##### Physics
General Knowledge

# Coulomb's law & Electric Charge

## 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 is a device which converts chemical energy into electrical energy. Cells are basically of two types :
1. Primary cell
2. 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.

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 rechargeable.

»   Production of electricity by chemical reaction was first discovered by Allexand 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 Mn02, 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.