OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the equations linking the potential differences and numbers of turns…
Full specification wording
Uses of magnetism: apply the equations linking the potential differences and numbers of turns in the two coils of a transformer (content found only in the physics separate science qualification) (Higher tier only)
The linking equation for an ideal transformer is Vp/Vs = Np/Ns, where Vp and Vs are the potential differences across the primary and secondary coils and Np and Ns are their numbers of turns.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 8 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The linking equation for an ideal transformer is Vp/Vs = Np/Ns, where Vp and Vs are the potential differences across the primary and secondary coils and Np and Ns are their numbers of turns. Rearranged, Vs = Vp × Ns/Np and Ns = Np × Vs/Vp. To apply it, list the three known quantities, rearrange for the unknown, substitute values with consistent units (potential differences in volts, turns as plain counts) and calculate.
For example: With Vp = 12 V, Np = 100 and Ns = 400, Vs = 12 × 400/100 = 48 V. Check that a step-up result is larger than Vp and a step-down result is smaller.
What earns the marks
- Selects the equation Vp/Vs = Np/Ns and rearranges it correctly for the unknown quantity before substituting values.
- Substitutes the known potential differences and numbers of turns into the rearranged equation with consistent units.
- Calculates the unknown potential difference or number of turns and gives the answer with an appropriate unit or as a whole number of turns.
- Checks the answer against the transformer type, confirming a step-up transformer gives a larger secondary potential difference and a step-down transformer a smaller one.
Common mistakes and how to put them right
Substitutes values into the ratio the wrong way round and gets a reciprocal answer.
Put it right by writing Vp/Vs = Np/Ns and matching each value to its coil.
Mixes units, for example entering a potential difference in millivolts beside one in volts.
Put it right by converting all potential differences to the same unit first.
Reports a fractional number of turns.
Put it right by rounding to a whole number of turns and checking the value is sensible.
Exam tips
- Write down Vp, Vs, Np and Ns and mark which one is unknown before rearranging the equation.
- Convert all potential differences to the same unit, usually volts, before substituting.
- Sanity-check the result: a step-up transformer must give Vs greater than Vp, and a step-down transformer Vs less than Vp.
Try the lesson
The start of the Uses of magnetism: apply the equations linking the potential differences and numbers of turns in the two coils of a transformer (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 step-up transformer has 400 turns on its primary coil and 1200 turns on its secondary coil. The primary potential difference is 230 V. Calculate the secondary potential difference.
Listen
The linking equation for an ideal transformer is Vp/Vs = Np/Ns, where Vp and Vs are the potential differences across the primary and secondary coils and Np and Ns are their numbers of turns. Rearranged, Vs = Vp × Ns/Np and Ns = Np × Vs/Vp. To apply it, list the three known quantities, rearrange for the unknown, substitute values with consistent units (potential differences in volts, turns as plain counts) and calculate. For example, with Vp = 12 V, Np = 100 and Ns = 400, Vs = 12 × 400/100 = 48 V. Check that a step-up result is larger than Vp and a step-down result is smaller. A strong answer earns marks for these points. Selects the equation Vp/Vs = Np/Ns and rearranges it correctly for the unknown quantity before substituting values. Substitutes the known potential differences and numbers of turns into the rearranged equation with consistent units. Calculates the unknown potential difference or number of turns and gives the answer with an appropriate unit or as a whole number of turns. Checks the answer against the transformer type, confirming a step-up transformer gives a larger secondary potential difference and a step-down transformer a smaller one. A common slip: Substitutes values into the ratio the wrong way round and gets a reciprocal answer. Instead, writing Vp/Vs = Np/Ns and matching each value to its coil. Exam tip: Write down Vp, Vs, Np and Ns and mark which one is unknown before rearranging the equation.
Key words
Say what each word means before you turn the card.
Applying the transformer equation
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Rearranging equations
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Unit consistency in calculations
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Step-up and step-down transformers
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See every definition
- Applying the transformer equation
- List the three known quantities, rearrange for the unknown, substitute values with consistent units and calculate.
- Rearranging equations
- The equation Vp/Vs = Np/Ns can be rearranged, for example as Vs = Vp × Ns/Np or Ns = Np × Vs/Vp.
- Unit consistency in calculations
- Potential differences are given in volts and turns as plain counts when values are substituted.
- Step-up and step-down transformers
- A step-up result is larger than Vp and a step-down result is smaller.
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