Magnetism and Electromagnetism

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Properties of magnetism

- has a magnetic field around it

- will attract magnetic materials by inducing magnetism in them

- will exert little or no force on a non magnetic material

- has two opposite poles which exert forces on other magnets

Induced Magnetism

Magnetic Induction is the process of inducing magnetism in an unmagnetised magnetic material

when a magnetic material is placed near or in contact with a permanent magnet, temporary magnetism occurs

ability to temporarily pass its magnetic effects to a magnetic material so that the latter can attract other magnetic materials

Induced magnet is not permanant - will lose it's magnetism once removed from permanant magnet

Magnetisation & Demagnetisation by electrical method

Magnetisation:

a solenoid is connected to a DC supply

a steel bar is placed in solenoid and current is passed for a while

a strong magnetic field produced - steel bar is magnetised

current clockwise - end is the induced S Pole

current anti clockwise - end is the induced N Pole

Demagnetisation:

solenoid connected to an AC Supply

magnet to to be demagnetized is placed inside, parallel to east west direction

following the east west direction, slowly withdraw the magnet far away through the solenoid while the ac current is still flowing

as this process is repeated, the magnet will be demagnetized

A Solenoid is a cylindrical coil of insulated copper wires

Law of magnetism

Unlike poles attract, like poles repel

Properties of hard magnets

- Harder to magnetise and demagnetise

( can retain its magnetism)

- permanant magnets

- Used as: DC Motors, AC Generators, Moving coil loudspeakers

- eg: Steel

Properties of soft magnets

- easy to magnetise and demagnetise

( can lose its magnetism easily)

- electromagnets

- Used as:

Transformers

Electric bells

Audio and Visual Tapes

- eg: Iron

Magnetic Screening

Material Used: Iron (soft magnet)

Use of magnetic screening:

- Electronic equipment easily upset by magnetic fields from nearby generators/motors/earth

- the equipment can be shielded by enclosing it in a layer of iron

- this redirects the field so that it does not pass the equipment