ELECTRICITY
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| 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 current 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=V✕I
or
P=I2R
(since:V=IR)
1 kilowatt hour =
3600000 joule,=3.6✕106 J
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.
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