magnetism basics

2. 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 iron
ii. Sets itself in a definite direction when
suspended freely
 These were known as magnets,derived from
the word Magnesia.

3. 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 shape
ii. Horse shoe magnet---bar magnet in U-
shaped.
iii. Magnetic needle-thin bar tapered from its
centre to its ends

4. 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 place
ii. To find the direction of magnetic field of a
place
iii. To plot the magnetic field lines
iv. To test the polarity of a magnet

5. Properties of a magnet
i. Attractive property
ii. Directive property
iii. Poles exist in pairs
iv. Like poles attract and unlike poles repel

6. 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.

7. 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

8. 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.

9. 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.

10. Like poles attract and unlike
poles repel

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. A freely suspended magnet
always rests in north south
direction.

19. Neutral points

20. 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.

21. 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.

22. 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 magnet
ii. A horse-shoe magnet
iii. Two unlike poles facing each other
iv. Two like poles facing each other

23. 1. Non-uniform magnetic field
lines :
i. When a magnet is placed with its north pole
pointing towards north.

24.  When a magnet has its north pole facing
towards south:

25. 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.