OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    (b) potential divider circuits with variable components e.g. LDR and…

    Full specification wording

    Potential dividers: (b) potential divider circuits with variable components e.g. LDR and thermistor

    A potential divider can include a variable component so that the output p.d. changes with a physical condition.

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

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A potential divider can include a variable component so that the output p.d. changes with a physical condition. A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases. A thermistor has a resistance that decreases as temperature increases (for an NTC thermistor). If the variable component is in series with a fixed resistor across a supply, the output p.d. across either component changes as the condition changes. For example, with an LDR in the upper position and a fixed resistor below, increasing light intensity lowers the LDR resistance, so the output p.d. across the fixed resistor rises. Such circuits are used in sensing and switching applications.

    What earns the marks

    • States that an LDR resistance decreases as light intensity increases.
    • States that an NTC thermistor resistance decreases as temperature increases.
    • Explains that changing the resistance of one component changes the p.d. across both components in the divider.
    • Applies V_out = V_in × R₂/(R₁ + R₂) with the variable resistance substituted appropriately.
    • Explains a sensing application, such as a light- or temperature-sensitive output p.d.

    Common mistakes and how to put them right

    • Stating that LDR resistance increases with light intensity.

      Put it right: LDR resistance decreases as light intensity increases.

    • Stating that an NTC thermistor resistance increases with temperature.

      Put it right: Its resistance decreases as temperature increases.

    • Assuming the output p.d. stays constant when one resistance changes.

      Put it right: The p.d.s redistribute as the resistance ratio changes.

    • Placing the variable component in parallel with the supply.

      Put it right: It must be in series with the other divider component for the ratio to operate.

    Exam tips

    • State the direction of resistance change for the sensor before working out the effect on output p.d.
    • Use the ratio with the resistance across which the output is measured in the numerator.
    • Check the answer against the limiting cases of very high and very low sensor resistance.

    Try the lesson

    The start of the Potential dividers: (b) potential divider circuits with variable components e.g. LDR and thermistor lesson, made from checked teaching notes for this specification point.

    Listen

    A potential divider can include a variable component so that the output p.d. changes with a physical condition. A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases. A thermistor has a resistance that decreases as temperature increases (for an NTC thermistor). If the variable component is in series with a fixed resistor across a supply, the output p.d. across either component changes as the condition changes. For example, with an LDR in the upper position and a fixed resistor below, increasing light intensity lowers the LDR resistance, so the output p.d. across the fixed resistor rises. Such circuits are used in sensing and switching applications. A strong answer earns marks for these points. States that an LDR resistance decreases as light intensity increases. States that an NTC thermistor resistance decreases as temperature increases. Explains that changing the resistance of one component changes the p.d. across both components in the divider. Applies V_out = V_in × R₂/(R₁ + R₂) with the variable resistance substituted appropriately. A common slip: Stating that LDR resistance increases with light intensity. Instead, LDR resistance decreases as light intensity increases. Exam tip: State the direction of resistance change for the sensor before working out the effect on output p.d.

    See it

    A top-down flowchart. The first node states the supply p.d. across the divider, which leads to a node describing a variable component in series with a fixed resistor. This branches into two nodes: one saying an LDR's resistance decreases as light intensity increases, and one saying an NTC thermistor's resistance decreases as temperature increases. Both lead to a node saying the resistance ratio changes, which leads to a node saying the p.d.s redistribute across both components, then to a node saying the output p.d. across the fixed resistor changes, and finally to a node saying sensing and switching applications.
    Shows how a variable component (LDR or thermistor) in a potential divider changes the output p.d. as the physical condition changes, leading to sensing and switching uses.

    Key words

    Say what each word means before you turn the card.

    LDR

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

    Thermistor

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

    Potential divider

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

    Sensing and switching applications

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

    See every definition
    LDR
    A light-dependent resistor has a resistance that decreases as light intensity increases.
    Thermistor
    An NTC thermistor has a resistance that decreases as temperature increases.
    Potential divider
    A circuit that can include a variable component so that the output p.d. changes with a physical condition.
    Sensing and switching applications
    Circuits in which a potential divider with a variable component is used, for example a light- or temperature-sensitive output p.d.

    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 node states the supply p.d. across the divider, which leads to a node describing a variable component in series with a fixed resistor. This branches into two nodes: one saying an LDR's resistance decreases as light intensity increases, and one saying an NTC thermistor's resistance decreases as temperature increases. Both lead to a node saying the resistance ratio changes, which leads to a node saying the p.d.s redistribute across both components, then to a node saying the output p.d. across the fixed resistor changes, and finally to a node saying sensing and switching applications.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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