Simple iron-core ideal transformers
Key idea: H2 Physics lessons on magnetic flux, induction laws, applications and ideal transformers.
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The core idea
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Learn the idea
Big question: How can changing flux transfer power between two circuits?
An alternating primary current creates changing flux in an iron core, linking a secondary coil. For an ideal transformer Vs/Vp = Ns/Np and input power equals output power, so current changes inversely with voltage. A steady direct current cannot maintain the required changing flux.
Use common changing core flux
An alternating primary current creates changing magnetic flux in a shared core. This links both coils and induces e.m.f.s. For an ideal transformer, V_s/V_p = N_s/N_p because both coils share the same rate of flux change per turn.
A transformer requires changing flux, so a steady d.c. input gives no sustained secondary e.m.f. after the switching transient. Step-up means N_s > N_p and V_s > V_p; it does not mean energy is created.
Check your understanding: A transformer has 200 primary and 50 secondary turns at 240 V. Find ideal output voltage.
V_s = 240(50/200) = 60 V.
Use the ideal power balance
For an ideal transformer, input and output powers are equal: V_pI_p = V_sI_s. Combining this with the turns ratio gives N_s/N_p = V_s/V_p = I_p/I_s, so stepping voltage up steps current down by the reciprocal factor.
The ideal model assumes the two coils share the changing core flux and that no energy is lost in the windings or core. Keep primary quantities together on one side of each ratio before substituting.
Check your understanding: An ideal transformer doubles the voltage. What happens to the current?
The current halves, so input and output power remain equal.
Key ideas to keep
- A transformer requires changing flux, normally from a.c.
- Step-up voltage means step-down current in the ideal model.
- The turns ratio compares corresponding primary and secondary quantities.
See the reasoning
Worked example
Explain energy transfer through a transformer
Question: Explain how an iron-core transformer transfers energy without conducting current between windings.
Step 1: Create changing core flux
Why: Only changing flux can induce a sustained e.m.f.
Working: Alternating primary current produces alternating magnetic flux in the iron core.
Step 2: Link the secondary
Why: The common core guides nearly the same changing flux through both windings.
Working: Faraday's law induces an alternating secondary e.m.f.; Vₛ/Vₚ = Nₛ/Nₚ ideally.
Step 3: Account for power
Why: The ideal model neglects winding and core losses.
Working: VₚIₚ = VₛIₛ, so stepping voltage up steps current down.
Answer: Alternating primary current produces changing core flux. The iron core links this flux through both windings; Faraday's law induces alternating secondary e.m.f. Turns ratio sets voltage ratio. In the ideal model, power is conserved and current ratio is inverse to voltage ratio.
Check: A steady d.c. primary produces no sustained secondary e.m.f. after switching.
Use a hint if needed
Practise with support
Try this
An ideal transformer steps voltage up by factor 5. State the current factor.
Hint: Ideal power is conserved.
Check your answer
Current steps down by factor 5 because VpIp = VsIs and Ip/Is = Ns/Np.
Now work without the hint
Practise independently
Your turn
State the ideal-transformer assumptions behind the voltage and current ratios and explain what a step-up device does.
Check your answer
All changing core flux links both windings, winding resistance and core losses are negligible, and input power equals output power. Ns/Np = Vs/Vp = Ip/Is. A step-up transformer raises voltage and lowers current by the same ratio; it does not create power.
Avoid these traps
Common mistakes
Common mistake
A step-up transformer creates power.
What is wrong with this reasoning?
Show better thinking
An ideal transformer conserves power and trades increased voltage for decreased current.
Common mistake
A steady d.c. primary produces a continuous secondary e.m.f.
What is wrong with this reasoning?
Show better thinking
After the switching transient, steady d.c. gives constant flux and no induced secondary e.m.f.; transformer operation requires changing flux.
Write for the examiner
Exam guidance
Write the turns ratio and ideal power equation separately before combining them.
Exam-style practice [7 marks]
An ideal 240 V transformer with 1500 primary and 75 secondary turns supplies 4.0 A. Find secondary voltage, primary current and output power.
Plan before you answer
- Use the turns ratio for voltage.
- Use ideal power for current.
- Check both powers agree.
Mark your answer and compare the model
Marking points
Tick each point only if your answer states it clearly.
Model answer
Vs = 240(75/1500) = 12 V. Ip = Is(Ns/Np) = 0.20 A. Output power is 12(4.0) = 48 W, equal to input power.
Come back in three days
Check what stayed with you
Recall question
An ideal transformer has Ns/Np = 0.10. State Vs/Vp and Ip/Is.
Check the answer
Vs/Vp = 0.10 and Ip/Is = 0.10; secondary current is ten times primary current.
Syllabus and review details
This lesson covers the listed H2 Physics 9478 outcomes. Flux uses area perpendicular to B; flux linkage is NΦ for N linked turns. Faraday's law uses the rate of change of linkage and Lenz's law fixes polarity from the change being opposed. The simple Blv motional-e.m.f. form requires mutually perpendicular conductor length, velocity and uniform field. Ideal transformer ratios assume common linked flux, alternating operation and no winding or core losses. More advanced induction applications are not required here.
- GCE A-Level H2 PhysicsTopic 18(g) · 2027Checked against the syllabus · partial topic coverageOfficial 9478 syllabus
Course and syllabus information
- Course
- GCE A-Level H2 Physics
- Edition
- GCE A-Level H2 Physics 2027