10 2011 2012 Magnetic Effect of Electric Current Notes 1

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Grade 10/Physics/2012-13

MAGNETIC EFFECT OF ELECTRIC CURRENTPART-1

MAGNET

A magnet is a piece of body which attracts pieces of iron, cobalt andnickel.Magnets come in different shapes and sizes.

Bar magnet

Horseshoe magnet

Magnetic needle

A freely suspended magnet always aligns itself along the north and south

poles of the earth. The end of the magnet pointing towards the north poleof the earth is called north-seeking pole or north pole and the other endis called south-seeking pole or south pole.

MAGNETIC FIELD

The space around a magnet in which it attracts or repels another magnetor attracts pieces of magnetic material (iron, cobalt, nickel) is calledmagnetic field of a magnet.

PLOTTING OF MAGNETIC FIELD LINES WITH THE HELP OF A

COMPASS NEEDLE. Take a small compass and a bar

magnet.

Place the magnet on a sheet of

white paper fixed on a drawingboard. Mark the boundary of themagnet.

Place the compass with the southpole of the needle points towardsthe north pole of the magnet.

Mark the position of two ends ofthe needle.

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Now move the needle to a new position such that its south pole

occupies the position previously occupied by its north pole.

In this way, proceed step by step till you reach the south pole of themagnet .

Join the points marked on the paper by a smooth curve. This curverepresents a field line.

Repeat the above procedure and draw as many lines as you can. Theselines are magnetic lines of forces.

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PROPERTIES OF MAGNETIC LINES OF FORCE

The magnetic field lines are

closed curves which emergefrom North Pole of a magnet andmerge at the south pole andInside the magnet, the directionof field lines is from its southpole to its north pole.

The magnetic lines of force comecloser to one another near thepoles of a magnet but they are

widely separated at other places.It means the field is stronger nearthe poles.

The magnetic lines of force do not intersect each other. This is due tothe fact that the resultant force on a north pole at any point canbe only in one direction.

Note

The following diagram depicts the magnetic lines of force between two north

pole, two south pole; North-South pole.

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MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR

OERSTEDS EXPERIMENT When current is passed through a conductor a

magnetic field is set up around the conductor.

This can be detected by placing a compass needlenear the conductor.

When the direction of current is reversed thedirection of deflection of the needle also reverses.

H.C Oersted was the first person to demonstrate that acurrent carrying conductor has a magnetic field associated with it.

DIRECTION OF DEFLECTION OF MAGNETIC NEEDLESNOW rule

If current flows from South to North on a straight conductor kept, Over thecompass needle, the north pole of the needle deflects towardsWest.

MAGNETIC FIELD DUE TO A CURRENT CARRYING STRAIGHTWIRE

When current is passed through a wire (straight conductor) it produces amagnetic field around the wire. This magnetic field is represented by

magneticlines of force.

Themagneticlines of force areconcentric

circles whose centre lies on the wire.

MAGNITUDE OF MAGNETIC FIELD PRODUCED BYA CURRENT CARRYING STRAIGHT WIRE.

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Grade 10/Physics/2012-13The magnitude of magnetic field produced by a straight current carryingwire at a given point is,

Directly proportional to the strength of the current in the wire.

Inversely proportional to the distance from the wire.

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DIRECTION OF MAGNETIC FIELD AROUNDA CURRENT CARRYING STRAIGHT WIRE.

RIGHT HAND THUMB RULE(STATEMENT)

Imagine that you are holding a current-carrying straight conductor in yourright hand such that the thumb points towards the direction of current,then your fingers will wrap around the conductor in the direction of thefield lines of the magnetic field.This is known as the right-hand thumb rule.

When the direction of current reverses, the direction of magnetic field alsoreverses.

MAGNETIC FIELD DUE TO CIRCULAR COIL CARRYING CURRENT.(Single loop)

When a current is passed througha circular coil, a magnetic field isproduced around it.

The lines of force are circular nearthe wire and parallel near thecentre of the loop.

Each small segment of the loop issurrounded by such magneticlines of force.

This current carrying loopbehaves as a magnet.

MAGNITUDE OF MAGNETIC FIELD PRODUCED BYA CURRENT CARRYING CIRCULAR COIL

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Grade 10/Physics/2012-13The magnitude of magnetic field produced by a current carrying circularcoil at its centre is,

Is directly proportional to the strength of the current passing through

the circular coil Is directly proportional to the number of circular loops

Is inversely proportional to its radius

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SOLENOID It is a long circular coil containing a

large number of close turns of

insulated copper wire. The magnetic field produced by a

current carrying solenoid is similar tothe magnetic field produced bay abar magnet.

One end of the solenoid acts like asouth pole (clockwise current) andthe other end acts like a north pole (anticlockwise current).

MAGNITUDE OF MAGNETIC FIELD PRODUCED BY A SOLENOID

The magnitude of magnetic field produced by a current carrying solenoidis,

directly proportional to the strength of the current in the solenoid

directly proportional to the number of turns (n) in the solenoid

nature of the core material used- The use of soft iron as core in asolenoid produces strong magnetic field.

CLOCK RULE TO DETERMINE THE POLARITY OF A CIRCULARCOIL CARRYING CURRENT

If a coil carrying current is suspended by thin elastic long conductors, itwill align itself as the compass needle, i.e., one end of the coil will pointin the north direction and the other end will point towards the south.

The polarity of the faces of the coil depends on the direction of currentand is obtained by a rule known as "The Clock Rule".

According to this rule "When an observer, looking at the face of thecoil, finds the current to be flowing in the anti-clockwise direction, then

the face of the coil will behave like the north pole. While if the currentis in the clockwise direction, the face of the coil will behave like SouthPole.

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ELECTROMAGNET

A temporary magnet produced by using electric current is called an

electromagnet.

PrincipleWhen current is passed through a long solenoid a strong magnetic field is

produced at its centre.

ConstructionIt consists of a long coil of insulated copper wire wound on a soft iron core.

WorkingWhen the key is closed, the current through the coil produces a strongmagnetic field within the coil (solenoid). Thus any magnetic material (softiron) placed inside the coil gets magnetized. When the key is opened themagnetic material demagnetizes.

USES OF ELECTRO MAGNETS In electrical appliances like electric bell, electric fan etc.,

In cranes to lift heavy iron objects.

In electric motors and electric generators.

In separating iron from other non-magnetic substances.

NOTEThe core of an electromagnet must be of soft iron, because soft iron losesall of its magnetism when current in the coil is switched off.On the other hand, if steel is used for making electromagnet, it does not

lose all its magnetism when current in the coil is switched off. Hence steelgets permanently magnetized.

DIFFERENCES BETWEEN BAR MAGNETS AND ELECTROMAGNETS

ELECTROMAGNET BAR MAGNET

It is a temporary magnet. It is a permanent magnet.

It produces strong magnetic field. It produces weak magnetic field.

Its polarity can be changed bychanging the direction of current.

Its polarity cannot be changed.

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Strength of an electromagnet canbe changed by changing thestrength of the current or thenumber of turns in the coil.

The strength of a permanentmagnet cannot be changed.

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10 2011 2012 Magnetic Effect of Electric Current Notes 1

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