Magnetism

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Magnetism
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ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole
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Breaking a magnet produces two magnets!
N
S
N
S
N
N
S
S
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Opposite poles attract and like poles repel
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Magnetic materials
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Magnetic materials
Iron (steel), Cobalt and Nickel
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Magnetic induction
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Magnetic induction

• When a magnetic material is close to a magnet,
  it becomes a magnet itself
• We say it has induced magnetism

magnet
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N
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Hard and Soft Magnetism
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Soft Magnetism

• Pure iron is a soft magnetic material
• It is easy to magnetise but loses its magnetism
  easily

before
after
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S
N
S
N
Not a magnet
Iron nail
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Hard Magnetism

• Steel is a hard magnetic material
• It is harder to magnetise, but keeps its
  magnetism (it is used to make magnets!)

before
after
S
N
N
S
N
N
Its a magnet!
S
Steel paper clip
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Magnetic field

• Magnets and electric currents produce magnetic
  fields around them.
• In a magnetic field, another magnet, a magnetic
  material or a moving charge will experience a
  magnetic force.

www.physchem.co.za
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Magnetic field lines

• We can represent the magnetic field around a
  wire or magnet using field lines.

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Magnetic field lines

• The arrows show the direction a compass needle
  would point at that point in the field.

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Magnetic field lines

• The arrows show the direction a compass needle
  would point at that point in the field.

Note that magnetic field is a vector quantity
The closer the field lines are, the stronger the
magnetic force felt
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Earths Magnetic Field
Remember the North of a compass needle points to
the geographic north pole (i.e. the geographic
North pole is a magnetic south pole!)
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Moving charges (currents)

• Moving charges (electric currents) also produce
  a magnetic field

Conventional current electrons flow in the
opposite direction
http//www.sciencebuddies.org
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Magnetic field around a straight wire
Stronger field closer to wire
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Magnetic field around a flat circular coil
http//physicsed.buffalostate.edu
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Magnetic field around a solenoid
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The Motor Effect

• When a current is placed in a magnetic field it
  will experience a force. This is called the motor
  effect.

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The Motor Effect

• The direction of the force on a current in a
  magnetic field is given by Flemmings left hand
  rule.

Thumb Motion
First finger Field direction
Centre finger Conventional Current
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Sample question

• In this example, which way will the wire be
  pushed? (red is north on the magnets)

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Another sample question!

• An electron approaches a bar magnet as shown.
  What is the direction of the force on the
  electron?

N
S
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D.C.Motor
Commutator ensures that every half rotaion the
current direction reverses in the coil