OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    Resistance: (b) Ohm’s law

    Ohm’s law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions such as temperature remain constant.

    Checked against the OCR H556 specification (opens in a new tab) on .

    • Point 2 of 5
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Ohm’s law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions such as temperature remain constant. In symbols, I ∝ V, so V/I is constant. A resistor that obeys Ohm’s law is called an ohmic conductor; its current–voltage graph is a straight line through the origin. For example, if a 4.0 Ω resistor has 2.0 V across it, the current is I = V/R = 2.0 V ÷ 4.0 Ω = 0.50 A; doubling the voltage to 4.0 V doubles the current to 1.0 A. A filament lamp does not obey Ohm’s law because its temperature rises, increasing its resistance, so its I–V graph curves. Ohm’s law is a special case, not a universal rule for all components.

    What earns the marks

    • Ohm’s law states that current is directly proportional to potential difference for a metallic conductor at constant temperature.
    • For an ohmic conductor, V/I is constant and the I–V graph is a straight line through the origin.
    • Ohm’s law applies only when physical conditions, especially temperature, remain constant.
    • Components such as filament lamps and diodes are non-ohmic because their resistance changes with current or voltage.
    • The law can be used to calculate current, potential difference or resistance for an ohmic conductor.

    Common mistakes and how to put them right

    • Stating that all conductors obey Ohm’s law.

      Put it right: Only ohmic conductors at constant temperature obey it; filament lamps and diodes do not.

    • Saying that V is proportional to I without specifying constant temperature.

      Put it right: Include the condition of constant physical conditions, especially temperature.

    • Treating a curved I–V graph as ohmic.

      Put it right: A straight line through the origin is required for ohmic behaviour.

    • Confusing Ohm’s law with the definition of resistance.

      Put it right: R = V/I defines resistance for any component, while Ohm’s law is the statement that V/I is constant for an ohmic conductor.

    Exam tips

    • Quote Ohm’s law with the condition of constant temperature to gain full credit.
    • When interpreting an I–V graph, check whether it is a straight line through the origin before calling the component ohmic.
    • Use the gradient of a V–I graph to find resistance only if the graph is linear through the origin.
    • For non-ohmic components, describe how resistance changes rather than saying the component has no resistance.

    Try the lesson

    The start of the Resistance: (b) Ohm’s law lesson, made from checked teaching notes for this specification point.

    Listen

    Ohm’s law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions such as temperature remain constant. In symbols, I ∝ V, so V/I is constant. A resistor that obeys Ohm’s law is called an ohmic conductor; its current–voltage graph is a straight line through the origin. For example, if a 4.0 Ω resistor has 2.0 V across it, the current is I = V/R = 2.0 V ÷ 4.0 Ω = 0.50 A; doubling the voltage to 4.0 V doubles the current to 1.0 A. A filament lamp does not obey Ohm’s law because its temperature rises, increasing its resistance, so its I–V graph curves. Ohm’s law is a special case, not a universal rule for all components. A strong answer earns marks for these points. Ohm’s law states that current is directly proportional to potential difference for a metallic conductor at constant temperature. For an ohmic conductor, V/I is constant and the I–V graph is a straight line through the origin. Ohm’s law applies only when physical conditions, especially temperature, remain constant. Components such as filament lamps and diodes are non-ohmic because their resistance changes with current or voltage. A common slip: Stating that all conductors obey Ohm’s law. Instead, only ohmic conductors at constant temperature obey it; filament lamps and diodes do not. Exam tip: Quote Ohm’s law with the condition of constant temperature to gain full credit.

    See it

    A top-down flowchart. The top node reads "Ohm's law: I proportional to V". From it, one branch goes to "Condition: metallic conductor", then to "Condition: temperature constant", then to "V/I is constant", then to "Ohmic conductor", and finally to "I–V graph: straight line through origin". A second branch from the top node goes to "Filament lamp: temperature rises", then to "Resistance increases", and finally to "Non-ohmic: I–V graph curves".
    It shows Ohm's law and its constant-temperature condition leading to an ohmic conductor with a straight-line I–V graph, alongside the filament lamp whose rising temperature makes it non-ohmic.

    Key words

    Say what each word means before you turn the card.

    Ohm’s law

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

    Ohmic conductor

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

    Current–voltage characteristics

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

    See every definition
    Ohm’s law
    The current through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions such as temperature remain constant.
    Ohmic conductor
    A resistor that obeys Ohm’s law, so its current–voltage graph is a straight line through the origin.
    Current–voltage characteristics
    The current–voltage graph of a component, which is a straight line through the origin for an ohmic conductor but curves for a filament lamp.

    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 top node reads "Ohm's law: I proportional to V". From it, one branch goes to "Condition: metallic conductor", then to "Condition: temperature constant", then to "V/I is constant", then to "Ohmic conductor", and finally to "I–V graph: straight line through origin". A second branch from the top node goes to "Filament lamp: temperature rises", then to "Resistance increases", and finally to "Non-ohmic: I–V graph curves".

    • 1Box 1

    • 2Box 2

    • 3Box 3

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