Resistors: Thermistors and LDRs Simplified Revision Notes for GCSE AQA Physics

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2.1.7 Resistors: Thermistors and LDRs

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Certain types of resistors are designed to respond differently to changes in current and potential difference.

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A thermistor is designed so that its resistance decreases as the temperature increases.

An LDR (light-dependent resistor) is designed so that its resistance decreases as the light intensity on the LDR increases.

These components can be used in sensing circuits. In such circuits, a thermistor or LDR is used to vary the resistance across another component, allowing the current through the component to increase under specific conditions. For instance, a thermistor might be used in a sensing circuit to switch on a heater only when it is cold or to activate a fan when it gets hot.

A different example of a sensing circuit is shown on the left. This circuit is designed so that when the room gets dark, the bulb switches on.

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When the room is dark, the resistance of the LDR is high. As a result, both the LDR and the bulb will have a high potential difference. This means the bulb receives more energy and thus shines brighter in darker conditions. When there is a bright light in the room, the LDR has a very low resistance. The bulb will also experience a low potential difference and will therefore be dimmer due to the lower potential difference.

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