Four Factors Affecting Resistance

Some old-fashioned electrical components

Resistance is a property of a DEVICE or a COMPONENT. like a lamp, a resistor, a thermistor, a diode and so on. Its value depends on four things.

1. What it’s made of. Metals are good conductors so have lower resistance than an insulator of the same dimensions.

2. Length – the longer, the greater resistance.

3. Area of cross section – the larger the lower.

4. Temperature (metals: higher temperature = higher resistance because the vibration of the metal lattice impedes the drift of the electrons, semiconductors:  like thermistors , resistance decreases with temperature)

Variable resistors or potentiometers control the length of wire we introduce into a circuit  – the volume control on a hi-fi

Thermistors have a high resistance in the cold and a low resistance in the warm. They are used in logic circuits to turn heaters on and off.

LDR’s (light dependent resistors have a high resistance in the dark but a low resistance in the light. As darkness falls, they are used in logic circuits to turn on street lights.

A diode is an electrical one way street, which is useful sometimes in electronic circuits. Current is allowed through a low resistance pathway in one direction, the resistance is very high in the reverse direction so no current flows.

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