Magnetism

MAGNETISM

HOW WAS MAGNET FOUND?

The first known magnet found was pieces of lodestone ,an ore of iron oxide found in large quantities in Magnesia in Asia minor.

It possessed two properties:i. Attracts small pieces of ironii. Sets itself in a definite direction when

suspended freely These were known as magnets,derived

from the word Magnesia.

NATURAL AND ARTIFICIAL MAGNETS

The pieces of lodestone found in nature are called natural magnet.

When apiece of iron is rubbed with lodestone it also acquires magnetic property. This piece of iron is called artificial magnet.

ARTIFICIAL MAGNETS:i. Bar magnet --- rectangular in shapeii. Horse shoe magnet---bar magnet in U-

shaped.iii. Magnetic needle-thin bar tapered from

its centre to its ends

A MAGNETIC COMPASS

The magnetic compass consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth's magnetic field

USES:i. To find the north south direction of a placeii. To find the direction of magnetic field of a

placeiii. To plot the magnetic field linesiv. To test the polarity of a magnet

Properties of a magnet

i. Attractive propertyii. Directive propertyiii. Poles exist in pairsiv. Like poles attract and unlike poles

repel

Attractive property

A magnet has a property of attracting magnetic particles towards it.

The poles of a magnet where the attraction seems to be maximum are called the poles of a magnet.

The poles are not exactly the ends of a magnet they are slightly inside.

Magnetic axis and effective length

Magnetic axis : the imaginary line joining the poles of a magnet.

Effective length:The distance

between the poles of amagnet

Directive property

When a magnet is suspended freely it is always seen to rest in the north south direction.

The end of the magnet which points towards the north is known as the north pole and the end which points towards the south is called the south pole.

Poles exist in pairs

On breaking a magnet into parts it is said to exist in dependently that is possessing the attractive and directive property.

If a magnet is cut into halves either along its axis or perpendicular to it its strength is half of the original strength.

Like poles attract and unlike poles repel

Induced magnetism

The magnetism acquired by a magnetic material when kept near (or in contact with)a magnet is called induced magnetism.

This process I called magnetic induction.

A magnetic pole induces opposite polarity on near end and similar polarity on farther end of the iron bar.

Induction precedes attraction. When a piece of iron is brought near one

end of a magnet , the nearer end of piece acquires an opposite polarity by magnetic induction.

Since unlike poles attract each other therefore iron piece is attracted towards the end of a magnet.

The piece of iron first becomes a magnet by induction and then is attracted.

Thus, induction precedes attraction.

Induced magnetism is temporary If one pole of a bar is brought near small

iron nails they form a chain of nails. One magnetised nail magnetises the

other ones . The process continues till force of

attraction of magnet on first nail continues to balance the total weight of all the nails.

On removing the uppermost nail we see that all the other nails lose magnetism.T his shows the magnetism acquired is purely temporary.

Magnetic field

When a magnetic compass is place near a magnet ,it rests in some other direction other than north south.

The space around the magnet in which the compass rests in direction other than north south direction , is called magnetic field of a magnet.

Magnetic field is a vector quantity.

Lines of magnetic field

If we place a magnet below a stiff paper and above them iron filings we find that the filings get arranged in a specific pattern like curved lines.

A magnetic field line is closed and continuous curve such that tangent at any point gives the direction of the magnetic field at that point.

Properties of magnetic field lines They are closed and continuous curves. They are directed from north pole towards

south pole of the magnet (outside the magnet)

The tangent at any point gives on a field line gives the magnetic field of the place.

They are non-intersecting. They are crowded near the poles and far

separated near the middle. They behave like a stretched elastic rubber.

MAGNETIC FIELD OF EARTH(EVIDENCES OF EARTHS MAGNETIC FIELD)

1. A freely suspended magnetic needle always rests in north-south direction.

2. A piece of iron buried inside the earth along north- south direction also becomes a magnet.

3. Neutral points are obtained on plotting the field lines of a magnet.

4. A magnetic needle rests making different angles with the horizontal when suspended at different places on earth.

A freely suspended magnet always rests in north south direction.

Neutral points

Magnetic field lines of the earth

Two places where the magnetic needle becomes vertical are the magnetic poles

The line joining places where the magnetic needle becomes horizontal is the magnetic equator.

The magnetic field lines are normal to the earths surface near the magnetic poles and parallel to eaths surface near magnetic equator.

Plotting of uniform magnetic field lines of the earth

Earth’s magnetic field is uniform in a limited space. Fix a sheet of paper on a brass pins .Place a small

compass needle at position and looking from top of the needle ,mark two pencil dots exactly at two ends of the needle. Then move the compass needle to position2 in such a way that one end of the needle coincides with the second pencil dot. Mark the position of the other needle with a dot .Repeat this process ,you will get a straight line .Thus, one line of magnetic field is traced.

These lines do not cross each other they are parallel and equidistant.

Plotting of non-uniform magnetic field of a strong bar magnet and neutral points.

The magnetic field around a bar magnet is non-uniform.

The magnetic field lines are curved .the closely spaced magnetic lines represent a strong magnetic field while the widely spaced represent a weak magnetic field

Non-uniform magnetic field lines are formed due to :

i. A bar magnetii. A horse-shoe magnetiii. Two unlike poles facing each otheriv. Two like poles facing each other

1. Non-uniform magnetic field lines :

i. When a magnet is placed with its north pole pointing towards north.

When a magnet has its north pole facing towards south:

Neutral points

Neutral points are points where the magnetic field of the magnet is equal in magnitude to the earths horizontal magnetic field , but it is in opposite direction . Thus the resultant magnetic field at the neutral point is zero.

If a compass needle is placed at a neutral point it moves in any direction that is it remains unaffected.

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