Magnetism Grade 12 Physics

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MagnetismGrade 12 Physics

• Chelstone High School
• Mr. S. S. Simfukwe

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Properties of Magnets

• They attract ferromagnetic materials
• They have magnetic poles
• They obey the first law of magnetism

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Ferromagnetic materials

• These are metals which are attracted by magnets
• Examples include Iron, Cobalt, Nickel and alloys
  of these metals.

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Magnetic Poles

• These are areas on the magnet where magnetism is
  concentrated (mostly on the ends of the magnet).
• These poles are called North are South poles
  respectively.

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Law of magnetism

• It states that Like Poles Repel and Unlike Poles
  Attract.

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Test for Magnets

• Remember that unmagnetised iron, steel, cobalt
  and nickel objects will be attracted to either
  pole of a magnet.
• This means you can only show that an object is a
  magnet if it repels a known magnet.
• Therefore Repulsion is the best test for
  Magnetism.

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Magnetic Properties of Iron and Steel.

• Iron is easier to Magnetise and Demagnetise,
  therefore it is called soft iron.
• Steel is hard to Magnetise and Demagnetise,
  therefore it is called hard iron.

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Making Magnets

• Hammering with a steel bar in a North-South
  direction can make a magnet.

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• Stroking a piece of unmagnetised iron or steel
  with a known magnet can make it into a magnet.

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• Placing a magnet alongside a non-magnetised steel
  knitting needle for some time will magnetise it.

Steel nail
Magnet
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• An electric current has a magnetic effect.
• Winding a nail with a wire and then connecting
  the wire to a cell produces a magnet.

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• This causes the molecules or dipoles to line up
  in the North-South orientation.

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 Demagnetizing

• A magnet can be destroyed by hammering.
• Heating a piece of magnetized iron or steel
  strongly in a flame will demagnetize it.

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• Placing a magnet in a solenoid (coil of wire)
  carrying alternating current will also cause it
  to demagnetize.

coil
magnet
a. c supply
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Magnetic Fields

• Magnetic field is the area around a magnet where
  its magnetic forces can be detected.
• The magnetic field can be seen by placing the
  magnetunder a piece of paper with small iron
  filings on top.

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Magnetic Fields

• Two like poles will repel each other.These may
  be two North poles or two South poles,they will
  both have the same magnetic field shape.
• The region in the centre of the poles shown by
  the red starhas no magnetic field,

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Magnetic Fields

• The poles of this free magnet are ATTRACTED to
  the OPPOSITE poles of the stationary' magnet.

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Uses of Magnets

• Electric Bell
• Motors
• Speakers etc

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Practice

• 1. State three properties of magnets?
• Attract ferromagnetic materials.
• They have magnetic poles.
• They obey the first law of magnetism.

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2. List down three ways you can make a magnet

• Hammering in North-South direction.
• By using direct current.
• Stroking with a permanent magnet.

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3. How can you demagnetize a magnet?

• Heating to hot-red
• Strongly hammering it.
• Placing it in a solenoid carrying alternating
  current.

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4. Define magnetic field?

• This is the area around a magnet where its
  magnetic forces can be detected.