Transcript Magnetite found in nature is the only natural magnet.

MAGNETISM
Samples of MAGNETITE
Magnetism is the property of attracting iron
by a magnet.
A magnet is an object made of certain
materials like iron which can create a
magnetic field around it. If there are things
made of iron in this field (region) they are
attracted by the magnet.
Magnetite found in nature is the only
natural magnet.
It is a weak magnet.
Using steel we can make powerful artificial
magnets.
Of various shapes and sizes.
Substances that get attracted to a magnet
are called magnetic substances.
And those which do not attract are nonmagnetic substances.
• Activity 1:
• We can use this property of a magnet
attracting iron to search for a needle in a
haystack or a stitching kit.
What are objects such as
needles, pin etc made up of?
Iron
When we bring a pin near a
magnet. The pin attracts
another pin and next pin.
Thus the pins themselves start
behaving like magnets attracting
other pins.
What happens when the
magnet is taken away? Pins lose
their magnetism.
Such magnets which lose their
magnetism as soon as the
magnetizing force is removed
are usually made of soft (Pure)
iron. These are called temporary
magnets.
But the magnets made of steel, cobalt and nickel are
capable of retaining magnetism for a longer period of
time. Even when magnetising force is removed. These are
called permanent magnets.
PERMANENT MAGNETS ARE USED TO MAKE MAGNETS.
DONE SO FAR
Magnetism is the property of attracting iron by a
magnet.
A magnet is an object made of certain materials like
iron which can create a magnetic field around it.
Magnetite found in nature is the only natural magnet.
It is a weak magnet.
Using steel we can make powerful artificial magnets.
Of various shapes and sizes.
Substances that get attracted to a magnet are called
magnetic substances.
And those which do not attract are non-magnetic
substances.
Home work
B 1 a,c,d,e,f
Ans b:
Making of magnets
Iron is called a soft magnetic material because it
loses its magnetism easily. It is used in making
electromagnets. Eg iron nail.
Steel is called a permanent material because it
does not lose its magnetism so easily. It is
used to make permanent magnets. Cobalt and
nickel are also used to make permanent
magnets.
1. Magnetic induction method
Activity : MAGNET
ATTRACTING NAILS
This property by which an
ordinary piece of iron (or any
magnetic substance)
acquires magnetic
properties, temporarily due
to the presence of another
magnet close to it is known
as magnetic induction.
2. SINGLE TOUCH METHOD
https://www.youtube.com/watch?v=eYSG5aeTy
-Y
https://www.youtube.com/watch?v=BJ7CwPuyv
Sg
https://www.youtube.com/watch?v=PDUqU4XN
AZg (DO NOT DOWNLOAD DOUBLE TOUCH
METHOD)
CARE ONE MUST TAKE WHILE MAKING MAGNETS
USE THE SAME POLE OF MAGNET EVERY TIME
AND RUB IT ON THE IRON PIECE ONLY IN ONE
DIRECTION FOR SEVERAL TIMES.
For double touch method each time, start from
the middle of the iron bar and rub up to its
end.
4. ELECTROMAGNETS BY ELECTRICAL METHOD
When a current flows through a wire a magnetic
field is produced around the wire. This field is called
and electromagnetic field and the device produced
is called an electromagnet. If the wire is wrapped
round into a coil the magnetic field is stronger.
It is very easy to detect the magnetic field by
putting a compass needle near a wire. When a
current is passed through the wire the compass
needle will turn. The greater the current in the wire
the more the compass needle moved.
This experiment was first done by a man called
Oersted in 1819 who made the discovery by
accident.
The direction of the magnetic field produced
depends on the direction of the current and
the shape of the field depends on the
arrangement of the wires.
A long straight coil of wire - called a solenoid
– produces a magnetic field very much like
that of a bar magnet. The shape of some
magnetic fields are shown in the diagrams
below.
The core of an electromagnet is made of iron
because this is easy to magnetise and demagnetise.
Steel would not be suitable because it stays
magnetised for much longer and so the
electromagnet could not be switched on and off
easily.
You can use a small compass to test the
polarity of an electromagnet.
 Uses of electromagnetism
Electric bell
Relay to allow one circuit to operate another
Electric motors
Recording heads in tape recorders, floppy discs and
hard discs
Electric analogue meters
In scrap yards for lifting metals
Deflector coils in television
For removing metal (ferrous) splinters from your eyes
Loudspeakers and telephone earpiece
Nuclear accelerators
Locking devices for safes
Permanent Magnets Vs Electromagnets.
TO INCREASE THE
Electromagnets have two
advantages over permanent
STRENGTH OF AN
magnets.
ELECTROMAGNET
They can be readily
magnetized or
demagnetized by turning
the current on or off in the
coil.
They can be made stronger
than any other permanent
magnet as per the need
We have only to do one of
the following.
By increasing the amount of
current.
By increasing the number of
turns used in a coil.
DEMAGNETISING A MAGNET
• By hammering
repeatedly
• By rough handling
• By heating it to a very
high temperature and
keeping it in the east
west direction
• By passing alternating
current around the
magnet
• Dropping it repeatedly
on a hard surface
• By self
demagnetisation, when
two poles of a magnet
are left free it loses its
magnetism
STORING OF MAGNETS
Hence the two magnets are
always kept in such a way
that their opposite poles
are joined.
And the two magnets are
separated by a piece of
wood.
Two pieces of soft iron
called magnetic keepers
should be placed across
their ends.
• For a single horse-shoe
magnet one piece of iron
is kept across the poles.
INFORMATION ON MAGNETS
• Ceramic magnets are
made from oxides of
iron, nickel and
berryllium
• Magnetic compass is
used by travellers,
sailors and navigators to
find directions
• The earth behaves as a
huge magnet.
Uses of magnets
• Magnetic compass,
door bells,
refridgerators,
cupboard doors, etc.
• Dynamos, motors,
loudspeakers
microphones, floppy
disk, stereo speakers
and televisions.
• Ceramic magnets are
used in computers
• Magnetic tapes are
used in tape recorders
and radio recorders.
• For giving magic effect
in toys.
STUDY THIS
CHAPTER VERY
WELL FOR FINAL
EXAM