OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) light-dependent resistor (LDR); variation of resistance with light…
Full specification wording
Resistance: (d) light-dependent resistor (LDR); variation of resistance with light intensity.
A light-dependent resistor, or LDR, is a semiconductor component whose resistance decreases as light intensity increases.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A light-dependent resistor, or LDR, is a semiconductor component whose resistance decreases as light intensity increases. In darkness its resistance is high, often megohms; in bright light it falls to a few hundred ohms or less. The light supplies energy that releases more charge carriers, so the current for a given p.d. rises and resistance falls. You can investigate this by placing an LDR in a circuit with an ammeter and voltmeter, varying the distance from a lamp or using different light intensities, and calculating resistance from V/I. The relationship is non-linear, so a graph of resistance against light intensity is a curve.
What earns the marks
- LDR resistance decreases as light intensity increases.
- In darkness the resistance is high; in bright light it is much lower.
- Light energy releases more charge carriers in the semiconductor, increasing current for a given p.d.
- Resistance is calculated from R = V/I using measured p.d. and current.
- The variation of resistance with light intensity is non-linear.
Common mistakes and how to put them right
Saying LDR resistance increases with light intensity.
Put it right: Resistance decreases as light intensity increases.
Treating the LDR as ohmic with constant resistance.
Put it right: Its resistance changes with light intensity, so it is non-ohmic.
Assuming a directly proportional relationship between resistance and light intensity.
Put it right: The graph is a curve, not a straight line.
Exam tips
- State the direction of change clearly: more light means less resistance.
- Use the equation R = V/I when calculating resistance from measurements.
- Describe the graph as a curve showing a non-linear decrease.
Try the lesson
The start of the Resistance: (d) light-dependent resistor (LDR); variation of resistance with light intensity. lesson, made from checked teaching notes for this specification point.
Listen
A light-dependent resistor, or LDR, is a semiconductor component whose resistance decreases as light intensity increases. In darkness its resistance is high, often megohms; in bright light it falls to a few hundred ohms or less. The light supplies energy that releases more charge carriers, so the current for a given p.d. rises and resistance falls. You can investigate this by placing an LDR in a circuit with an ammeter and voltmeter, varying the distance from a lamp or using different light intensities, and calculating resistance from V/I. The relationship is non-linear, so a graph of resistance against light intensity is a curve. A strong answer earns marks for these points. LDR resistance decreases as light intensity increases. In darkness the resistance is high; in bright light it is much lower. Light energy releases more charge carriers in the semiconductor, increasing current for a given p.d. Resistance is calculated from R = V/I using measured p.d. and current. A common slip: Saying LDR resistance increases with light intensity. Instead, resistance decreases as light intensity increases. Exam tip: State the direction of change clearly: more light means less resistance.
See it
Key words
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Light intensity as a variable
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Non-ohmic behaviour
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Practical measurement of resistance
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See every definition
- Light intensity as a variable
- It can be varied by changing the distance from a lamp or by using different light intensities when investigating an LDR.
- Non-ohmic behaviour
- The resistance changes with light intensity, so the LDR is not ohmic with constant resistance.
- Practical measurement of resistance
- Place an LDR in a circuit with an ammeter and voltmeter and calculate resistance from V/I.
Label the diagram
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