Recognize magnetising coils and transformer connections
Interpret magnetic components while distinguishing one current-carrying coil from two magnetically coupled circuits.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Describe an electromagnet.
- Distinguish a.c. and steady d.c.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What produces a coil magnetic field in the simple electromagnet account?
What you will learn
- Draw coil and transformer symbols.
- Explain current-produced magnetisation and changing-input transformer behaviour qualitatively.
Draw a magnetising coil
A coil symbol has a succession of curved loops with terminal wires. Current through a magnetising coil produces a magnetic field; a suitable iron core can strengthen it. This is the switchable electromagnetic action already studied in magnetism. A coil is not represented by the same symbol as a battery.
Opening its current path removes the sustained coil current in the simple switching account. A real switch-off can involve a transient induced voltage, so this school diagram is not an instruction to handle arbitrary coils. The magnetic effect is not obtained by consuming the iron core as fuel.
Identify a transformer
A transformer symbol shows two coils alongside a common core, often represented by two parallel lines. Each coil has its own terminal pair. The core couples their changing magnetic flux; it does not represent an electrical wire joining the two winding circuits.
A changing primary input, ordinarily a.c., can produce a secondary p.d. A steady d.c. input does not sustain an ideal transformer output after switching transients. Step-up or step-down requires winding information; the symbol alone does not fix which output is larger. Numerical turns ratios are taught in the later transformer section, not assumed here.
Sources for this lesson
Pause and explain
An ideal transformer is connected to steady d.c. What sustained secondary output is expected after transients?
Worked example
A diagram shows two coils separated by core lines. Are the two sets of terminals necessarily electrically joined?
Show the worked solution
- Recognize a transformer symbol.
- Core lines represent magnetic coupling, not joining wires.
- Each winding has its own terminal pair.
Answer No; the winding circuits are distinct and magnetically coupled.
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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.
Draw a magnetising coil in a switched low-voltage path and explain the closed-switch action.
Give me a hint
Use a looped coil symbol with two terminals.
Compare my reasoning
- Draw the coil curves and terminal wires.
- Place it in a complete source-and-switch loop.
- Explain that current creates a magnetic field, strengthened by a suitable iron core.
A current-carrying coil produces a magnetic field; a suitable core strengthens it.
Look for these in your work
- Draw the coil rather than a source.
- Connect the switch within the path.
Draw a transformer symbol and identify its winding terminals and core.
Give me a hint
Keep two electrical circuits distinct.
Compare my reasoning
- Draw two adjacent winding symbols.
- Add common core lines between them.
- Label two terminals for each winding and explain magnetic coupling.
Two winding terminal pairs, with core lines representing magnetic coupling.
Look for these in your work
- Show four terminals.
- Do not connect the circuits through the core lines.
An unlabelled transformer drawing is claimed to triple voltage from a steady d.c. source. What two problems does this claim have?
Give me a hint
Check input variation and winding information separately.
Compare my reasoning
- Steady d.c. gives no sustained ideal secondary induction after transients.
- A numerical voltage ratio needs winding or other supplied information.
- The symbol alone establishes neither the claimed input operation nor factor three.
It needs changing input for sustained output, and the factor three requires winding data.
Look for these in your work
- Identify both missing conditions.
- Avoid inventing a turns ratio.
Common mistakes
- Core lines are not joining wires.
- A transformer needs a changing magnetic input for sustained output.
- The unlabelled symbol does not determine a turns ratio.
What can you explain now?
What identifies a transformer rather than one magnetising coil?
Compare with the explanation
Two windings coupled through a core.
The windings have distinct terminal pairs and a magnetic coupling.
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Make it stick
Draw coils and transformers and distinguish magnetic coupling from an electrical connection.
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.