Potential Dividers (Edexcel International A Level Physics)

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Potential Dividers

  • The electrical voltages rule is defined as:

The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop

  • Therefore, when two resistors are connected in series,  the potential difference across the power source will be divided across the two resistors

  • Potential dividers are circuits that produce an output voltage as a fraction of the input voltage
  • This is done by using two resistors in series to split or divide the voltage of the supply in a chosen ratio
  • Potential dividers have three 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

  • The link between the input voltage and the output voltage across each resistor is linked in an equation

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

Potential divider diagram and equation

  • The input voltage Vin is applied across both resistors, which are in series
  • The output voltage Vout is measured across one of the resistors, in this case resistor R2
  • The potential difference V across each resistor depends upon its resistance R:
    • The resistor with the largest resistance will have the greater potential difference across it
    • This is shown as a greater Vout
    • This is 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

  • Since potential divider circuits are based on the ratio of voltage between components, and since V=IR, this is equal to the ratio of the resistances of the resistors
  • Therefore, the ratios of the potential differences and resistances across each resistor can be linked

  • Where:
    • V1 = potential difference of R1 (V)
    • V2 = potential difference or R2 (V)
  • Using Ohm's Law, with a constant current, I, these can also be written as:
    • V1 = IR1
    • V2 = IR2

Worked example

The circuit shown is designed to light up a lamp when the input voltage exceeds a preset value.

WE - potential divider question image, downloadable AS & A Level Physics revision notes

Vout is equal to 5.3 V when the lamp lights.

Calculate the input voltage Vin.

WE - potential divider answer image, downloadable AS & A Level Physics revision notes

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