magnetism and electricity - ppt useful for grade 6,7 and 8

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Magnetism and Electricity

Magnetite

HISTORY OF MAGNETISM

What are Natural Magnets?• The naturally occurring materials which

have the property of attracting iron. • These are weak magnets.

What are Artificial Magnets?• They are made of materials like iron,

cobalt and nickel which can be easily magnetized.

• They are strong magnets.• They can be made in different shapes

and sizes.

Artificial Magnets

What is magnetism?

• Magnetism is a force of attraction or repulsion that acts at a distance. It is due to magnetic field, which is caused by moving electrically charged particles or is inherent in magnetic objects such as a magnet.

MAGNETIC FIELD

THE SPACE AROUND A MAGNET WHERE ITS INFLUENCE CAN BE DETECTED IS CALLED

MAGNETIC FIELD.

Why does a magnet suspended freely comes to rest only in North-South direction?

This is because the earth behaves like a huge bar magnet.

Earth’s Magnetic Field

http://phet.colorado.edu/en/simulation/magnet-and-compass

Magnetic lines of force

• The imaginary lines which represent the direction of magnetic field, are known as magnetic lines of force.

• Magnetic lines of force are closed curves. Outside the magnet their direction is from north pole to south pole and inside the magnet these are from south to north pole.

PROPERTIES OF MAGNETIC LINES OF FORCE

•They are continuous closed curves

•They travel from North to South pole outside and from South to North pole inside.

•They curve towards the magnet.

•They never intersect each other . •They are more concentrated near the poles of the magnet where the magnetic field is the strongest

• When electric current is passed through a conductor, a magnetic field is produced around it

• The branch of physics which deals with the magnetic effect of electric current is called electromagnetism.

What is electromagnetism?

Electromagnet• An electromagnet is a solenoid with a soft

iron core.• When using iron, the magnet is temporary.• When using steel, the magnet is

permanent

What is an electromagnet ?

• An electromagnet is a solenoid with a soft iron core.

The strength of an electromagnet depends on:Number of turns in its coils &

Amount of current passing through the coil

USES OF ELECTROMAGNETThey are used in appliances such as electric motors, electric fans and electric bells.• They are used in lifting heaving loads of iron and scrap

and are also used in cranes to lift vehicles.• They are used to deflect beams of electrons in TVs.• They are used to separate iron ores from impurities in

mining industries.• They are used in relays in telephone exchanges and in

computers.• They are used by doctors to cure certain diseases.

Bar Magnet Electromagnet• Made of steel • Made of soft iron

• Poles cannot be reversed • Poles can be reversed

• It produces a permanent magnetic field

• It produces a temporary magnetic field

• Strength of magnetic field cannot be changed

• Strength of magnetic field can be changed

• Does not need electric current to act as a magnet

• Requires electric current to act as an electromagnet

Comparison between a Bar Magnet and an Electromagnet

PARTS OF AN ELECTRIC BELL

An electric bell includes:U- shaped electromagnet (it is made of soft iron)

Armature (soft iron bar which faces the poles of the magnet)

Contact spring

Contact adjustment screw HammerGong SwitchBattery

WORKING

When the switch is pressed the circuit is complete.The current begins to flow through the electromagnet.The magnet gets magnetised and attracts the armature.The armature gets attracted to the magnet and due to the motion of the armature towards the poles of the solenoid and the hammer moves and strikes the gong, thereby causing the bell to ring.