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Light-Dependent Resistors

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A series of free GCSE/IGCSE Physics Notes and Lessons.

Light-Dependent Resistors

In this lesson, we will
• Recognise the symbol for light-dependent resistor (LDR).
• Describe how the resistance of an LDR changes in light and dark conditions.
• Explain how LDRs can be used in circuits.

How does the mobile phone turn the screen off during conversation?
In the light, the resistance of the LDR is very low. It takes very little energy for the current to pass through the LDR. The potential difference across the LDR is low. Because the potential difference is shared between components in series, the potential difference across the lamp is large. The lamp now lights up the screen.
If the phone is held to a person's ear, the LDR is in darkness. The resistance of the LDR rises sharply. Less electrical energy is available for the lamp. Now it takes a great deal of energy for the current to pass through the LDR so the potential energy across the LDR is very high. The potential difference across the lamp is now very low. The lamp becomes very dim.

Light Dependent Resistors and Potential Dividers - OCR Gateway P6 Q14 - GCSE Physics Revision
In this question, you need to apply your understanding of Light Dependent Resistors (LDRs) to a simple series circuit and to share potential difference between two resistors in a simple ratio.

(a) Dan builds a circuit using an LDR.
The torch is not switched on.
Describe what happens to the resistance of the LDR and the speed of the motor when Dan switches the torch on.

(b) Dan uses a potential divider in a circuit.
The input voltage = 5 V.
Calculate the output voltage

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