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Electromagnets and field patterns
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A current makes a magnetic field. Coiling the wire round an iron core makes a strong magnet you can switch on and off.
Field patterns
Straight wire: circles around the wire, centred on it. The field is strongest close to the wire. Use the right-hand grip rule: thumb along the current, fingers curl the way the field goes.
Flat circular coil: circles around each side of the coil, with nearly straight lines through the middle.
Solenoid (long coil): outside, the field is like that of a bar magnet; inside, the lines are parallel, so the field is strong and uniform.
Flat circular coil: circles around each side of the coil, with nearly straight lines through the middle.
Solenoid (long coil): outside, the field is like that of a bar magnet; inside, the lines are parallel, so the field is strong and uniform.
Building an electromagnet
Wind a coil of insulated wire around a soft iron core and pass a current through it.
The iron core makes the field much stronger. Because iron is magnetically soft, the magnetism disappears when the current is switched off.
Stronger electromagnet: more current, more turns on the coil, an iron core.
Uses: cranes in scrapyards, relays, electric bells.
The iron core makes the field much stronger. Because iron is magnetically soft, the magnetism disappears when the current is switched off.
Stronger electromagnet: more current, more turns on the coil, an iron core.
Uses: cranes in scrapyards, relays, electric bells.
Moving charges
There is a force on a charged particle when it moves through a magnetic field, as long as it is not moving parallel to the field.
An electric current is a flow of charged particles, which is why a current-carrying wire feels a force in a magnetic field (the motor effect).
An electric current is a flow of charged particles, which is why a current-carrying wire feels a force in a magnetic field (the motor effect).
Give three ways to make an electromagnet stronger.
- Increase the current.
- Add more turns of wire to the coil.
- Put a soft iron core inside the coil.
Answer: Increase the current, increase the number of turns, use an iron core.
A straight wire has circular field lines; a solenoid has a bar-magnet field outside and a uniform field inside. An electromagnet is a coil round a soft iron core; more current, more turns and an iron core make it stronger. Charged particles moving across a magnetic field feel a force.
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