figure(b) some of the free electrons leave the
sphere and distribute themselves on the much larger earth. If the grounding
wire is then removed, followed by the ebonite rod, the sphere is left with a
net positive charge.
è The
process of giving one object a net electric charge without touching the object
to a second charged object is called charging by induction.
Coulomb’s
Law
The electric force Fe exerted by one-point
charge on another acts along the line between the charges. It varies inversely
as the square of the distance separating the charges and is proportional to the
product of charges. The force is repulsive if the charges have the same sign
and attractive if the charges have opposite sign.
Note: -
(i) When two charges exert forces simultaneously on a
third charge, the total force acting on that charge is the vector sum of the
forces that the two charges would exert individually. This property is called
the principle of superposition of forces.
Thus,
(ii) The electric force is an action reaction pair,
i.e. the two charges exert equal and opposite forces on each other.
(iii) The electric force is conservative in nature.
(iv) If some dielectric(insulator) is present in the
space between the charges, the net force acting on each charge is altered. The
force decreases K times if the medium extends till infinity.
Here K is a dimensionless constant which depends on
the medium and called dielectric constant of the medium.
(v) Lami’s Theorem
According to this theorem, “if three concurrent
forces F1, F2, and F3 are in equilibrium or if
F1+F2+F3=0, then
(vi) Suppose the position vectors of two charges q1
and q2 are r1 and r2, then electric force on
charge q1 due to charge q2 is,
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