(Magnetism is a class of physical phenomena that includes forces
exerted by magnets)
1. Magnet and its types :
A piece of substance, which possesses the property of attracting small
pieces of iron towards it, is called a magnet.
There are two types of magnets:
a. Natural: One that occurs naturally e.g., lodestone.
b. Artificial: One which is made by magnetizing small pieces of iron &
nickel.
2. Bar magnet & its properties :
A magnet with a regular shape of a parallelepiped of small thickness
is called a bar magnet. A bar magnet has two poles: North Pole & South
Pole. A bar magnet possesses the following properties:
a. Directive property: When a magnet is suspended freely, it always
orients itself in the north-south direction. The north pole of a magnet
is usually colored red or a red dot is placed on its side.
i. The end of a free to rotate magnet that points north direction is
called North Pole (N-pole)
ii. The end of a free to rotate magnet that points south direction is
called South Pole (S-pole).
b. Attractive property: A magnet can attract small pieces of magnetic
substances like iron, steel, cobalt, alnico, etc.
i. The attraction is maximum at the two ends/poles of the magnet.
ii. The attraction is minimum or zero at the center of the magnet.
Magnetic field
pattern of a bar
magnet as
displayed by
iron fillings on
a sheet of paper
iii. Between two magnets, opposite poles are attracted towards
each other.
iv. Between two magnets, like poles are repelled away from each
other.
c. Isolated poles do not exist: If a magnet is cut into small pieces, each
piece by itself is a magnet.
i. If a magnet is cut parallel to its length then its pole strength
becomes half.
ii. If a magnet is cut perpendicular to its length there is no change
in its pole strength.
3. Magnetic lines of force (or) magnetic field lines:
The path (straight or curved) along which unit North Pole moves in a
magnetic field (if it is free to do so) is called magnetic lines of force.
These lines of forces are actually imaginary lines.
4. Types of magnetic field:
There are two types of magnetic fields:
a. Uniform: If the magnitude of the magnetic field is equal & direction
is same at every point in the space.
b. Non-uniform: If the magnitude of the magnetic field is not equal &
direction is not same at every point in the space.
5. Right Hand Thumb Rule:
According to this rule, “if we imagine that we are holding a wire
carrying current and thumb is stretched in the direction of current then the
direction in which the fingers will be wrapped gives the direction of
magnetic lines of force”
i. If the current is flowing in upward direction then the direction
of magnetic field will be anticlockwise.
ii. If the current is flowing in downward direction then the
direction of magnetic field will be clockwise.
6. Magnetic field due to a straight current carrying conductor:
When current (I) is passed through a straight conductor, magnetic
field (B) forms concentric circles around the conductor & its magnitude is:
Where,
SI unit of magnetic field = Tesla
= circumference of a circle
= permeability of free space or vacuum
[Value of
in SI is
tesla metre per ampere as
current (I) is measured in ampere (A) and the distance r is
measured in metre (m)]
7. Magnetic field due to a current carrying circular coil:
When current (I) is passed through a circular coil, magnetic field (B)
forms almost a straight line near the centre of the coil & its magnitude is:
Where,
= permeability of free space or vacuum
= number of turns of the wire
= radius of the coil
8. Magnetic field due to a current carrying solenoid:
A solenoid is a cylindrical coil of many tightly wound turns of
insulated wires with generally diameter of the coil smaller than its length.
When current is passed through a solenoid, a magnetic field (B) is
developed in it.
i. The end of the solenoid at which the current flows in
anticlockwise direction acts as a North Pole.
ii. The end of the solenoid at which the current flows in clockwise
direction acts as a South Pole.
The magnetic field around a current carrying solenoid is similar to the
magnetic field produced by a bar magnet & its magnitude is:
Where,
= permeability of free space or vacuum
= number of turns in the solenoid
= strength of current passed through the solenoid
9. Electromagnets:
It is a piece of magnetic material like soft iron or hard steel which is
placed inside a solenoid through which current is flowing.
There are two types of electromagnet:
i. Permanent electromagnet:
It is obtained by placing a magnetic material like hard
steel in the strong magnetic field produced by a coil.
ii. Temporary electromagnet:
It is obtained by placing soft iron in a coil through which
current is flowing.
It only behaves like a magnet till the current flows
through it.
10. Magnetic force on a current carrying conductor :
Oersted found that a current carrying conductor produces a magnetic
field around itself. When this current carrying conductor is placed in the
magnetic field of a magnet, then the magnetic fields of the conductor & the
magnet interact. This interaction leads to a force. This force is experienced
by both the magnet and the current carrying conductor, in accordance with
Newton’s third law of motion.
The force experienced by the current carrying conductor is
perpendicular to both the direction of current & the magnetic field.
This force reverses when
i. either direction of current is reversed or
ii. the direction of the magnetic field is reversed.
The direction of force experienced by a current carrying conductor
can be found by Fleming’s left hand rule.
11. Fleming’s left hand rule:
According to this rule, “stretch out the thumb, the fore finger & the
central finger of left hand in such a way that these fingers are perpendicular
to each other. If the fore finger is placed in the direction of the magnetic
field & central finger in the direction of the current then the thumb will give
the direction of the force experienced by the current carrying conductor”
Experimentally it has been found that the magnitude of the force (F)
experienced by the current carrying conductor, in a magnetic field (B) is:
Where,
= current flowing through the conductor
= length of the conductor inside the magnetic field
= sine of the angle between the conductor & the magnetic
field
12. Alternating current & Direct current:
The electric current which changes
its direction after certain fixed
interval of time is called alternating
current (AC).
The electric current which always
flows in the same direction is called
direct current (DC).
13. Electric motor:
An electric motor is a device that converts electric energy into
mechanical energy.
Principle: When a current carrying conductor capable of moving
freely is placed in a magnetic field, it experiences a force & begins to move
in a direction given by Fleming’s left hand rule.
There are two kinds of motor that we use in our daily life:
i.
ii.
AC motor that uses alternating current (AC) supply e.g., motor
of a fan.
DC motor that uses direct current (DC) supply e.g., motors of
battery operated toys.
Basic components of a DC motor
14. Electromagnetic induction:
Michael Faraday discovered that when a straight conductor moves in a
magnetic field, current is generated in the conductor. This phenomenon of
production of electric current is known as electromagnetic induction & the
current so obtained is called induced current.
Strength of the induced current depends upon the following factors:
i. Strength of the magnetic field.
ii. Number of turns in the coil
iii. Relative speed between the coil & the magnet
iv. Resistance of the coil.
The direction of force experienced by a current carrying conductor
can be found by Fleming’s left hand rule.
15. Fleming’s right hand rule:
According to this rule, “stretch out the thumb, the fore finger & the
central finger of the right hand in such a way that these fingers are
perpendicular to each other. If the thumb is placed in the direction of the
motion of the conductor (force) & forefinger in the direction of the magnetic
field then the central finger will give the direction of induced current”
16. Electric generator (dynamo):
An electric generator is a machine that converts mechanical energy
into electrical energy.
There are two kinds of generators:
i. DC generators that convert mechanical energy into direct
electrical energy (DC).
Principle: When a coil rotates in a uniform magnetic field, a current
is induced in the coil. The direction of induced current is determined
by Fleming’s right hand rule.
ii.
AC generators that convert mechanical energy into alternating
electrical energy (AC). It is similar to a DC generator, with one
major difference the split ring commutator is replaced by slip
rings.
Principle: When a coil rotates in a uniform magnetic field, a current
is induced in the coil. The direction of induced current is determined
by Fleming’s right hand rule.
17. Domestic electric circuit:
i.
ii.
iii.
iv.
v.
At our home we receive 220 V alternating current (AC).
Each core cable is a copper wire capable of withstanding about
15A of current.
The power is supplied through a cable consisting of three wires:
a. the live/ phase wire
b. the neutral wire &
c. the earth wire
In the old color convention:
a. Red wire: live wire
b. Black wire: neutral wire
c. Green wire: earth wire
In the new color convention:
a. Brown wire: live wire
b. Light blue wire: neutral wire
c. Green/ yellow wire: earth wire
18. Short circuit:
It occurs when the live and neutral wire come in contact with each
other. This can happen due to the
i. Damage to the appliance or
ii. Wearing out of the wires carrying current.
19. Overloading:
If the current in the circuit exceeds the predetermined value, then the
circuit is said to be overloaded.
20. Electric fuse:
i.
ii.
iii.
iv.
It is a short length of easily fusible wire connected to the live wire
in the beginning of the circuit in series for protection purposes.
It has low melting point. It is arranged to melt (or blow) in order to
break the circuit easily if the current in the circuit exceeds its
safety limit, due to short circuit or overloading.
It has high resistivity.
It is an alloy of lead & tin (63% tin + 37% lead).