CIE A Level Physics (9702) 2019-2021

Revision Notes

12.2.1 Potential Dividers

Potential Divider Circuit

  • When two resistors are connected in series, through Kirchoff’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. This splits the potential difference of a power source between two components
  • They 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:

Potential divider diagram and equation, downloadable AS & A Level Physics revision notes

Potential divider diagram and equation

  • The input voltage Vin is applied to the top and bottom of the series resistors
  • The output voltage Vout 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

Worked example

Worked example - potential divider, downloadable AS & A Level Physics revision notes

Exam Tip

Always make sure the correct resistance is in the numerator of the potential divider equation. This will be the resistance of the component you want to found the output voltage of.

The Potentiometer

  • A potentiometer is similar to a variable resistor connected as a potential divider to give a continuously variable output voltage
  • It can be used as a means of comparing potential differences in different parts of the circuit
  • The circuit symbol is recognised by an arrow next to the resistor

Potentiometer symbol, downloadable AS & A Level Physics revision notes

Potentiometer circuit diagram

 

  • 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 diagram, downloadable AS & A Level Physics revision notes

A potentiometer is a type of variable resistor

 

  • It is recognised on a circuit diagram with a resistor fitted with a sliding contact
  • 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

Potentiometer circuit diagram, downloadable AS & A Level Physics revision notes

Moving the slider (the arrow in the diagram) changes the resistance (and hence potential difference) 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
  • Therefore, the variable resistor obtains a maximum or minimum value for the output voltage
  • If the resistance is 3 Ω:
    • Maximum voltage is when the resistance is 3 Ω
    • Minimum voltage is when the resistance is 0 Ω

Worked example

Worked example - potentiometer (1), downloadable AS & A Level Physics revision notes
Worked example - potentiometer (2), downloadable AS & A Level Physics revision notes

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