MAGNETISM

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MAGNETISM
Chapter 21
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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.
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Earths Magnetic Field
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Earths Magnetic Field
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Properties of Magnets
1. North and South Poles
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Properties of Magnets
2. Law of magnetic poles
Unlike poles attract
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Properties of Magnets
2. Law of magnetic poles
Like poles repel
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LAWS OF ATTRACTION

• LIKE POLES REPEL
• UNLIKE POLES ATTRACT

N S
S N
N S
N S
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LAWS OF ATTRACTION

• LIKE POLES REPEL

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

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Induced Magnetism
The two iron nails become induced magnets and
show a repulsion between the far ends
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Induced Magnetism
Repulsion between the two iron nails increases as
a bar magnet is brought towards the nails
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MAGNETIC POLES

• A magnet is a dipole (two pole)
• There is no monopole (single pole)

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MAGNETIC POLES

• A magnet is a dipole (two pole)
• There is no monopole (single pole)

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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

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Magnetic Domains
These are small regions in a FERROMAGNETIC
Material where all the magnetic fields of the
atoms are aligned
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Magnetization
S
N

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

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Magnetization
Unmagnetised bar
A magnetised bar
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Methods of Magnetisation Demagnetisation
Making a magnet by stroking
Single Touch
Divided Touch
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Methods of Magnetisation Demagnetisation
Making a magnet by stroking
Single Touch
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Methods of Magnetisation Demagnetisation
Making a magnet by stroking
Single Touch
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Methods of Magnetisation Demagnetisation
Making a magnet by electrically
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Methods of Magnetisation Demagnetisation
Making a magnet by electrically
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Methods of Magnetisation Demagnetisation
Polarity of the magnet determined by
(i) Right-Hand Grip Rule

• Polarity of the
• magnetised specimen

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Methods of Magnetisation Demagnetisation
Methods of demagnetising magnets

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

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Methods of Magnetisation Demagnetisation
Methods of demagnetising magnets
3. Alternating Current
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Methods of Magnetisation Demagnetisation
Methods of demagnetising magnets
3. Alternating Current
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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.
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Magnetic Fields the Plotting Compass
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Magnetic Fields the Plotting Compass
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Magnetic Fields the Plotting Compass
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Magnetic Fields the Plotting Compass
X neutral point
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Magnetic Fields the Plotting Compass
X neutral point
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Magnetic Fields the Plotting Compass
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Earths Magnetic Field
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Earths Magnetic Field
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Magnetic Properties ofIron Steel
Question Comment on the magnetic properties of
Iron Steel
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Magnetic Properties ofIron Steel
 
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Magnetic Properties ofIron Steel
Iron (soft Magnet) Steel (Hard Magnet)
Easy to magnetize Can be magnetized by a weak magnetic field Harder to magnetize Required a strong magnetic field to magnetise
Easy to demagnetize Harder to demagnetize
Used in Electromagnets, transformer cores and magnetic shields Good in making permanent magnets and bar magnets
 
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Magnetic Field Near A Wire
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Magnetic Field Near A Wire
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Magnetic Field Near A Wire
Right-Hand Grip Rule
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Magnetic Field Near A Wire
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Magnetic Field Near A Wire
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Magnetic Field Near A Wire
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Magnetic Field Near A Wire
Iron Filings around a current-carrying wire
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Magnetic Field Near A Wire
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Magnetic Field Near A Loop
Iron Filings around a current-carrying loop
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Magnetic Field due to a solenoid
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Magnetic Field due to a solenoid
Iron Filings around a current-carrying coils of
loops
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Magnetic Field due to a solenoid
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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

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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

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Uses of electromagnets
The electric Bell
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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.

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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.
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Uses of electromagnets
Magnetic Relay
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Uses of electromagnets
Magnetic Relay
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Uses of electromagnets
Magnetic Relay
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Uses of electromagnets
Magnetic Relay