OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Apply: potential difference across primary coil (V) × current in primary…

    Full specification wording

    Powering Earth: apply: potential difference across primary coil (V) × current in primary coil (A) = potential difference across secondary coil (V) × current in secondary coil (A)

    This criterion asks you to apply the transformer power relationship: Vp × Ip = Vs × Is, where V is potential difference in volts and I is current in amperes.

    Checked against the OCR J249 specification (opens in a new tab) on .

    • Point 1 of 9
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This criterion asks you to apply the transformer power relationship: Vp × Ip = Vs × Is, where V is potential difference in volts and I is current in amperes. The equation expresses that, for an ideal transformer, the power input to the primary coil equals the power output from the secondary coil. To use it, identify the three known quantities and rearrange for the unknown. For example, if the primary is at 230 V and 0.5 A and the secondary potential difference is 23 V, then 230 × 0.5 = 115 W, so Is = 115 ÷ 23 = 5 A. A step-down transformer lowers potential difference and raises current in the same ratio. Keep units as V and A so the product is in watts, and check that your answer is sensible: a lower secondary potential difference should give a higher secondary current.

    What earns the marks

    • States the relationship Vp × Ip = Vs × Is with potential difference in volts and current in amperes.
    • Substitutes the three known values correctly into the equation with consistent units.
    • Rearranges the equation to isolate the unknown quantity before calculating.
    • Calculates the unknown current or potential difference and gives the answer with the correct unit.
    • Checks that the result is consistent with a step-up or step-down transformer.

    Common mistakes and how to put them right

    • Mixes up primary and secondary values.

      Put it right: Label each quantity as primary or secondary before substituting.

    • Adds the currents or potential differences instead of multiplying.

      Put it right: The relationship uses products, not sums.

    • Gives the answer without a unit.

      Put it right: State the current in A or the potential difference in V.

    Exam tips

    • Write the equation first, then substitute, so method marks are visible.
    • Convert any values given in mA or kV to A or V before calculating.
    • Check the direction: a step-down transformer should show a larger secondary current.

    Try the lesson

    The start of the Powering Earth: apply: potential difference across primary coil (V) × current in primary coil (A) = potential difference across secondary coil (V) × current in secondary coil (A) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A transformer has 6 V and 3 A in its primary coil. The secondary coil has a current of 9 A. What is the potential difference across the secondary coil?

    Listen

    This criterion asks you to apply the transformer power relationship: Vp × Ip = Vs × Is, where V is potential difference in volts and I is current in amperes. The equation expresses that, for an ideal transformer, the power input to the primary coil equals the power output from the secondary coil. To use it, identify the three known quantities and rearrange for the unknown. For example, if the primary is at 230 V and 0.5 A and the secondary potential difference is 23 V, then 230 × 0.5 = 115 W, so Is = 115 ÷ 23 = 5 A. A step-down transformer lowers potential difference and raises current in the same ratio. Keep units as V and A so the product is in watts, and check that your answer is sensible: a lower secondary potential difference should give a higher secondary current. A strong answer earns marks for these points. States the relationship Vp × Ip = Vs × Is with potential difference in volts and current in amperes. Substitutes the three known values correctly into the equation with consistent units. Rearranges the equation to isolate the unknown quantity before calculating. Calculates the unknown current or potential difference and gives the answer with the correct unit. Exam tip: Write the equation first, then substitute, so method marks are visible.

    See it

    A top-to-bottom flowchart with seven steps. The first step is to write the relationship Vp × Ip = Vs × Is. The second is to label each quantity as primary or secondary. The third is to convert any values in mA or kV to A or V. The fourth is to substitute the three known values. The fifth is to rearrange to isolate the unknown. The sixth is to calculate the unknown and give the unit, in A or V. The seventh is to check that a step-down transformer gives a larger secondary current.
    The sequence of steps for applying the transformer relationship Vp × Ip = Vs × Is to find an unknown current or potential difference.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Check: step-down gives a larger secondary current
    2. 2. Calculate the unknown and give the unit (A or V)
    3. 3. Convert any values in mA or kV to A or V
    4. 4. Rearrange to isolate the unknown
    5. 5. Write the relationship: Vp × Ip = Vs × Is
    6. 6. Label each quantity as primary or secondary
    7. 7. Substitute the three known values

    Key words

    Say what each word means before you turn the card.

    Transformer power relationship

    Try answering aloud or in your head before turning the card.

    Potential difference and current in coils

    Try answering aloud or in your head before turning the card.

    Rearranging equations

    Try answering aloud or in your head before turning the card.

    Step-up and step-down transformers

    Try answering aloud or in your head before turning the card.

    See every definition
    Transformer power relationship
    Potential difference across primary coil multiplied by current in primary coil equals potential difference across secondary coil multiplied by current in secondary coil.
    Potential difference and current in coils
    For an ideal transformer, the power input to the primary coil equals the power output from the secondary coil.
    Rearranging equations
    Identify the three known quantities and rearrange for the unknown before calculating.
    Step-up and step-down transformers
    A step-down transformer lowers potential difference and raises current in the same ratio.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with seven steps. The first step is to write the relationship Vp × Ip = Vs × Is. The second is to label each quantity as primary or secondary. The third is to convert any values in mA or kV to A or V. The fourth is to substitute the three known values. The fifth is to rearrange to isolate the unknown. The sixth is to calculate the unknown and give the unit, in A or V. The seventh is to check that a step-down transformer gives a larger secondary current.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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