OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Recall and apply: potential difference (V) = current (A) × resistance (Ω)

    Full specification wording

    Simple circuits: recall and apply: potential difference (V) = current (A) × resistance (Ω)

    For a component, potential difference, current and resistance are linked by V = I × R, where V is in volts (V), I in amperes (A) and R in ohms (Ω).

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

    • Point 1 of 14
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    For a component, potential difference, current and resistance are linked by V = I × R, where V is in volts (V), I in amperes (A) and R in ohms (Ω). Potential difference is the energy transferred per unit charge, current is the rate of charge flow and resistance opposes that flow. To apply the relationship, identify the two known values, keep units as volts, amperes and ohms, then multiply current by resistance to find potential difference, or rearrange to I = V ÷ R or R = V ÷ I. For example, a current of 0.5 A through a 12 Ω resistor gives V = 0.5 × 12 = 6 V. In a multiple-choice question, calculate first, then match your value and unit to an option, checking whether the question asks for V, I or R.

    What earns the marks

    • Selects the option that applies V = I × R with current in amperes and resistance in ohms.
    • Rearranges the relationship correctly to find current as potential difference divided by resistance.
    • Rearranges the relationship correctly to find resistance as potential difference divided by current.
    • Matches the calculated value to the correct unit of volts, amperes or ohms as requested.

    Common mistakes and how to put them right

    • Adds current and resistance instead of multiplying them.

      Put it right: Multiply the two values to find potential difference.

    • Divides resistance by current when finding potential difference.

      Put it right: Multiply current by resistance instead.

    • Reports resistance in volts.

      Put it right: Give resistance in ohms and potential difference in volts.

    Exam tips

    • Write V = I × R and substitute the given values before checking the options.
    • Decide which quantity the question asks for, then rearrange before calculating.
    • Check that the unit of your answer matches the quantity requested.

    Try the lesson

    The start of the Simple circuits: recall and apply: potential difference (V) = current (A) × resistance (Ω) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A wire carries a current of 2 A and has a resistance of 5 Ω. What is the potential difference across the wire?

    Listen

    For a component, potential difference, current and resistance are linked by V = I × R, where V is in volts (V), I in amperes (A) and R in ohms (Ω). Potential difference is the energy transferred per unit charge, current is the rate of charge flow and resistance opposes that flow. To apply the relationship, identify the two known values, keep units as volts, amperes and ohms, then multiply current by resistance to find potential difference, or rearrange to I = V ÷ R or R = V ÷ I. For example, a current of 0.5 A through a 12 Ω resistor gives V = 0.5 × 12 = 6 V. In a multiple-choice question, calculate first, then match your value and unit to an option, checking whether the question asks for V, I or R. A strong answer earns marks for these points. Selects the option that applies V = I × R with current in amperes and resistance in ohms. Rearranges the relationship correctly to find current as potential difference divided by resistance. Rearranges the relationship correctly to find resistance as potential difference divided by current. Matches the calculated value to the correct unit of volts, amperes or ohms as requested. Exam tip: Write V = I × R and substitute the given values before checking the options.

    See it

    A top-down flowchart. The first box says identify the two known values, leading to a box saying keep units as volts, amperes and ohms, which leads to a decision box asking which quantity is asked for. Three branches leave the decision: potential difference leads to V = I × R, current leads to I = V ÷ R, and resistance leads to R = V ÷ I. All three of these boxes lead to a final box saying match value and unit to the option.
    Shows the steps for applying V = I × R: identify the known values, keep the correct units, choose the right rearrangement for the quantity asked for, then match the value and unit to an option.

    Key words

    Say what each word means before you turn the card.

    Potential difference

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

    Current

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

    Resistance

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

    Using and rearranging equations

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

    See every definition
    Potential difference
    The energy transferred per unit charge, measured in volts (V).
    Current
    The rate of charge flow, measured in amperes (A).
    Resistance
    Opposes the flow of charge and is measured in ohms (Ω).
    Using and rearranging equations
    Identify the two known values, keep units as volts, amperes and ohms, then multiply current by resistance to find potential difference, or rearrange to find current or resistance.

    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-down flowchart. The first box says identify the two known values, leading to a box saying keep units as volts, amperes and ohms, which leads to a decision box asking which quantity is asked for. Three branches leave the decision: potential difference leads to V = I × R, current leads to I = V ÷ R, and resistance leads to R = V ÷ I. All three of these boxes lead to a final box saying match value and unit to the option.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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