Explain magnetic relay action and its applications
Connect control-current magnetisation to a mechanical contact change in a separate switched circuit.
Section P4.5.3 · Magnetic effect of a current
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Explain electromagnet magnetisation.
- Trace separate control and load circuits.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to a suitable soft-iron core when its coil current is removed in this model?
What you will learn
- Connect control-current magnetisation to a mechanical contact change in a separate switched circuit.
- Explain relay switching through current, magnetisation and armature motion, with separate loop and source roles.
An electromagnet moves the contacts
A simple normally open relay has a control coil, a magnetisable core, a movable armature and separate switching contacts. Current in the control coil magnetises its core and attracts the armature, closing the stated contacts. When current is removed, the field falls and a spring can return the armature, reopening them.
This is the described normally open model; relays can also have normally closed or changeover contacts. The control loop and switched load loop are distinct. A drawn dashed mechanical link from coil to contacts is not a conducting wire or a transfer of charge into the load circuit.
Explain a use rather than only naming it
A low-current control signal can operate a relay that switches a separate suitably rated lamp, bell or motor circuit. For example, a temperature-control circuit can energise the coil to switch a separate fan circuit. The control provides a command; the load’s electrical energy comes from its own circuit supply.
Real contacts have current and voltage limits, isolation requirements and protective needs, especially for inductive loads. The school diagram uses stated low-voltage circuits and does not approve a relay for mains control. A soft-iron core’s temporary magnetisation supports reliable release in the ideal model.
Sources for this lesson
Pause and explain
A dashed coil-to-contact link in a relay drawing is what?
Worked example
A normally open relay coil is energised by one low-voltage source and closes a separately supplied lamp path. Where does lamp energy come from?
Show the worked solution
- Control current creates the magnetic action.
- The armature changes the separate contacts.
- The lamp receives electrical energy from its own connected supply.
Answer From the lamp circuit supply; the coil provides the switching action.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain a normally open relay through energise, attract, close and release stages.
Give me a hint
Track magnetic action and contact state.
Compare my reasoning
- Control current magnetises the core and attracts the armature.
- The normally open contacts close the stated load path.
- Removing current allows the return mechanism to reopen them.
Control current changes magnetisation, armature position and separate contact state.
Look for these in your work
- State normally open.
- Include release.
Describe a relay use in a supplied low-voltage temperature-controlled fan model.
Give me a hint
Separate sensing control and switched power.
Compare my reasoning
- The control condition energises the coil.
- Magnetic action closes the fan circuit contacts.
- The fan’s own source supplies its operating energy.
The relay converts the control signal into a separate fan-circuit switching action.
Look for these in your work
- Explain why the relay is useful.
- Name the load energy supply.
A diagram joins the coil loop and load loop by replacing a mechanical dashed link with a wire and claims the relay creates the load energy. Correct both claims.
Give me a hint
Separate topology from command and energy source.
Compare my reasoning
- The dashed link is mechanical action.
- The current loops remain separate in the stated model.
- Load energy comes from the load source, not from the switching label.
Retain separate loops and mechanical action; attribute load energy to its connected source.
Look for these in your work
- Correct the false connection.
- Correct the energy account.
Common mistakes
- Separate electrical loops from the mechanical link.
- Contact type must be stated.
- Ratings are not established by the relay name.
What can you explain now?
Why does energising the relay coil move its armature?
Compare with the explanation
The coil’s magnetic effect attracts the magnetisable armature.
Control current produces magnetisation and the resulting mechanical action.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain relay switching through current, magnetisation and armature motion, with separate loop and source roles.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.