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Magnets, magnetic materials and fields
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Magnets attract magnetic materials and attract or repel other magnets. Field lines show the forces around them.
Magnets and materials
Like poles repel; unlike poles attract. Magnets also attract magnetic materials such as iron, steel, nickel and cobalt.
Magnetically hard materials (such as steel) are hard to magnetise but keep their magnetism: used for permanent magnets.
Magnetically soft materials (such as iron) are easily magnetised but lose their magnetism quickly: used for electromagnet cores and transformers.
Induced magnetism: a magnetic material placed in a magnetic field becomes a magnet itself, for example an iron nail near a magnet can pick up paper clips.
Magnetically hard materials (such as steel) are hard to magnetise but keep their magnetism: used for permanent magnets.
Magnetically soft materials (such as iron) are easily magnetised but lose their magnetism quickly: used for electromagnet cores and transformers.
Induced magnetism: a magnetic material placed in a magnetic field becomes a magnet itself, for example an iron nail near a magnet can pick up paper clips.
Magnetic field lines
A magnetic field line shows the direction of the force on a north pole at that point. Lines go from north to south outside a magnet.
The closer the lines, the stronger the field.
Investigating: place a bar magnet under paper and sprinkle iron filings, or move a small plotting compass around it and mark the direction of its needle. Try this with two bar magnets, with like poles and then unlike poles facing.
The closer the lines, the stronger the field.
Investigating: place a bar magnet under paper and sprinkle iron filings, or move a small plotting compass around it and mark the direction of its needle. Try this with two bar magnets, with like poles and then unlike poles facing.
A uniform field
Place two flat magnets close together and parallel, with opposite poles facing.
Between them the field lines are parallel and equally spaced: the field is uniform (the same strength and direction everywhere).
Between them the field lines are parallel and equally spaced: the field is uniform (the same strength and direction everywhere).
Why is iron, not steel, used as the core of an electromagnet?
- Iron is magnetically soft.
- It is magnetised easily when the current is on.
- It loses its magnetism when the current is switched off.
Answer: Iron is magnetically soft, so the electromagnet can be switched off.
Like poles repel and unlike poles attract. Hard materials (steel) keep magnetism; soft materials (iron) lose it quickly. Magnetic materials become magnets in a field (induced magnetism). Field lines go north to south; closer lines mean a stronger field. Opposite poles facing give a uniform field.
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