Electromagnets

Electromagnets

1. Ends of a magnets are called magnetic poles. There is a north pole and a south pole. The region around a magnet which is where a piece of iron or steel will be attracted to is called its magnetic field.

   • Iron filings placed near a magnet will form a pattern of lines that loop from one pole to another. These are lines of force.

2. An electromagnet is made of insulated wire wrapped round an iron bar. When a current is passed along the wire, a magnetic field is created around the wire. So the magnetic field magnetises the iron bar strongly.

   • When current is switched off the iron bar loses most of its magnetism. Iron easily loses magnetism when current is switched off. Steel would be a bad core as it doesn't lose its magnetism easily.

3. Electromagnets are used in many devices:

   • Scrapyard crane - scrap vehicles are lifted in a scrap yard using powerful electromagnets. Steel frame of vehicle attracts to crane when current passes through coil. When current stops, vehicle drops from electromagnet.
   • Circuit breaker - switch in series circuit with electromagnet. When too much current passes through electromagnet, switch is pulled open by electromagnet and stays open till reset manually.

4. A hospital always has its own electricity genertator on standby just incase there is a power-cut.

5. A generator contains coils of wire the spin in a magnetic field. A potential difference is created across the ends of a wire when its cut across the magnetic field lines - called ELECTROMAGNETIC INDUCTION.

   • If wire or coil is part of a complete circuit, a current passes through it.

6. If direction of movement of wire or coil is reversed or the polarity of magnet is reversed the direction of induced potential difference is reversed. Potential difference is only induced while there is movement.

   • Can create same effect by turning magnet end to end to create a current that lasts along as you spin the magnet which is how a generator works.
     • As turn the magnet, the magnetic field through coil changes which induces potential difference so makes current flow through wire. When turned magnet through half turn the direction of magnetic field through coil reverses, so potential difference reverses and current flows in opposite direction. If keep turning magnet in same direction then the potential difference will keep reversing every half turn making it an AC current.

7. Size of induced potential difference is increased by increasing the speed of movement, the strength of magnetic field and number of turns on the coil.

8. A magnetic field is a region where magnetic materials and also wires carrying currents experience a force acting on them.

9. The magnetic field inside a coil of wire is strong and uniform. Outside the coil the magnetic field is just like the one round a bar magnet.

   • Can increase strength of magnetic field around a solenoid by adding a magnetically "soft" iron core through middle of core.
   • A magnetically soft material magnetises and demagnetises very easily. As soon as you turn off current the magnetic field disappears - iron doesn't stay magnetised. Which makes it useful for something that needs to switch its magnetism off and on.

10. If you keep the magnet moving forwards and backwards, you produce a potential difference that keeps swapping direction - how you produce an alternating circuit.