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Chapter 21. MAGNETISM. Magnetic Levitation Train ( Shanghai) Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it with little noise or vibration. Earth’s Magnetic Field. Earth’s Magnetic Field. Properties of Magnets. 1. North and South Poles.

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MAGNETISM


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slide2

Magnetic Levitation Train (Shanghai)

Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it with little noise or vibration.

slide5

Properties of Magnets

1. North and South Poles

slide6

Properties of Magnets

2. Law of magnetic poles

Unlike poles attract

slide7

Properties of Magnets

2. Law of magnetic poles

Like poles repel

laws of attraction
LAWS OF ATTRACTION

LIKE POLES REPEL

UNLIKE POLES ATTRACT

N S

S N

N S

N S

laws of attraction1
LAWS OF ATTRACTION

LIKE POLES REPEL

slide10

Properties of Magnets

Testing a magnet

1. Bring one end of an unknown object to a suspended magnet.

2. If repulsion occurs, then the unknown object is a magnet since repulsion occurs between like poles.

slide11

Magnetic Induction

Magnetic Induction is the process of inducing magnetism in an unmagnetised ferromagnetic material without any contact with the magnet.

slide12

Induced Magnetism

  • Making of magnetic materials (e.g. steel and Iron) into Magnets
  • Inducing magnetism in an ordinary piece of magnetic material
  • Induced magnetism is a temporary process
slide13

Induced Magnetism

The two iron nails become induced magnets and show a repulsion between the far ends

slide14

Induced Magnetism

Repulsion between the two iron nails increases as a bar magnet is brought towards the nails

magnetic poles
MAGNETIC POLES
  • A magnet is a dipole (two pole)
  • There is no monopole (single pole)
magnetic poles1
MAGNETIC POLES
  • A magnet is a dipole (two pole)
  • There is no monopole (single pole)
magnetic materials
Magnetic Materials
  • Most materials are NOT magnetic
  • Naturally occurring magnetic materials are called FERROMAGNETIC (Iron-Like)
  • FERROMAGNETIC MATERIALS are
    • Iron, Steel
    • Cobalt
    • Nickel
    • Alloys of above metals
magnetic domains
Magnetic Domains

These are small regions in a FERROMAGNETIC

Material where all the magnetic fields of the atoms are aligned

magnetization
Magnetization

If the domains are aligned, the material is polarized and becomes magnetized

S

N

magnetization1
Magnetization

Unmagnetised bar

A magnetised bar

slide21

Methods of Magnetisation & Demagnetisation

Making a magnet by stroking

Single Touch

Divided Touch

slide22

Methods of Magnetisation & Demagnetisation

Making a magnet by stroking

Single Touch

slide23

Methods of Magnetisation & Demagnetisation

Making a magnet by stroking

Single Touch

slide24

Methods of Magnetisation & Demagnetisation

Making a magnet by electrically

slide25

Methods of Magnetisation & Demagnetisation

Making a magnet by electrically

slide26

Methods of Magnetisation & Demagnetisation

Polarity of the magnet determined by:

(i) Right-Hand Grip Rule

  • Polarity of the
  • magnetised specimen
slide27

Methods of Magnetisation & Demagnetisation

Methods of demagnetising magnets

  • Heating
  • - misalignment of “tiny” magnets, increase vibration
  • of atoms
  • 2. Hammering
slide28

Methods of Magnetisation & Demagnetisation

Methods of demagnetising magnets

3. Alternating Current

slide29

Methods of Magnetisation & Demagnetisation

Methods of demagnetising magnets

3. Alternating Current

slide30

Magnetic Fields & the

Plotting Compass

A magnetic field is the region where a magnetic force is exerted on any magnetic object placed within the influence of the field.

slide31

Magnetic Fields & the

Plotting Compass

slide32

Magnetic Fields & the

Plotting Compass

slide33

Magnetic Fields & the

Plotting Compass

slide34

Magnetic Fields & the

Plotting Compass

X: neutral point

slide35

Magnetic Fields & the

Plotting Compass

X: neutral point

slide36

Magnetic Fields & the

Plotting Compass

slide39

Magnetic Properties ofIron & Steel

Question:

Comment on the magnetic properties of Iron & Steel

magnetic field near a wire2
Magnetic Field Near A Wire

Right-Hand Grip Rule

magnetic field near a wire6
Magnetic Field Near A Wire

Iron Filings around a current-carrying wire

magnetic field near a loop
Magnetic Field Near A Loop

Iron Filings around a current-carrying loop

magnetic field due to a solenoid1
Magnetic Field due to a solenoid

Iron Filings around a current-carrying coils of loops

magnetic field due to a solenoid3
Magnetic Field due to a solenoid
  • The strength of the magnetic field can be
  • increased by:
  • Increasing the current
  • Increasing the number of turns per unit length of the solenoid
  • Using a soft-iron core within the solenoid
magnetic field due to a solenoid4
Magnetic Field due to a solenoid
  • Electromagnets are far more useful than permanent magnets because:
  • They can be switched on and off.
  • 2. The strength of the magnetic field can be changed, by altering the current.
  • 3. They can easily be made into a variety of shapes and are less expensive to make
uses of electromagnets
Uses of electromagnets

The electric Bell

uses of electromagnets1
Uses of electromagnets

The electric Bell

-The current flows in the coil

making an electromagnet

-The armature is attracted to

the core

-As it moves, the circuit is

broken at the contact screw.

  • -The electromagnet is switched
  • off, so the armature moves back.
  • The armature keeps vibrating
  • back and forth making the
  • hammer strike the gong.
uses of electromagnets2
Uses of electromagnets

Magnetic Relay

Relays are used as safety devices.

A large current circuit can be switched on by a small current circuit

When the small current/voltage (that is safe) in the input circuit is switched on, the electromagnet becomes magnetic and attracts the iron armature.

The armature rotates towards the electromagnet, pushing the contacts together. This switches on the large current/voltage (that is dangerous) in the output circuit.

This type of relay circuit is used in the ignition of a car.