Potential Dividers (Cambridge (CIE) O Level Physics): Revision Note

Input Sensors

Thermistors

• A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature

• Most thermistors are negative temperature coefficient (NTC) components.

  • This means that if the temperature increases, the resistance of the thermistor decreases (and vice versa)

Resistance-Temperature Graph for a NTC Thermistor

A graph of temperature against resistance for a thermistor shows that as temperature increases, resistance decreases

• 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

Relationship Between Resistance and Temperature for a Thermistor

The resistance through a thermistor is dependent on the temperature of it

Light-Dependent Resistors

• A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor

• Its resistance automatically changes depending on the light energy falling onto it (illumination)

• As the light intensity increases, the resistance of an LDR decreases

Resistance-Temperature Graph for a Light-Dependent Resistor

A graph of resistance against light intensity for an LDR shows that as light intensity increases, resistance decreases

• 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)

Resistance of an LDR depends on the light intensity falling on it

Variable Potential Dividers

• When two resistors are connected in series, the potential difference across the power source is shared between them

Potential Divider

A potential divider splits the potential difference of a power source between two components

• The potential difference across each resistor depends upon its resistance:

  • The resistor with the largest resistance will have a greater potential difference than the other one

  • If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share

• A potentiometer is a single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it

 Potentiometer

A potentiometer is a kind of variable resistor

• The sliding contact has the effect of separating the potentiometer into two parts – an upper part and a lower part – both of which have different resistances

Circuit Diagram Using a Potentiometer

Moving the slider (the arrow in the diagram) changes the resistances (and hence potential differences) of the upper and lower parts of the potentiometer

• If the slider in the above diagram is moved upwards, the resistance of the lower part will increase and so the potential difference across it will also increase

Resistors as Potential Dividers

• When two resistors are connected in series, through Kirchhoff’s Second Law, the potential difference across the power source is divided between them

• Potential dividers are circuits which produce an output voltage as a fraction of its input voltage

• Potential dividers have two main purposes:

  • To provide a variable potential difference

  • To enable a specific potential difference to be chosen

  • To split the potential difference of a power source between two or more components

• Potential dividers are used widely in volume controls and sensory circuits using LDRs and thermistors

• Potential divider circuits are based on the ratio of voltage between components. This is equal to the ratio of the resistances of the resistors in the diagram below, giving the following equation:

Diagram Illustrating the Potential Divider Equation

Potential divider diagram and equation

• The input voltage V_in is applied to the top and bottom of the series resistors

• The output voltage V_out is measured from the centre to the bottom of resistor R₂

• The potential difference V across each resistor depends upon its resistance R:

  • The resistor with the largest resistance will have a greater potential difference than the other one from V = IR

  • If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share

• In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR