When two bodies are rubbed together, they acquire the property of
attracting light objects like small bits of paper, dust particles etc. The
bodies which acquire this property are said to be electrified or charged with
electricity.
Charge : Charge is the basic property associated
with matter due 10 which it produces and experiences electrical and magnetic
effects.
·
Benjamin Franklin named the two types of
charges as positive and negative.
·
Similar charges repel each other and
opposite charges attract each other.
·
Charging of bodies takes place due to
transfer of electrons from one body to other body.
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A list of materials has been given below.
The list is such that any of the material in the list will be positively
charged when rubbed with any other material coming later in the list. The other
material will naturally be negatively charged.
1. Fur 2. Flannel
3. Shellac 4. Sealing Wax
5. Glass 6. Paper
7. Silk 8. Human body
9. Wood 10. Metals
Surface density of charge : Surface density of
charge is defined as the amount of charge per unit area on the surface of
conductor.
·
The surface density of charge at a point on
the surface of conductor depends upon the shape of conductor and presence of
other conductors or insulators near the given conductor.
·
The surface density of charge at any part of
the conductor is inversely proportional to the radius of curvature of the
surface of that part. This is why surface density of charge in maximum at the
pointed parts of the conductor.
Conductor : Conductors are those materials which allow
electricity (charge) to pass through themselves.
Examples : (a) Metals like silver, iron, copper (b) Earth (especially
the moist part) acts like a huge conductor.
·
Silver is the best conductor.
Insulator or Dielectric : Insulators are those
materials which do not allow electricity to flow through themselves. Examples :
Wood, paper, mica, glass, ebonite.
Coulomb’s law According to Coulomb’s law, the force of attraction or
repulsion between two point charges at rest is directly proportional to the
product of the magnitudes of the charges and inversely proportional to the
square of the distance between them. This force acts on the line joining the
two charges.
Electric Field : Region in space around a charge
or charged body where the charge has its electrical effect is called electric
field of the charge.
Electric Field Intensity: Electric field
intensity at a point in an electric field is the force experienced by a unit
positive charge placed at that point.
Electric Field of hollow conductor:
Electric field intensity inside a charged hollow conductor is zero.
Charge given to such a conductor (or conductor of any shape) remains on its
surface only. This explains why a hollow conductor acts as an electrostatic
shield. It is for this reason that it is safer to sit in a car or bus during
lightning.
Electric Potential: Electric potential at a point in
an electric field is the work done in bringing a unit positive charge from
infinity to that point. SI unit of electric
potential is volt. It is a scalar quantity.
Potential Difference : Work done in
bringing a unit positive charge from one point to other point is the potential
difference between the two points. Its SI unit is volt and is a scalar
quantity.
Electric Capacity : Electric capacity of a conductor
is defined as the charge required to increase the potential of the conductor by
unity. If potential of a conductor is increased by V when a charge Q is given
to it, capacity of the conductor is Q/V
. Its SI unit is farad. (F)
Electrochemical Cell: Electrochemical cell
is a device which converts chemical energy into electrical energy.
Cells are basically of two types: (i) Primary cell (ii) Secondary cell.
Primary Cell : In primary cell electrical
energy is obtained from the irreversible chemical reaction taking inside the
cell. After complete discharge, primary cell becomes unserviceable.
Examples : Voltaic Cell, Leclanche Cell, Daniel Cell, Dry Cell etc.
Secondary Cell : A secondary cell is that which
has to be charged at first from an external electric source and then can be
used to draw current. Such cells are rechargable.
·
Production of electricity by chemical
reaction was first discovered by Allexandro de volta (voltaic cell is named
after him) in 1794. In voltaic cell zinc rod is used as cathode and copper rod
is used as anode. These rods are placed in sulphuric acid kept in a glass
vessel.
·
In a Leclanche cell, carbon rod acts as
anode and zinc rod acts as cathode. These rods are placed in amonium chloride
kept in a glass vessel.
·
The emf of Leclanche cell is 1.5 volt.
·
Leclanche cell is used for intermittent
works. i.e. works in which continuous electrical energy is not required like
electric bell.
In a dry cell, mixture of MnO2, NH4C1 and carbon
is kept in a zinc vessel. A carbon rod is placed in the mixture which acts as
anode. The zinc vessel itself acts as cathode. The emf of dry cell is 1.5 volt.
