Resistors (OCR GCSE Physics A (Gateway))

Constant & Variable Resistance

• Resistors come in two types:

  • Fixed resistors

  • Variable resistors

• Fixed resistors have a resistance that remains constant

• Variable resistors can change the resistance by changing the length of wire that makes up the circuit

  • A longer length of wire has more resistance than a shorter length of wire

• For fixed and variable resistors, once the resistance is set, it will stay at this value no matter how the current changes

• However, the resistance of components such as lamps, diodes (diodes and LEDs), thermistors and LDRs (light dependent resistors) changes with the current through the component

Fixed and variable resistor circuit symbols

Components of Variable Resistance

Thermistors

• A thermistor is a temperature-dependent resistor

• It is represented by the following circuit symbol:

Thermistor circuit symbol

• The resistance of a thermistor changes depending on its temperature

  • As the temperature increases the resistance of a thermistor decreases and vice versa

The resistance through a thermistor is dependent on temperature

• Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers

  • As the thermistor gets hotter, its resistance decreases

  • As the thermistor gets cooler, its resistance increases

A digital thermometer uses a thermistor

LDRs

• A light-dependent resistor (LDR) represented by the following circuit symbol:

LDR circuit symbol

• The resistance of an LDR changes depending on the light intensity on it

  • As the light intensity increases the resistance of an LDR decreases and vice versa

The resistance of an LDR is dependent on the amount of light intensity on it

• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights

  • In the dark, its resistance is very large (millions of ohms)

  • In bright light, its resistance is small (tens of ohms)

LDRs are used for automatic street lights