# ELECTRICITY

 WHAT IS ELECTRICITY

When two bodies are rubbed together,they acquire the property of attracting light objects like small bits of paper dust particles.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 to which it produces and experience electrical and magnetic effects.
Similar charges repel each other and opposite charges attract each other.
Conductors are those substances which allow passage of electrical charges to flow through them and have very low electrical resistance e.g., copper,aluminum,gold,silver etc.
Resistors offer high resistance to flow of electricity through them e.g., eureka,nichrome etc.
Insulators have infinite resistance and do not allow the passage of current e.g., rubber,glass etc.

## ELECTRIC POTENTIAL

The electric potential at a point is the work done in bringing a unit positive charge from infinity to that point.
Electric potential = Work done/Charge
Its SI unit is Volt (V).
Potential difference (VA-VB) between two points A and B is the work done in bringing a unit charge from point B to point A.
Potential difference is a scalar quantity and is measured by means of voltmeter (a high resistance device).

## ELECTRIC CURRENT

Flow of electric charges is called as electric current.It is measured by ammeter.
There are two types of electric current
(i) Alternating Current (AC)
(ii) Direct Current (DC)
Alternating current is used in houses and factories and its frequency is 50 Hz.
The direction of positive charges is same as direction of conventional current.
Current = Charge/Time
I = Q/t

## OHM’s LAW

According to it,the current flowing through a conductor is directly proportional to the potential difference across it.
I = V/R
or                                 V = IR, where R is resistance.

## RESISTANCE

The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area.If I and A are respectively length and cross-sectional area of a conductor and R is its resistance,then
R I/A
R = ρ I/A
Unit of resistance is ohm.
where, ρ is a constant of material of conductor called specific resistance or resistivity. Its SI unit is ohm-metre.
The reciprocal of resistance of a conductor is called the electrical conductor of the conductor.
Conductance = 1/Resistance
Unit of conductance is mho or siemen.
The reciprocal of resistivity of a conductor is called its conductivity.Its SI unit is mho m-1 or siemen/metre (S m-1).
The specific resistance of the material depends only on the material of conductor and its temperature.

## COMBINATION OF RESISTANCES

Series combination: R = R1+R2+R3 and here current flows through each conductor is same.
Parallel combination:1/R = 1/R1+1/R2+1/R3 and here potential across each conductor is same.

## ELECTRIC POWER

Electrical power is the electrical work done per unit time.
P=W/t
Its SI unit is watt (W).
or                                P=VI
or                                P=I2R                (since:V=IR)
1 kilowatt hour = 3600000 joule,=3.610J
1 Horse power  =746 watt
1 Horse power  =550 foot-pound/second

## HEATING EFFECT OF ELECTRICITY

Heat is produced when electric current is passed through a conductor.
Heat produced = I2Rt =VIt=V2t/R

## IMPORTANT POINTS BASED ON HEATING EFFECT

In home appliances like as electric iron,electric heater and heating rod,the heating element used is of a nichrome (an alloy of Ni and Ci) wire. Nichrome has high melting point and high resistivity. To avoid the risk of electric shock, the metal body of electrical appliances is earthed.
A electric fuse is generally prepared from tin-lead alloy (63% tin + 37% lead).It is should have high resistance and low melting point.It is connected in the series.
Filament of electric bulb is made of tungsten has high melting point (35000C) and high resistivity.
Tube light contains a long tube of glass which is linked internally with a fluorescent substance.It is filled with an inert gas like argon along with some mercury.

## MAGNETS

Magnet is a piece of iron or other materials that can attract iron containing objects.
The magnets which do not lose their magnetism with normal treatment are called permanent magnets.
The permanent magnets are made of certain alloys of nickel,cobalt and alloys of iron with some carbon.
Permanent magnets are made of steel,cobalt-steel,ticonal, alcomax and alnico.
The materials which retain their magnetism for a long time are called hard magnetic materials.
In bar,rod and horse-shoe magnets,north or south poles are either indicated by the letter N or S.
When poles of two magnets are brought close together,they exert force of each other.This force is called intersection between the poles.
If we cut a magnet in two parts,then each separate part will behave as a magnet.

## MAGNETIC FIELD

The space in the surrounding of a magnet or a current carrying conductor in which its magnetic effect can be experience is called magnetic field.

## MAGNETIC FLUX

The magnetic flux linked with a surface is equal to the total number of magnetic lines of force passing through that surface normally.Its unit is weber.

## ELECTROMAGNETIC INDUCTION

When a change occurs in the magnetic flux lined with the coil, an emf is induced in the coil.The phenomenon is called electromagnetic induction.
The phenomenon of production of indued emf in a circuit due to change in magnetic flux in its neighboring circuit is called mutual induction.