AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    The resistance of components such as lamps, diodes, thermistors and LDRs…

    Full specification wording

    Resistors: The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.

    Several common components have a resistance that shifts as conditions change.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 4 of 13
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Several common components have a resistance that shifts as conditions change. A filament lamp resists more as the current through it heats the filament. A diode resists almost nothing in the forward direction and hugely in the reverse. A thermistor resists less when its surroundings are hotter, and a light dependent resistor resists less when the light on it is brighter. Their graphs are therefore curved, and you compare two such components at a chosen potential difference. At any given potential difference, the component carrying the smaller current has the larger resistance and its line has the smaller gradient; the one carrying the larger current transfers more energy each second, so a lamp with the larger current is the brighter one.

    What earns the marks

    • Identify which component carries the smaller current at the same potential difference when comparing graphs.
    • Conclude that the component with the smaller current has the greater resistance, using the relationship R = V / I.
    • Refer to the lower gradient of the line on the current-potential difference graph to indicate higher resistance.
    • Link a larger current at the same potential difference to a greater power output and therefore a brighter lamp.

    Common mistakes and how to put them right

    • Comparing two curved characteristics at different potential differences, which makes the comparison meaningless.

      Put it right: Compare at a fixed potential difference.

    • Claiming the steeper line has the greater resistance.

      Put it right: The steeper line has the greater current for a given pd and so the smaller resistance.

    • Stating a lamp is brighter because it has more resistance.

      Put it right: Brightness should be linked to higher current and greater power output.

    • Treating the resistance of a filament lamp as fixed and quoting one value for the whole graph.

      Put it right: Resistance increases as current increases.

    Exam tips

    • Draw a vertical line on the graph at one potential difference and compare the currents where it cuts each curve.
    • Answers phrased in terms of R = V / I are accepted, so quote the equation when comparing resistance.

    Try the lesson

    The start of the Resistors: The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which statement correctly describes the resistance of a filament lamp as the current through it changes?

    Listen

    Several common components have a resistance that shifts as conditions change. A filament lamp resists more as the current through it heats the filament. A diode resists almost nothing in the forward direction and hugely in the reverse. A thermistor resists less when its surroundings are hotter, and a light dependent resistor resists less when the light on it is brighter. Their graphs are therefore curved, and you compare two such components at a chosen potential difference. At any given potential difference, the component carrying the smaller current has the larger resistance and its line has the smaller gradient; the one carrying the larger current transfers more energy each second, so a lamp with the larger current is the brighter one. A strong answer earns marks for these points. Identify which component carries the smaller current at the same potential difference when comparing graphs. Conclude that the component with the smaller current has the greater resistance, using the relationship R = V / I. Refer to the lower gradient of the line on the current-potential difference graph to indicate higher resistance. Link a larger current at the same potential difference to a greater power output and therefore a brighter lamp. Exam tip: Draw a vertical line on the graph at one potential difference and compare the currents where it cuts each curve.

    See it

    A top-down flowchart. The top node says compare two components at the same potential difference, and it branches into two nodes: the component with the smaller current, and the component with the larger current. From the smaller-current node, two arrows lead to a node saying greater resistance, using R = V / I, and to a node saying the line has the smaller gradient. From the larger-current node, an arrow leads to a node saying it transfers more energy each second, which then leads to a final node saying the lamp with the larger current is the brighter one.
    Shows how comparing two components at the same potential difference links the smaller current to greater resistance and the larger current to a brighter lamp.

    Key words

    Say what each word means before you turn the card.

    non-ohmic components

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

    curved characteristics

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

    comparing at fixed pd

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

    gradient and resistance

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

    See every definition
    non-ohmic components
    Components such as lamps, diodes, thermistors and LDRs whose resistance is not constant but changes with the current through the component.
    curved characteristics
    The graphs of such components are curved rather than straight because their resistance changes as conditions change.
    comparing at fixed pd
    Two such components are compared at a chosen potential difference, since comparing them at different potential differences makes the comparison meaningless.
    gradient and resistance
    At any given potential difference the component carrying the smaller current has the larger resistance and its line has the smaller gradient.

    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 says compare two components at the same potential difference, and it branches into two nodes: the component with the smaller current, and the component with the larger current. From the smaller-current node, two arrows lead to a node saying greater resistance, using R = V / I, and to a node saying the line has the smaller gradient. From the larger-current node, an arrow leads to a node saying it transfers more energy each second, which then leads to a final node saying the lamp with the larger current is the brighter one.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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