Magnetism

Magnetism

1. Attraction

   • Magnets can attract IRON, COBALT and NICKEL only. They can also attract STEEL as it is at least 98% iron.
   • The force acts at a distance, and weakens as we get further from the magnet.
   • The region around a magnet where the force is felt is called the field. This force can also act through other materials.

2. Field

   • The field lines go from NORTH to SOUTH and never cross.
   • Like magnetic poles repel and unlike poles attract.

3. Domains

   • Magnetisable materials such as steel contain regions called domain that behave like tiny magnets. When the material is not magnetised, the domains are randomly aligned and cancel each other out.
     • By stroking with a magnet, the domains can be aligned so the tiny magnets act together making the material magnetic.
       • Heating or banging the object can jumble up the domains again and the object loses its magnetism.

4. Compasses

   • A freely suspended magnet will align itself with the magnets North pole pointing towards the earth's magnetic North pole. A compass aligns itself with the Earth's magnetic field. The Earth's magnetic field is due to the movement of liquid iron and nickel in the Earth's core. A compass is simply a freely suspended magnet.

5. The Earth's Magnetic Field

   • The Earth's magnetic field reverses itself every 250,000 years or so. The north magnetic pole becomes the south pole, and vice versa. Scientist have no clear theory about why the reversals happen. We're long overdue for a reversal as the last one happened about 780,000 years ago.

6. Cores

   • Steel cores can be made permanently magnetic quite easily as they retain their magnetism when the current is turned of.
     • Iron is therefore a better core to use for an electromagnet because it looses the magnetism when the current is turned off

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