Magnet
* A magnet is an object which attracts pieces of magnetic
materials like iron, steel, nickel and cobalt.
* Magnets come in various shapes and sizes depending on their
intended use. Most common magnets are the bar magnet and horse shoe
magnet ( U – shaped ) .
Bar Magnet
* A bar magnet is a long, rectangular bar of uniform
cross-section which attracts pieces of iron, steel, nickel and cobalt.
Characterstics
Of Magnet
* A magnet has two poles near its ends : north pole and South pole .
* The end of a freely suspended magnet which points towards
North direction of earth is called the
north pole of the magnet .
* The end of a freely suspended magnet which points towards the south direction of
earth is called the south pole of the
magnet.
* like magnetic poles repel each other whereas unlike magnetic
poles attract each other.
Uses Of Magnet :
Magnets are used for a variety of purposes.
* Magnets are used in radio, television, and stereo speakers.
* Magnets are used in refrigerator doors, in audio and video
cassette tapes.
* Magnets are used in
hard discs and floppies for computers, and in children's toys.
* Magnets are used in motors , Alternator.
* The Magnetic Resonance Imaging (MK) technique which is used
to scan inner human body parts in hospitals also uses magnets .
* Magnets are used for
making electric generators and electric motor.
Magnetic Field
* The region around a magnet in which magnetic material
experience a force is called magnetic field.
* When A compass needle placed near a magnet , it gets
deflected due to the magnetic force exerted by the magnet.
* Iron filings also cling to the magnet due to magnetic force.
Magnetic Field Lines:
* The curve drawn around the magnet along which a small
magnetic material moves is called Magnetic Field Line.
* The magnetic field lines are also known as magnetic lines of
force.
Properties of Magnetic Field Lines:
* Outside the magnet ,magnetic field lines always emerge from
the N-pole and end on the S-pole of the magnet .
* Inside the magnet the direction of magnetic field lines are
from the S-pole to the N-pole of the
magnet.
* Each magnetic field
line form closed loop .
* The magnetic field lines come closer to one another near the
poles but they are widely separated at other places.
* When magnetic field
lines are closer together, it indicates a stronger magnetic field. On the other
hand, when magnetic field lines are widely separated, then it indicates a weak
magnetic field.
* Tangent drawn at any point
on magnetic field line show
direction of field at that point .
* No two field lines intersect each other. If they intersect
then at point of intersection magnetic field show two direction which is not
possible.
Plot Magnetic Field Lines Pattern of Bar Magnet:
* Plot the Magnetic Field Pattern Due to a Bar Magnet by Using
Iron Filings.
* Place a card (thick, stiff paper) over a strong bar magnet .
* Sprinkle a thin layer of iron filings over the card with the
help of a sprinkler, and then tap the card gently.
* The iron filings arrange themselves in a regular pattern.
This happens as follows :
* The bar magnet produce a magnetic field all around it. The
iron filings experience the force of magnetic field of the bar magnet.
* The force of magnetic field of bar magnet makes the iron
filings to arrange themselves in a particular pattern.
* Under the influence of the magnetic
field of the bar magnet, the iron filings behave like tiny magnets and align
themselves along the directions of magnetic field lines.
Earth Magnetism :
* It is believed that
the earth's magnetism is due to the magnetic effect of current (which is
flowing in the liquid core at the centre of the earth). Thus, earth is a huge
electromagnet.
* The shape of the earth's magnetic fields resembles that of
an imaginary bar magnet buried at its center whose length is one-fifth of earth's diameter.
* A freely suspended magnet or magnetic needle always points
in the north-south direction. It indicate that
earth itself behaves as a magnet .
* The south pole of earth's magnet is in the geographical
north because it attracts the north pole of the suspended magnet. Similarly,
the north pole of earth's magnet is in the geographical south because it
attracts the south pole of the suspended magnet.
* The axis of earth's magnet and the geographical axis do not
coincide with each other.
* The axis of earth's magnetic field is inclined at an angle
of 150 with the geographical
axis.
Magnetic Effect Of Electric Current
A current carrying conductor produces a magnetic field
around it. This Phenomenon is called 'magnetic effect of electric current.
* The magnetic effect of current is also called
electromagnetism which means electricity produces magnetism.
* The electric motor, electric generator, telephone and radio,
all utilize the magnetic effect of current.
* The magnetic effect of current was discovered by Oersted in
1820.
* Oersted found that a
current carrying conductor was
able to deflect a compass needle. the compass needle is a tiny magnet which can
be deflected only by a magnetic field. Since a current carrying wire was able
to deflect a compass needle, it was concluded that a current flowing in a wire
always produce magnetic field around it.
Experiment-1 ( to
Demonstrate the Magnetic Effect of Current )
* Take a thick insulated copper wire and fix it in such a way
that the portion AB of the wire is in the north-south direction .
* A plotting compass M is placed under the wire AB.
* The two ends of the wire are connected to a battery through
a switch. When no current is flowing the wire AB, the compass needle is
parallel to the wire AB .
* On passing the current that compass needle is deflected from
its north-south position. And when the current is switched off, the compass
needle returns to its original position.
* If we reverse the direction of electric current flowing in
the wire AB by battery connections, the compass needle is deflected in the
opposite direction. This shows that when we reverse the direction of electric
current, then the direction of magnetic field produced by it is also reversed.
Experiments-2 :
* Take long insulated copper wire and wind it around the large iron nail to form many closed turns * Connect the ends of the wire to a battery. The large iron nail now become magnet and attract tiny iron nails towards it . This has happened because an electric current flowing in the wire has produced a magnetic field which has turned the large iron nail into a magnet.
* The current-carrying straight electric wires (like an
electric iron connecting cable) do not attract the nearby iron objects towards
them because the strength of magnetic field produced by them is quite weak.
Magnetic Patterns Produced by Current-Carrying
Conductors Having Different Shapes
* The pattern of magnetic field (or shape of magnetic field
lines) produced by a current-carrying conductor depends on its shape.
* current-carrying conductors having different-shapes produce
Different magnetic field patterns .
1. Magnetic Field Pattern due to Straight
Current-Carrying Conductor:
* The magnetic field lines around a straight current carrying
conductor are concentric circles whose centers lie on the conductor.
* When current in the wire flows in the upward direction then
the lines of magnetic field are in the anticlockwise direction.
* When current in the
wire flows in the downward direction then the lines of magnetic field are in
the clockwise direction
* the magnitude of magnetic field produced by a straight
current- carrying conductor at a given
point is :
(i) Directly
proportional to the current passing in the wire
(ii) Inversely proportional to the distance of that
point from the wire.
* So, greater the current in the wire, stronger will be the
magnetic field produced. And greater the distance of a point from the current-carrying
wire, weaker will be the magnetic field produced at that point.
* Direction of magnetic field lines produced by a straight
current- carrying conductor can be known
by Maxwell right hand rules or Maxwell's corkscrew rule.
Maxwell's right- hand thumb rule
Grasp the
current-carrying conductor in right hand
so that thumb points in the direction of
current, then the direction in which
fingers encircle the conductor
will give the direction of magnetic field lines around the
conductor .
Maxwell's corkscrew rule:
Imagine driving a corkscrew in the direction of
current, then the direction in which we turn its handle is the direction of
magnetic field.
2. Magnetie Field Pattern due to a Circular Loop (or Circular Wire) Current Carrying conductor :
* When a current is passed through the circular loop of wire,
a magnetic field is produced around it.
* The magnetic field lines are circular near the
current-carrying loop.
* As we move toward centre of circular loop the concentric circles representing magnetic
field lines become bigger and bigger.
* At the centre of the circular loop, the magnetic field lines
are nearly straight .
* Each segment of circular loop carrying current produces
magnetic field lines in the same direction near the centre of loop.
* At the centre of the circular loop, all the magnetic field
lines are in the same direction and aid each other, due to which the strength
of magnetic field increases .
* The magnitude of magnetic field produced by current-carrying
circular loop at its centre is :
( i ) Directly proportional to the current passing
through the circular loop
( ii ) Inversely
proportional to the radius of circular loop wire .
* The strength of magnetic field can be increased by taking a
circular coil consisting of a number of turns of insulated copper wire closely
wound together.
* Thus, if there is a circular coil having n turns, the
magnetic field produced by this current-carrying circular coil will n times as
large as that produced by a circular loop of a single turn of wire.
Direction of magnetic field at any point on the axis
of circular coil
Right hand fist rule
* According to this rule,
hold the axis of the coil in the right hand fist in such a way that
fingers point in the direction of current in the coil. Then outstretched thumb gives the direction
of magnetic field at any point on the
axis of coil.
Solenoid
A coil having large number of close turns
of insulated copper wire is called
solenoid .
* The magnetic field produced by a current-carrying solenoid
is similar to the magnetic field produced by a bar magnet.
* The magnetic field lines inside the solenoid are in the form
of parallel straight lines. This indicates that the strength of magnetic field
is the same at all the points inside the solenoid.
* One end of the current-carrying solenoid acts like a
north-pole (N-pole) and the other end a south pole (S-pole). So, if a
current-carrying solenoid is suspended freely, it will come to rest pointing in
the north and south directions.
Electromagnet :
When a soft iron core is placed in Current carrying solenoid then system of solenoid and core is called electromagnet.
* The core of an electromagnet must be of soft iron because
soft iron loses all of its magnetism when current in the coil is switched off.
On the other hand, if steel is used for making the core of an electromagnet,
the steel does not lose all its magnetism when the current is stopped and it
becomes a permanent magnet. This is why steel is not used for making
electromagnets.
* Electromagnets can be made in different shapes and sizes
depending on the purpose for which they are to be used.
Factors Affecting the Strength of an Electromagnet
The strength of an electromagnet depends on:
(i) The number of turns in the coil. If we increase
the number of turns in the coil, the strength of electromagnet increases.
(ii) The current flowing in the coil. If the current
in the coil is increased, the strength of electromagnet increases.
(iii) The length of air gap between its poles. If we
reduce the length of air gap between the poles of an electromagnet, then its
strength increases.
For example, the air gap between the poles of a
straight, bar type electromagnet is quite large, so a bar type electromagnet is
not very strong. On the other hand, the air gap between the poles of a U-shaped
electromagnet is small, so it is a very strong electromagnet
Determine the polarity of electromagnet
1. Clock face rule
* If direction of current flowing in the coil looks anticlockwise
from one end . Then that end will be
North pole (N-pole).
* If direction of current flowing in the coil looks clockwise
from one end . Then that end will be
South pole (S-pole).
2. Ampere's right hand rule
Imagine to
grasp the solenoid with right hand so that the fingers are curled in the
direction of current. Then the thumb stretched parallel to the axis of the
solenoid will point towards the N-pole end of the solenoid .
No comments:
Post a Comment