OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply: potential difference across primary coil (V) / potential difference…
Full specification wording
Uses of magnetism: apply: potential difference across primary coil (V) / potential difference across secondary coil (V) = number of turns in primary coil / number of turns in secondary coil (content found only in the physics separate science qualification) (Higher tier only)
This transformer equation links the two coil voltages to the two turn counts.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This transformer equation links the two coil voltages to the two turn counts. Write it as Vp/Vs = Np/Ns, where Vp is the potential difference across the primary coil, Vs the potential difference across the secondary coil, Np the number of turns in the primary coil and Ns the number of turns in the secondary coil. A step-down transformer has fewer secondary turns, so Vs is smaller than Vp. Example: Np = 400, Ns = 20, Vp = 230 V gives Vs = 230 × 20/400 = 11.5 V. Rearrange by cross-multiplying: Vp × Ns = Vs × Np. Keep voltages in volts and turns as whole counts; the ratio is dimensionless. This content appears only in the separate physics qualification and is Higher tier only.
What earns the marks
- Selects the ratio Vp/Vs = Np/Ns and substitutes the two known voltages and one known turn count correctly.
- Rearranges the equation by cross-multiplication to isolate the unknown quantity before substituting numbers.
- Calculates the secondary voltage as Vp × Ns/Np and states the unit in volts.
- Identifies a step-down transformer from Ns being smaller than Np and predicts a smaller secondary voltage.
- Checks that the ratio of voltages equals the ratio of turns and that the answer is physically sensible.
Common mistakes and how to put them right
Inverts the turn ratio and divides by the wrong coil count.
Put it right: Keep primary quantities on the same side as each other.
Mixes the two voltages when substituting.
Put it right: Label each value as primary or secondary before calculating.
Omits the unit or writes the voltage without stating volts.
Put it right: Attach the correct unit to the final answer.
Exam tips
- Write the equation first, then substitute, so the method is visible even if arithmetic slips.
- Label every substituted value with its coil to avoid swapping primary and secondary.
- Sanity-check whether the answer should be larger or smaller than the input voltage for a step-down or step-up transformer.
Try the lesson
The start of the Uses of magnetism: apply: potential difference across primary coil (V) / potential difference across secondary coil (V) = number of turns in primary coil / number of turns in secondary coil (content found only in the physics separate science qualification) (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A transformer has 1000 turns on its primary coil and 250 turns on its secondary coil. The potential difference across the primary coil is 40 V. Calculate the potential difference across the secondary coil.
Listen
This transformer equation links the two coil voltages to the two turn counts. Write it as Vp/Vs = Np/Ns, where Vp is the potential difference across the primary coil, Vs the potential difference across the secondary coil, Np the number of turns in the primary coil and Ns the number of turns in the secondary coil. A step-down transformer has fewer secondary turns, so Vs is smaller than Vp. Example: Np = 400, Ns = 20, Vp = 230 V gives Vs = 230 × 20/400 = 11.5 V. Rearrange by cross-multiplying: Vp × Ns = Vs × Np. Keep voltages in volts and turns as whole counts; the ratio is dimensionless. This content appears only in the separate physics qualification and is Higher tier only. A strong answer earns marks for these points. Selects the ratio Vp/Vs = Np/Ns and substitutes the two known voltages and one known turn count correctly. Rearranges the equation by cross-multiplication to isolate the unknown quantity before substituting numbers. Calculates the secondary voltage as Vp × Ns/Np and states the unit in volts. Identifies a step-down transformer from Ns being smaller than Np and predicts a smaller secondary voltage. Exam tip: Write the equation first, then substitute, so the method is visible even if arithmetic slips.
Key words
Say what each word means before you turn the card.
Transformer turns ratio
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Potential difference across coils
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Step-down transformer
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Rearranging ratio equations
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See every definition
- Transformer turns ratio
- The number of turns in the primary coil divided by the number of turns in the secondary coil, equal to the ratio of the two coil voltages.
- Potential difference across coils
- The voltage across the primary coil and the voltage across the secondary coil, related to the two turn counts by the transformer equation.
- Step-down transformer
- A transformer with fewer secondary turns, so the secondary voltage is smaller than the primary voltage.
- Rearranging ratio equations
- Cross-multiplying so that the primary voltage times the secondary turns equals the secondary voltage times the primary turns, isolating the unknown quantity.
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