Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

8.4Chapter 8: Magnetism and Electromagnets

Using electromagnets — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 6 2 quizzes

📚 Alhussan Physics - Stage 8 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

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 8Relays

Relay
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 8Relay 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 8Other 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 8The electric bell

1Make and break
  • A doorbell circuit keeps making and breaking contact.
2Sequence
  • 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.
3Compared with a relay
  • In a bell the armature moves backwards and forwards while the switch is pressed, whereas in a relay it moves once.
4Common feature
  • Both use an electromagnet to move an iron armature that opens or closes contacts.

Section 5 of 8Magnets 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 8Why 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.

Section 7 of 8Key terms

TermMeaning
RelayA switch operated by an electromagnet.
Reed switchTwo magnetic strips that touch when a coil around them carries a current.
ArmatureAn iron piece attracted by an electromagnet.
Circuit breakerA switch that opens when current is too large and can be reset.
MRI scannerA machine using strong magnets to image the inside of the body.

Section 8 of 8Exam technique

1
Thinking a relay carries the same current as the circuit it switches — it uses a small current to switch a large one.
2
Thinking a bell rings from one movement — it repeatedly makes and breaks the circuit.
3
Thinking fire doors close by a spring only — the electromagnet holds them open until the current stops.
4
Thinking MRI uses X-rays — it uses strong magnets.

?What do you want to solve now?

Practice questionsTraining questions with instant feedback - not an exam. Official exam - gradedCheckpoint Quiz in MasterStudy: your mark is saved to your progress.

Last updated: