Explain induction in an iron-cored transformer
Trace changing primary current, linked core field and induced secondary e.m.f. without joining the separate winding circuits.
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Explain changing-field induction.
- Distinguish voltage from current.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does an open secondary guarantee zero induced voltage?
What you will learn
- Trace changing primary current, linked core field and induced secondary e.m.f. without joining the separate winding circuits.
- Explain linked-field induction and distinguish normal alternating output from open-load and d.c.-transient cases.
Follow a changing linked field
An alternating primary current produces a changing magnetic field in the shared soft-iron core. That changing field links the secondary turns and induces a secondary e.m.f. With a suitable closed secondary load, the e.m.f. can drive output current. An open secondary can have voltage without sustained load current.
The primary and secondary circuits remain electrically separate in the stated simple two-winding arrangement. Energy is supplied through the primary source and transferred to the secondary load through the changing magnetic interaction; electrons do not need to cross from one insulated winding to the other.
Distinguish steady input from a switching transient
A steady d.c. current would produce a steady field after the switching transient, so it does not sustain the normal induced secondary e.m.f. of this simple transformer. Switching d.c. on or off can briefly change linking and induce a transient. Saying d.c. can never cause any transient is as inaccurate as saying steady d.c. provides normal continuous transformer output.
A real primary winding connected improperly to d.c. can heat and exceed operating limits; this is not an experiment instruction. The soft-iron core and suitable construction support the changing linked field, while winding and core losses make real behaviour differ from an ideal model.
Pause and explain
After a steady d.c. primary current has settled, what happens to normal sustained secondary induction in the simple model?
Worked example
Explain why an open secondary can have a.c. e.m.f. while a steady d.c. primary state does not sustain normal output.
Show the worked solution
- Changing primary current makes linked field change.
- Induced e.m.f. does not require a closed output load.
- A steady d.c. field has no sustained linking change after its transient.
Answer Changing linking can give open-output e.m.f.; steady d.c. does not sustain normal induction.
Try a topic test
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 the transformer’s primary-to-secondary induction chain.
Give me a hint
Follow changes rather than a direct electrical bridge.
Compare my reasoning
- Alternating primary current produces changing core field.
- The field links the secondary turns.
- Changing linking induces secondary e.m.f.
Changing primary current → changing linked field → secondary e.m.f.
Look for these in your work
- Name the primary source role.
- Keep the circuits electrically separate.
Compare open and closed secondary states under changing linked field.
Give me a hint
Ask separately about voltage and a complete load path.
Compare my reasoning
- Both states can have induced secondary e.m.f.
- A closed suitable load permits output current.
- An open load does not sustain that external current.
E.m.f. can exist in both; sustained load current requires a closed path.
Look for these in your work
- Separate voltage and current.
- State the load-path condition.
A report says steady d.c. gives normal continuous output, while switching d.c. can never induce anything. Correct both parts.
Give me a hint
Separate the settled state from the change at switching.
Compare my reasoning
- Steady d.c. gives steady linking after settling.
- Switching changes linking and can induce a transient.
- Normal sustained operation needs the specified changing-input condition.
No normal settled d.c. output; switching can briefly induce a transient.
Look for these in your work
- Correct both claims.
- Retain real operating and heating limits.
Common mistakes
- Link current change to field change.
- Separate e.m.f. from load current.
- Retain switching-transient limits.
What can you explain now?
What induces the secondary e.m.f.?
Compare with the explanation
A changing magnetic field linking its turns
The secondary voltage arises from changing linked field, not direct charge crossing the core.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain linked-field induction and distinguish normal alternating output from open-load and d.c.-transient cases.
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.