magnetism has been known for thousands of years, though until

8/14/2019 Magnetism Has Been Known for Thousands of Years, Though Until

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Magnetism

Magnetism has been known for thousands of years, though until relatively recently (ca.1820) the only magnets were naturally occurring magnetic rocks called "lodestones"

(magnetite). The word "magnetism" derives from the district of Magnesia in Asia Minor

where lodestones were found. These lodestones were observed to attract bits of iron, justlike a kitchen magnet which is attracted to your refrigerator door. The space around themagnet is the site of the magnetic field, where the magnetic force is felt by the bit of iron.

At each point around the magnet, the magnetic field has both strength and a direction.

# MagnetAny substance that-

attract iron towards itself

When suspended freely always rest in N-S direction, is referred to

as a magnet.

#MAGNETISM

The phenomenon of attracting iron towards itself and pointing in M-s direction is called

magnetism.

# DIFFERENCE BETWEEN MAGNETIC AND NON MAGNETIC

SUBSTANCE

Magnetic substance Non magnetic substance

Magnetic substance are those that can beattracted by a magnet and can be converted into

a magnet

Non magnetic substances are those that cannotbe attracted by a magnet and cannot be

converted into a magnet.

Eg Iron, Nickel cobalt etc. Eg silver, gold, aluminium

#Natural magnet: - Example MAGNETITE (Fe3O4)

#Artificial Magnet: - Horse shoe magnet and Bar magnet

HORSE SHOE MAGNET

Q Why magnet is referred to as lodestone or leading stone?
http://images.google.co.in/imgres?imgurl=http://www.a3bs.com/imagelibrary/U19550/electricity-and-magnetism/U19550_pair-of-bar-magnets-80-mm.jpg&imgrefurl=http://www.a3bs.com/electricity-and-magnetism/magnetism,pg_83_671_623_0.html&usg=__YdPgYoAH770MWFZl_rQOBk6cePQ=&h=438&w=400&sz=27&hl=en&start=1&tbnid=5_erY2DpNAoZdM:&tbnh=127&tbnw=116&prev=/images%3Fq%3DBar%2Bmagnet%26gbv%3D2%26hl%3Denhttp://www.pieces-zine.com/200902magnet/images/horshoemagnet.jpg


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PROPERTIES OF A MAGNET

1. A FREELY SUSPENDED MAGNET ALWAYS REST IN N-S DIRECTION.The pole facing north is called north pole or north seeking pole and the pole facing

south is called South Pole or south seeking pole.

2. MAGNETISM IS THE STRONGEST AT THE ENDS OF THE MAGENTS

AND THESE ENDS ARE CALLED POLES.

3. LIKE POLES REPEL EACH OTHER AND UNLIKE POLES ATTRACT EACHOTHER.

4. POLES ALWAYS EXIST IN PAIRS AND CANNOT BE ISOLATED.

Q Desha wishes to separate the poles of a magnet. Will she be able to do? If yes then support

your answer and if not, then also support your answer.

Q What conclusion will she be able to draw from it?

EARTH AS A MAGNET

Q Explain, Why does earth behave as a magnet?

OR

Why a freely suspended magnet always rest in N-S direction?

The earth behaves as a huge magnet having its north pole near the geographical South Pole.

Thus, when a magnet is suspended freely, the north pole of suspended magnet attractstowards the earth magnetic south pole. So, it points towards geographical North Pole and vice

versa.


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# DIFFERNTIATE BETWEEN TEMPORARY AND PERMANENT

MAGNETS

Temporary Magnet Permanent Magnet

When am iron bar is magnetized by

MAGNETIC INDUCTION, the magnet formedis temporary magnet.

If the magnet used is very strong, it can

permanently align the molecular magnets iniron bar. Such magnets formed are called

permanent magnets.

It retains the magnetism for a very short period

of time, till the magnet is kept in CONTACT

with the iron bar.

It retains the magnetism for a longer period

even after the magnet is REMOVED.

Example; Electromagnet Example: ALNICO, Ferrite

Q Why ferrite made magnets are used in magnetic toys?

______________________________________________________________________________

# CONCEPT OF ORESTED

In 1820, Hans Christian Orested discovered the correlation between electric current and

magnetism. He found that a current carrying wire behaves like magnet and when suspended

freely, it would always rest in N-S direction.

THE WORK OF ORESTED WAS CONFIRMED BY AMPERE

# FACTORS AFFECTING MAGNETISM OF A CURRENT CARRYING CONDUCTOR

(WIRE)

An increase in current flowing through the wire

A decrease in distance from the wire

#ELECTROMAGENT

CONSTRUCTION

A coil of insulated wire wounded around a piece of soft iron, acts as a magnet as along as

current flows through it, is called an electromagnet.

SOLENOID

A coil of insulated wire is called a solenoid, which behaves as a magnet as long as currentflows through it.

When a magnetic compass of brought near it, there is a deflection of the compass needle

showing the presence of magnetic field.

CORE

The magnetic substance around which a solenoid is bounded is called core


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# FACTORS INFLUENCING THE STRENGTH OF SOLENOID

The number of turns of wire.

The amount of current flowing through the wire.

# USES OF ELECTROMAGNET

Electric bell

In telegraphs and telephones

Electric Motors in fans, washing machine.

For lifting and carrying heavy steel and iron sheets in the

factories.

For removing iron bits from wounds by surgeons.

# ELECTRIC BELL

Principle

An electric bell is based on electromagnetism. When electric current is passed,

through the electromagnet, it attracts the armature causing the hammer to strike the gong.

CONSTRUCTION

Electric bell consist of an electromagnet, armature and contact adjusting screw, Gong and a

hammer. The one end of the iron presses against the contact adjusting screw.

WORKING

When the switch is pressed, the current flows in the electromagnet, it then

attracts the iron rod causing the hammer to strike the gong.

At the same time, the armature loses its contact with the screw mad the current I

switched off.

This causes the electromagnet to loose its magnetism.

The armature spring backs to close the circuit again.

The cycle is repeated again and again till the switch is off.


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# ELECTROMAGNETIC INDUCTION (BY FARADAY)

PRINCIPLE

It states that current is induced in a coil of a wire as long asthere is a relative motion between a coil of wire and a magnet.

CONCEPT OF FARADAY

In 1831, Michael Faraday introduced the concept of electromagnetic induction, which

is the reverse of electromagnetism (concept of orested).

He performed an experiment. He took a coil of wire and connected it to a Galvanometer

(Galvanometer is a device that detects the flow of current through the circuit.). He moved

the magnet in and out of the coil and observed the following:

Current flows through the coil, and the bulb glows, when there is relative motion

between the coil and the magnet.

The direction of the current reverses as the direction of relative motion reverses.

Faster is the relative motion, more the current flows.

The current stops flowing when there is no relative motion.

# APPLICATIONS OF ELECTROMAGNETIC INDUCTION

Working of electric generator

In dynamos

magnetism has been known for thousands of years, though until

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