Magnet
MAGNETIC EFFECTS OF CURRENT
A magnet is an object, which attracts pieces of iron, steel,
nickel and cobalt.
Naturally Occurring Magnet
Lodestone is a naturally occurring magnet. It is actually a
black coloured, oxide ore of iron called magnetite (Fe 3 O 4 ),
which behaves like a magnet.
1.
2.
3.
4.
Used in radio and stereo speakers, refrigerator doors.
In audio and video cassettes, tapes, hard disks.
In children’s toys.
In hospitals to scan inner human body parts.
Magnetic Field
Space surrounding a magnet in which magnetic force
(attraction or repulsion) is exerted is called magnetic field.
Direction of magnetic field at a point is the direction of the
resultant force acting on a hypothetical north pole placed at
that point.
Bar Magnet
A bar magnet is a long rectangular bar of uniform cross section
that attracts pieces of iron, steel, nickel and cobalt.
Magnetic Needle (Plotting Needle, Plotting Compass,
Magnetic Compass)
It is a tiny magnet which is free to move in the horizontal
plane. It is in the form of an arrow. The tip of compass
represents its north pole and the tail of compass represents its
south pole.
Magnetic Lines Of Force
Magnetic lines of force are the lines drawn in a magnetic field
along which a north magnetic pole would move. The direction
of magnetic lines of force at any point gives the direction of
the magnetic force on a north pole placed at that point.
Method To Plot Magnetic Lines Of Force
Horse Shoe Type Magnet
1. By Using Iron Filings
If iron filings are sprinkled around a bar magnet placed on a
horizontal plane, then the iron filings arrange themselves in a
regular pattern giving a rough picture of magnetic lines of
force.
For A Bar Magnet
A magnet has two poles: 1. North Pole Of Magnet
The end of a freely suspended magnet which points
towards the geographic north direction of earth is called
the north pole of magnet.
2.
South Pole Of Magnet
The end of freely suspended magnet which points towards
the geographic south direction of earth is called the south
pole of magnet.
Properties Of The Poles Of Magnet
1. Like magnetic poles repel each other.
2. Unlike magnetic poles attract each other.
Uses Of Magnet
For A U Shaped Magnet
2.
3.
2.
By Using A Plotting Compass
1.
2.
Place a bar magnet on a sheet of paper.
Bring north pole of the plotting compass near the north
pole of the bar magnet. Due to the repulsion between the
two north poles the tip of the compass moves away from
the north pole of the magnet and tail of the compass
comes near to the north pole of magnet.
Mark the positions of tip and tail of compass.
Now move the plotting compass to the marked point and
get another point.
In this manner we get various dots denoting the path in
which the north pole of the plotting compass moves.
Joining the various dots we get a smooth curve
representing a magnetic line of force. In this way we can
draw a large number of lines of forces by starting from
different points near the magnet.
Steps
3.
4.
5.
6.
Properties Of Magnetic Lines Of Force
1. They travel from north pole to south pole outside the
magnet and south to north inside the magnet.
2. They are continuous closed curves.
3. They emerge out normally from the magnetized surfaces.
4. The tangent drawn at any point of magnetic lines of force
represents the direction of the magnetic field at that point.
5. Two magnetic lines of force do not intersect each other.
6. They contract in length and repel each other laterally.
7. The field lines of a uniform magnetic field are parallel to
each other.
8. The more crowded the magnetic field lines, the stronger is
the magnetic field.
Magnetic Effects Of Current (Electromagnetism)
Magnetic effects of current means that, the current flowing in
a wire produces a magnetic field around it. In 1820 Oersted
was able to show that an electric current flowing through a
wire produces a magnetic field around it. It was the first
observation that indicates a connection between electricity and
magnetism. Electrically produced magnetism is also called
electromagnetism.
Experiment Demonstrating Magnetic Effects Of Current
1. Take a thin insulated copper wire connected to a battery.
4.
Place the compass needle directly under the wire. The
needle points north - south when there is no current.
Pass the current towards the north, the needle deflects
towards the west. When the current is passed towards the
south the needle deflects towards the east.
Place the compass directly above the wire, now the needle
deflections are reversed.
Magnetic Field Due To A Straight Current Carrying
Conductor
Procedure
1. Take a copper wire AB.
2. Pass it through a cardboard. Connect the wire to a battery
through a key.
3. Sprinkle some iron fillings on the cardboard.
4. Switch on the key and hold the cardboard gently. The iron
fillings will arrange themselves in the form of concentric
circles.
5. Reverse the direction of current by changing polarity of
the battery. This time too the iron fillings arrange
themselves in concentric circles but in opposite direction.
This shows that the magnetic lines of force of a straight
current carrying conductor are circular in nature.
Right Hand Thumb Rule To Find Direction Of Magnetic
Field Produced By Straight Current Carrying Conductor
According To Maxwell’s Right Hand Thumb Rule:
Hold the conductor in your right hand in such a way that the
thumb points in the direction of current flowing in it and then
the direction in which fingers encircle (curve) gives the
direction of magnetic field.
Magnetic Field Pattern Due To A Circular Coil Carrying
Current
Circular Coil
A circular coil consists of twenty or more turns of insulated
copper wire closely wound together. It has been found that
the magnetic effect of current increases if, instead of using a
straight wire, the wire is converted into a circular coil. To make
things simple circular coil consisting of only one turn is taken.
Procedure
1. Take a short circular coil and fixed it to a thin cardboard
sheet.
2. Pass the current through the circular coil, a magnetic field
is produced around it. The pattern of magnetic lines of
force is circular near the wire, but they become straight
and parallel at the middle of coil.
Factors On Which Strength Of Magnetic Field Produced
By A Current Carrying Solenoid Depends: 1. The number of turns in the solenoid.
2. Strength of current in solenoid.
3. The nature of core material used in making solenoid.
Electromagnet
The combination of a solenoid and a soft iron core is
called electromagnet.
The magnetic field produced by a current carrying circular wire
behaves as a thin disc of magnet, whose one face is a north
pole and other face is a south pole.
Magnetic Field Due To A Current Carrying Solenoid
The solenoid is a long coil containing a large number of close
turns of insulated copper wire.
Thus, an electromagnet consists of a long coil of insulated
copper wire wound on a soft iron core.
Procedure
1. Connect the two ends of a solenoid to a battery through a
switch.
2. When electric current is passed through it, it produces a
magnetic field around it. The magnetic field produced by a
current carrying solenoid is similar to the magnetic field
produced by a bar magnet. If a current carrying solenoid
is suspended freely, it comes to rest pointing north and
south.
How Electromagnets Are Made?
1. To make an electromagnet take a rod of soft iron.
2. Wind a coil of insulated copper wire round it.
3. Connect the two ends of the copper coil to a battery. An
electromagnet is formed.
A solenoid containing soft iron core in it acts as a magnet only
as long as the current is flowing in the solenoid. If we switch
off the current in the solenoid, it no more behaves as a
magnet. All the magnetism of the soft iron core disappears as
soon as the current in the coil is switched off. It is the iron
piece inside the coil which becomes a strong
electromagnet on passing the current.
Factors Affecting Strength Of An Electromagnet
Strength of electromagnet is directly proportional to: 1. The number of turns in the coil.
2. Strength of current flowing in the coil.
And,
The strength of electromagnet is inversely proportional to the
length of air gap between its poles. A U shaped electromagnet
has small air gaps hence it acts as a strong electromagnet.
Bar Magnet (Permanent Magnet)
Permanent magnets are usually made up of alloys such as :
carbon steel, chromium steel, tungsten steel, cobalt steel,
alnico (alloy of aluminium, nickle, cobalt, iron) nipermag (alloy
of iron, nickle, aluminium, titanium). Permanent magnets of
these alloys are much stronger than those made up of ordinary
steel. Such strong permanent magnets are used in
microphones, loudspeakers, electric clocks, ammeter,
voltmeter and speedometer etc.
When a current carrying conductor is placed in a magnetic
field, it experiences a force and begins to rotate continuously
in a direction given by Flemings Left Hand rule.
Force On Current Carrying Conductor Placed In A
Magnetic Field
Oersted’s experiment shows that a current carrying wire exerts
a force on a freely suspended magnetic needle and deflects it.
We can also say that a current carrying wire exerts a
mechanical force on a magnet and this force produce motion
in the magnet.
1. Armature Coil
It consists of large number of turns of copper wire wound over
a rectangular core of soft iron.
In reverse we can say that a magnet exerts a mechanical
force on a current carrying wire, and if the wire is free to move
this force can produce motion in the wire.
How The Direction Of Force On A Current Carrying
Conductor Placed In Magnetic Field Can Be Reversed?
1. Reversing the direction of flow of current in the conductor.
2. Reversing the direction of magnetic field.
Fleming’s Left Hand Rule To Find The Direction Of
Force Applied On The Conductor
According to Fleming’s Left Hand Rule: Hold the forefinger, the centre finger and thumb of your left
hand at right angles to one another. Adjust your hand in such
a way that the forefinger points in the direction of magnetic
field and the centre finger points in the direction of current,
then the direction in which the thumb points, gives the
direction of force acting on a conductor.
Electric Motor
An electric motor is a device that converts electrical
energy into mechanical energy. Every motor has a shaft or
spindle which rotates continuously when current is passed into
it. The rotation of its shaft is used to derive the various types
of machines in homes and industry e.g. electric fans, washing
machines, refrigerator, mixer and grinder etc.
Types Of Motor
1.
AC Motor (Alternating Current Motor)
Uses AC (Alternating Current) supply e.g. motor of a fan.
2.
DC Motor (Direct Current Motor)
Uses DC (Direct Current) supply e.g. motors of battery
operated toys.
Direct Current Motor (DC Motor)
Principle Of A Motor
Construction
It consists of following components: -
2. Strong Field Magnets
The coil is mounted between the curved poles of a U-shaped
permanent magnet in such a way that it can rotate between
the poles N and S.
3. Split Rings
The two ends of the coil are welded to two semicircular
metallic rings. These rings are called split rings or half rings or
commutator.
Function Of Split Rings
Split rings are meant to change the direction of current flowing
through the coil after each half rotation.
4. Carbon Brushes
Two carbon brushes B1 and B2 make a contact with the slip
rings of the commutator and through them the current is
supplied to the coil. The system of two half rings and the
associated brushes are referred to as a split ring commutator.
Working
1. When the current is passed through the coil of copper
wire from the battery, it enters the coil through the left
brush and half ring, goes around the coil and then leaves
through the right half ring and brush.
2. As the direction of current is perpendicular to magnetic
field, a torque (one force in upward direction and one in
downward direction) acts on it. According to Fleming’s Left
Hand Rule, applied on the left side AB of the coil we find
that it will experience a force in upward direction and the
right side DC of the coil will experience a force in
downward direction.
3. This turns the coil due to which the coil starts rotating in
the anticlockwise direction.
4. While rotating when the coil reaches the vertical position
then the brushes will touch the gap between the two
commutator rings and current to the coil is cut off, but the
coil does not stop rotating due to gain of momentum.
5. When the coil goes beyond the vertical position and the
half rings are again connected to brushes, but now on
opposite sides, this reverses the direction of current in the
coil which in turn reverses the direction of forces acting on
the two sides of the coil.
6. In this position also couple of forces act on the coil
(torque) which rotate it in the same anticlockwise
direction.
7.
This process is repeated again and again and the coil
continues to rotate as long as the current is passing.
Note
The coil experiences maximum force at 0° and 180° and no
force at 90° and 270°.
Electromagnetic Induction
(Electricity From Magnetism)
The phenomenon of producing electric current by
moving a magnet near a coil or a coil near a magnet is
called electromagnetic induction. If magnetic field through
a circuit changes an emf and a current is induced in the circuit,
the emf is called induced emf (voltage) and the current is
called induced (brought about) current.
Condition For Electromagnetic Induction To Take Place
The condition is that there must be a relative motion between
magnetic field and coil.
Faraday’s Experiment
There are a number of ways a magnetic field can be used to
generate an electric current.
First Method
1. Place the coil in the field of a bar magnet to which
galvanometer is connected.
2. When there is no relative motion between the bar magnet
and the coil, the galvanometer reads zero, indicating there
is no current.
3. However, when the magnet moves towards the coil, a
current appears in the coil. As the magnet approaches the
magnetic field that it creates at the location of the coil
becomes stronger and stronger and it is this changing
magnetic field which produces current.
4. When the magnet moves away from the coil a current also
exists but the direction of the current is reversed.
Second Method
A current would be created if the magnet were held stationary
and the coil were moved because the magnetic field at the coil
would be changing as the coil approaches or recede from the
magnet. Only relative motion between the magnet and the coil
is needed to generate a current it does not matter which one
moves.
Factors On Which Induced Current In The Coil
Depends: 1. Strength of the magnetic field
2. Number of turns in the coil
3. Relative speed between the coil and the magnet
Direction Of Induced EMF
Induced Current changes its direction with the motion of the
magnet in opposite direction. The direction of induced current
can be obtained by Fleming’s Right Hand Rule.
Fleming’s Right Hand Rule
Hold the forefinger, the middle finger and the thumb of your
right hand mutually perpendicular to each other. Now position
your hand in such a way that the thumb represents the
direction of motion of conductor, and the fore finger gives the
direction of magnetic field, then the direction in which the
middle finger points gives the direction of induced current.
Electric Generators (Dynamo)
An electric generator converts mechanical energy into
electrical energy. An electric generator is a machine, which
generates electricity from mechanical energy by using the
principle of electromagnetic induction.
Types Of Generators
1.
AC Generator (Alternating Current Generator)
Induces AC (Alternating Current) supply.
2.
DC Generator (Direct Current Generator)
Induces DC (Direct Current) supply.
DC Generator
Also called DC dynamo, converts mechanical energy into
electrical energy. The DC generator produces direct current
and not alternating current.
Principle
When a straight conductor is moved in a magnetic field, then
current is induced in the conductor. In electric generator a
rectangular coil having straight sides is made to rotate rapidly
in the magnetic field between the poles of a horse shoe type
magnet. When the coil rotates, it cuts the lines of magnetic
force, due to which a current is induced in the generator coil.
This current can be used to run various electrical appliances.
Construction Of D.C. Generator
A simple D.C. Generator consists of the following: An armature coil of copper wire wound on a soft iron core. The
armature coil rotates between the pole pieces of a strong
magnet. The free ends of the coil are connected to the
commutator or slip rings R1 and R2. Two metallic brushes B1
and B2 are in contact with the slip rings. These brushes are
called contact brushes. When the coil is rotated, the two half
rings R1 and R2 touch the two carbon brushes B1 and B2 one
by one. In this way the current produced in the rotating coil
can be tapped out through the commutator half rings into the
carbon brushes. From the carbon brushes B1 and B2 we can
take the current into the various electrical appliances.
1.
As the coil rotates in the anticlockwise direction, the side
AB moves down and CD moves up cutting the magnetic
lines near N pole and S pole respectively. Due to this the
current is induced in side AB and DC of the coil. On
applying Fleming’s Right Hand rule to the sides AB and
CD, the direction of induced current is found out to be
BADC.
2.
After half rotation the sides AB and CD of the coil
interchange their positions, arm AB moves up and arm CD
moves down. Now the direction of induced current in each
side of the coil is reversed after half revolution. So the
polarity of the two ends of the coil also changes after half
revolution. Thus, in one complete rotation of the coil, the
current changes its direction two times.
Working Of A D.C. Generator
Suppose that generator coil ABCD is initially in the horizontal
position and is being rotated in the anticlockwise direction.
1.
2.
As the coil rotates in the anticlockwise direction, i.e., the
side AB moves downwards and side CD upwards cutting
the magnetic lines of force near the N – Pole and S – Pole
of the magnet respectively. Due to this motion of the coil
in the magnetic field, current is induced in the coil from D
to C and B to A, according to the Fleming’s right hand rule
respectively. Thus, the induced current in the two sides of
coil are in the same direction i.e. BADC. Due to this the
brush b1 becomes positive and brush B2 becomes
negative.
After half rotation, the arms of the coil interchange their
positions. Arm AB comes to the right and arm CD to the
left. But when sides of the coil interchange their positions
then the two commutator half rings R1 and R2
automatically change. This keeps current flowing in the
same direction i.e., B1 always remain +ve and B2 always
remain –ve. Thus a DC generator supplies current always
in same direction.
Comparison Of D.C. Generator with D.C. Motor
DC Generator
Coil is rotated to produce
direct current.
Electric Motor
Direct current is supplied to
rotate the coil.
AC Generator
AC Generator produces alternating current which changes its
polarity continuously.
Construction
A simple AC generator consists of an armature coil of copper
wire of many turns wound over a soft iron core. The armature
is placed between the poles a strong magnet. The ends of the
coil are connected to two cylindrical full rings called slip rings
R1 and R2 mounted on the shaft. Each slip ring is permanently
in contact with a carbon brush. The brushes are connected to
the fixed terminals P and Q. As the slip rings R1 and R2 rotate
with the coil, the two carbon brushes B1 and B2 keep contact
with them and the current can be tapped out through the slip
rings into the carbon brushes and further into the circuit.
Working
Suppose that the generator coil ABCD is initially in the
horizontal position and then is being rotated in the
anticlockwise direction between the poles N and S of a horse
shoe type magnet.
Household Electric Circuit (Domestic Wiring)
Electricity is generated in a power station. It is usually
alternating. Its voltage is 11000 volts to 22000 volt. The power
stations are located far away from the cities and industrial area
and hence transmission of electricity is done with the help of
high tension underground or overhead cables. From electric
poles situated in our street, two insulated wires, live wire
(red in color) and neutral wire (black in color) are used.
Live wire is at high potential of 220 volts whereas neutral
wire is at ground potential of 0 volt. Thus the potential
difference between the live wire and neutral wire in India is
220 volts. There is no harm when we touch the neutral wire
but we will get an electric shock if we touch live wire directly.
The two insulated wires L and N coming from the electric pole
enter a box fitted inside our house. In this box, a main fuse is
put in the live wire. This fuse has a high rating of about 50 A.
The two line wires then enter the electric meter which records
the electrical energy consumed in Kilowatt Hours. The two
wires coming out of the meter are connected to the main
switch from which the electrical supply can be switched off
when required. After the main switch there is another fuse in
the live wire. This is called consumer’s fuse.
Circuit In House
There are usually two separate circuits in a house.
1.
2.
The lighting circuit with a 5 A fuse for lighting.
The power circuits with a 15 A fuse for radios, fans,
lighting electric iron, room heater, electric stove,
refrigerator etc.
Each distribution circuit is provided with a separate fuse so
that if a fault like short circuiting or overloading occurs in one
circuit, its corresponding fuse blows off but the other circuits
remain unaffected. Also various distribution circuits are
connected in parallel so that if a fault occurs in one circuit, its
fuse will melt but the other circuits remain operational.
Wiring In Rooms
Along with the live wire and neutral wire, a third wire called
Earth wire (green in colour) also goes into our rooms. Earth
wire is usually an uncovered copper wire having no plastic
insulation on it. One end of the earth wire is connected to a
copper plate which is burried deep under the earth or at the
nearest electric sub station.
All Switches Are Put In The Live Wire. Why?
Usually all the electrical appliances are provided with separate
switches. So, if we switch off the electrical appliance, then
connection with the live wire is cut off and there will be no
danger of the electric shock if we touch metal case of electrical
appliance.
What Is Earthing Of Electrical Appliance?
Earthing means to connect the metal case of an electrical
appliance to the earth (at zero potential) by means of a metal
wire called earth wire. One end of the earth wire is burried in
the earth. Earth wire is connected to the metal case of the
appliance by using a three-pin plug. The metal casing of the
appliance will now always remain at the zero potential of the
earth. This is called grounding or earthing of appliance.
Why An Electric Fuse Is Needed?
Electric fuse is very useful in case of short circuiting or
overloading to avoid the circuit or device from getting
damaged by any undue hike in the electric current.
Short Circuiting
Touching of live wire and neutral wire directly is known as
short circuiting. It occurs if the plastic insulation of the live
wire and neutral wire gets torn, then the two wires touch each
other. When the two wires touch each other, the resistance of
the circuit becomes very small. Since the resistance is very
small the current flowing through the wire becomes very large
and heats the wire to a dangerously high temperature and
may cause the fire.
Overloading
If too many electrical appliances of high power rating like
electric iron, water heater and air conditioner etc. are switched
on at the same time, they draw extremely large current from
the circuit. This is known as overloading of the current flowing
through the circuit. Since the current flowing through the wire
becomes very large it heats the wire to a dangerously high
temperature and may cause the fire.
Why Earthing Is Done?
To avoid the risk of electric shocks, the metal body of an
electrical appliance is earthed. The appliances which draw
heavy current and which are liable to touch like electric iron,
electric heater, room cooler, geyser, refrigerator should all be
provided with earth connections so that the user will not get
any kind of electric shock.
Electric Fuse
A fuse is a safety device having a short length of a thin wire
made of tin or tin lead alloy having low melting point, which
melts and breaks the circuit if a current exceeds a safe value.
An electric fuse works on the principle of the heating effect of
current. A fuse wire is always connected in series in the
electrical circuit or with the device.
It is obvious that we should have some devices which may
disconnect the electricity supply when a short circuit or
overloading occurs. To avoid this danger electric fuse is used.
Miniature Circuit Breaker (MCB) (Fuse)
These days MCBs are used to protect household wiring from
the excessive flow of electric current through it. If the current
becomes too large, the MCB puts off a switch cutting off the
electric supply. The MCB can be reset when the fault has been
corrected.
Fuse Used In Electrical Appliances
The fuses which are put on the main switch-board in our
houses are used to protect the whole wiring of the house.
Fuses are also used to protect domestic electrical appliances
from damage which may be caused due to excessive current
flowing through them. Costly electrical appliances like T.V.,
refrigerators have their own fuses which protect them against
damage by too much current.
Precautions While Harnessing Electricity
1. If a person accidentally touches a live wire or if an electric
fire starts in a house, the main switch should be turned off
at once as to cut off the electric supply. This will prevent
fire from spreading.
2. Use wires of high quality, proper amperage and good
insulating material.
3. All connections at plugs, switches, sockets must be tight.
4. Replace any defective plugs, switches and sockets.
5. Never touch any part of the circuit being wet or without
putting on rubber shoes or rubber gloves.
6. Use fuse of proper rating and material.
7. All electrical appliances must be earthed.
8. Connect switches and fuse to live wire.