Based on Tamilnadu samacheer

1. MAGNETISM AND MAGNETIC
EFFECTS OF ELECTRIC CURRENT

2. INTRODUCTION TO MAGNETISM
• Magnets! no doubt, its behaviour will attract
everyone
• The world enjoys its benefits, to lead a
modern luxurious life.
• The study of magnets fascinated scientists
around our globe for many centuries and even
now, door for research on magnets is still
open

3. Magnetic levitation

5. • Magnetism is everywhere from tiny particles
like electrons to the entire universe.
• Historically the word ‘magnetism’ was derived
from iron ore magnetite (Fe3 O4).
• In olden days, magnets were used as
magnetic compass for navigation, magnetic
therapy for treatment and also used in magic
shows

6. • Earlier, both electricity and magnetism were
thought to be two independent branches in
physics.
• In 1820, H.C. Oersted observed the deflection
of magnetic compass needle kept near a
current carrying wire.
• This unified the two different branches,
electricity and magnetism as a single subject
‘electromagnetism’ in physics

7. Uses of magnets in modern world

8. Earth’s magnetic field and
magnetic elements

9. Earth’s magnetic field

10. • The needle in a magnetic compass or freely
suspended magnet comes to rest in a position
which is approximately along the geographical
north-south direction of the Earth.
• William Gilbert in 1600 proposed that Earth
itself behaves like a gigantic powerful bar
magnet.
• But this theory is not successful because the
temperature inside the Earth is very high and
so it will not be possible for a magnet to retain
its magnetism.

11. • Gover suggested that the Earth’s magnetic
field is due to hot rays coming out from the
Sun.
• These rays will heat up the air near equatorial
region. Once air becomes hotter, it rises above
and will move towards northern and southern
hemispheres and get electrified.
• This may be responsible to magnetize the
ferromagnetic materials near the Earth’s
surface.
• Till date, so many theories have been
proposed. But none of the theory completely
explains the cause for the Earth’s magnetism

12. • The north pole of magnetic compass needle is
attracted towards the magnetic south pole of
the Earth which is near the geographic north
pole.
• Similarly, the south pole of magnetic compass
needle is attracted towards the geographic
north pole of the Earth which is near magnetic
north-pole.
• The branch of physics which deals with the
Earth’s magnetic field is called
Geomagnetism or Terrestrial magnetism

13. • There are three quantities required to specify
the magnetic field of the Earth on its surface,
which are often called as the elements of the
Earth’s magnetic field. They are
• (a) magnetic declination (D)
• (b) magnetic dip or inclination (I)
• (c) the horizontal component of the Earth’s
magnetic field (BH)

17. • Day and night occur because Earth spins about
an axis called geographic axis.
• A vertical plane passing through the
geographic axis is called geographic meridian
and a great circle perpendicular to Earth’s
geographic axis is called geographic equator.
• The straight line which connects magnetic
poles of Earth is known as magnetic axis.
• A vertical plane passing through magnetic axis
is called magnetic meridian and a great circle
perpendicular to Earth’s magnetic axis is called
magnetic equator.

18. • When a magnetic needle is freely suspended, the
alignment of the magnet does not exactly lie
along the geographic meridian
• The angle between magnetic meridian at a
point and geographical meridian is called the
declination or magnetic declination (D).
• At higher latitudes, the declination is greater
whereas near the equator, the declination is
smaller.
• In India, declination angle is very small and for
Chennai, magnetic declination angle is -1o 8’
(which is negative (west)).

19. • The angle subtended by the Earth’s total
magnetic field with the horizontal direction
in the magnetic meridian is called dip or
magnetic inclination (I) at that point
• For Chennai, inclination angle is 140 16’. The
component of Earth’s magnetic fie ld along
the horizontal direction in the magnetic
meridian is called horizontal component of
Earth’s magnetic field, denoted by BH.

20. • Let BE be the net Earth’s magnetic field at a
point P on the surface of the Earth. BE can be
resolved into two perpendicular components.
• Horizontal component BH = BE cos I
• Vertical component BV = BE sin I

21. (i) At magnetic equator
• The Earth’s magnetic field is parallel to the
surface of the Earth (i.e., horizontal) which
implies that the needle of magnetic compass
rests horizontally at an angle of dip, I = 0o
• BH = BE
• BV = 0
• This implies that the horizontal component is
maximum at equator and vertical component
is zero at equator.

22. Needle of magnetic compass rests
horizontally at an angle of dip – at
magnetic equator

23. (ii) At magnetic poles
• The Earth’s magnetic field is perpendicular to the
surface of the Earth (i.e., vertical) which implies
that the needle of magnetic compass rests
vertically at an angle of dip, I = 90o . Hence,
• BH = 0
• BV = BE
• This implies that the vertical component is
maximum at poles and horizontal component is
zero at poles.

24. Needle of magnetic compass rests
vertically at an angle of dip – at magnetic
poles