4) Circuit Devices

What is an LDR?

A Light Dependent Resistor, whose resistance changes with light intensity.

How does the resistance of an LDR vary with light?

It decreases in bright light and increases in darkness.

Give an example use of an LDR.

In automatic night lights or outdoor lighting.

What is a thermistor?

A temperature-dependent resistor.

How does a thermistor’s resistance change with temperature?

It decreases in hot conditions and increases in cool conditions.

Where might a thermistor be used?

In car engine temperature sensors or thermostats.

How does a sensing circuit work with an LDR or thermistor?

It adjusts the pd or power to components based on light or temperature changes.

Describe a circuit that uses a thermistor to control a fan.

As the temperature rises, the thermistor’s resistance drops, increasing pd across the fan, making it speed up.

How can an LDR control a bulb’s brightness?

In darkness (high resistance in LDR), pd across the bulb increases, making it brighter.

Why does pd across a component increase when its resistance is high in a sensing circuit?

Higher resistance means it takes a larger share of the total pd.