OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(i) simple laminated iron-cored transformer; Ns/Np = Vs/Vp = Ip/Is for an…
Full specification wording
Electromagnetism: (i) simple laminated iron-cored transformer; Ns/Np = Vs/Vp = Ip/Is for an ideal transformer
A simple transformer has two coils wound on a laminated iron core.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 7 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A simple transformer has two coils wound on a laminated iron core. Alternating current in the primary coil produces a changing magnetic flux in the core, which links the secondary coil and induces an alternating e.m.f. there. Laminations reduce eddy currents and energy loss. For an ideal transformer, power in equals power out, so Ns/Np = Vs/Vp = Ip/Is, where N is the number of turns, V the voltage and I the current in the secondary (s) and primary (p). A step-up transformer has more secondary turns and raises voltage while lowering current; a step-down transformer does the reverse.
What earns the marks
- A transformer consists of primary and secondary coils on a laminated iron core.
- Alternating current in the primary produces a changing flux that induces an alternating e.m.f. in the secondary.
- Laminations reduce eddy currents, improving efficiency.
- For an ideal transformer, Ns/Np = Vs/Vp = Ip/Is.
- A step-up transformer has Ns greater than Np and increases voltage while decreasing current; a step-down transformer does the opposite.
Common mistakes and how to put them right
Using the turns ratio the wrong way round, for example writing Np/Ns = Vs/Vp.
Put it right: The correct relation is Ns/Np = Vs/Vp.
Assuming current also increases in a step-up transformer.
Put it right: For an ideal transformer, voltage rises while current falls so power is conserved.
Thinking a transformer works with direct current.
Put it right: A steady d.c. produces no changing flux and so no induced e.m.f.
Ignoring the role of laminations and treating the core as solid, which would allow large eddy currents and energy loss.
Exam tips
- Write the ratio as Ns/Np = Vs/Vp = Ip/Is and substitute values carefully, checking which coil is primary.
- Use power conservation to check your answer: VpIp should equal VsIs for an ideal transformer.
- State that the input must be alternating so that the flux changes and induces an e.m.f.
Try the lesson
The start of the Electromagnetism: (i) simple laminated iron-cored transformer; Ns/Np = Vs/Vp = Ip/Is for an ideal transformer lesson, made from checked teaching notes for this specification point.
Listen
A simple transformer has two coils wound on a laminated iron core. Alternating current in the primary coil produces a changing magnetic flux in the core, which links the secondary coil and induces an alternating e.m.f. there. Laminations reduce eddy currents and energy loss. For an ideal transformer, power in equals power out, so Ns/Np = Vs/Vp = Ip/Is, where N is the number of turns, V the voltage and I the current in the secondary (s) and primary (p). A step-up transformer has more secondary turns and raises voltage while lowering current; a step-down transformer does the reverse. A strong answer earns marks for these points. A transformer consists of primary and secondary coils on a laminated iron core. Alternating current in the primary produces a changing flux that induces an alternating e.m.f. in the secondary. Laminations reduce eddy currents, improving efficiency. For an ideal transformer, Ns/Np = Vs/Vp = Ip/Is. Exam tip: Write the ratio as Ns/Np = Vs/Vp = Ip/Is and substitute values carefully, checking which coil is primary.
See it
Key words
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Transformer action and turns ratio
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Power conservation in ideal transformers
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Energy losses and laminated cores
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Step-up transformer
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See every definition
- Transformer action and turns ratio
- Alternating current in the primary coil produces a changing magnetic flux in the core, which links the secondary coil and induces an alternating e.m.f. there, with Ns/Np = Vs/Vp = Ip/Is for an ideal transformer.
- Power conservation in ideal transformers
- For an ideal transformer, power in equals power out, so Ns/Np = Vs/Vp = Ip/Is.
- Energy losses and laminated cores
- Laminations reduce eddy currents and energy loss.
- Step-up transformer
- A transformer with more secondary turns, which raises voltage while lowering current.
Label the diagram
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1Box 1
2Box 2
3Box 3
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