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PHYSICS · Stage 8 · Chapter 8: Magnetism and Electromagnets

Physics 8.4 Using electromagnets — Electric Bells, Relays and Circuit Breakers

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Physics 8.4 Using electromagnets — Electric Bells, Relays and Circuit Breakers — Alhussan International Academy — Physics (Stages 7–9) notes: Describe uses of electromagnets and explain why they are used instead of permanent magnets.Relay = a switch operated by an electromagnet, …

  • Reed switch
  • X-ray machines
  • Fire doors
  • Make and break
  • Compared with a relay
  • Common feature

Section 1 of 6Relays

  • Relay = a switch operated by an electromagnet, in which a small current in one circuit switches on a circuit that could be dangerous.
  • Reed switch
    • A simple relay is made from two pieces of magnetic material (reeds) with a gap between them.
    • A coil is wound around the outside and a current passed through it.
    • The reeds are magnetised, attract each other and touch.
    • This completes the second circuit.

Section 2 of 6Relay in a car starter motor

  1. The driver turns the key and a current flows in the coil.
  2. A magnetic field is produced.
  3. The iron armature is magnetised and attracted towards the contacts.
  4. The contacts close and complete the starter motor circuit.
  5. The car starts.
  • Safety
    • The starter motor takes a dangerously high current from the car battery, so the driver switches only a small current in the relay circuit.

Section 3 of 6Other uses of relays and electromagnets

  • X-ray machines
    • They use a high-voltage circuit and X-rays damage cells, so radiographers switch them with a relay from a safe distance.
  • Fire doors
    • An electromagnet on the wall holds the door open while a current flows.
    • A piece of iron on the door is attracted to it.
    • When the fire alarm breaks the circuit, the current stops and the door closes.
  • Circuit breakers
    • If the current is too large the electromagnet becomes strong enough to pull a lever and break the circuit.
  • Reset
    • A circuit breaker is reset by pressing a button, so it does not need replacing like a fuse.

Section 4 of 6The electric bell

  • Make and break
    • A doorbell circuit keeps making and breaking contact.
  • Sequence
    • Pressing the bell switch completes the circuit and current flows through the coil.
    • The iron core is magnetised and attracts the iron armature.
    • The hammer hits the gong.
    • The armature moves away from the contact at X, breaking the circuit.
    • The core stops being magnetised and the armature springs back, so the cycle repeats.
  • Compared with a relay
    • In a bell the armature moves backwards and forwards while the switch is pressed, whereas in a relay it moves once.
  • Common feature
    • Both use an electromagnet to move an iron armature that opens or closes contacts.

Section 5 of 6Magnets in medicine

  • MRI scanner
    • It contains very strong magnets that produce an intense field interacting with hydrogen in water molecules in the body.
  • Advantages
    • Scans are safe and painless and give much more detailed images than X-rays.
    • Doctors see structures that X-rays do not show without cutting the patient open.
    • They can see how an organ works under normal conditions.
  • Disadvantages
    • Scanners are very expensive, noisy and cramped.
    • The patient must lie very still in a small space.

Section 6 of 6Why electromagnets are used

  • Switchable
    • They can be turned on and off, unlike permanent magnets.
  • Strong
    • They can be much stronger than permanent magnets.
  • Maglev trains
    • They need strong controllable magnets, so electromagnets are used rather than permanent magnets.

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