4PH1

Components in Series & Parallel Circuits

Electricity

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable
High
Stable12%

Calculating resistance, current and voltage in series and parallel circuits tested every series.

Voltage in series

  • The total voltage of the supply is shared between the components in a series loop:

V_supply = V₁ + V₂ + V₃ + …

  • The share that each component takes is set by its resistance:
    • Two identical components in series each take half the supply voltage
    • A component with higher resistance takes the larger share of the supply voltage
    • A component with lower resistance takes the smaller share
  • This is why a high-resistance bulb in series with a low-resistance wire glows brightly while the wire stays cool: the bulb is dissipating most of the supply's energy because most of the voltage is dropped across it
Series circuit with a 12 V supply driving two identical components on the single loop; the same current I flows all the way round and the 12 V is shared as 6 V across each component
Source: Voltage in Series & Parallel by Save My Exams

Voltage on parallel branches

  • Every parallel branch sits between the same two junction points; the voltage across one branch therefore matches the voltage across any other branch, and both equal the supply voltage:

V_supply = V_branch1 = V_branch2 = …

  • This is because the two ends of every branch are joined to the same two points (the two junctions), and the potential difference between two fixed points is one number whatever route you take between them
  • A consequence: any single branch in a parallel circuit will run at its normal "rated" voltage if it is rated to match the supply; this is why household appliances are wired in parallel
Parallel circuit with a 12 V supply; the supply current I₁ + I₂ splits between two branches, and each branch sits across the full 12 V of the supply
Source: Voltage in Series & Parallel by Save My Exams

Advantages and disadvantages of each wiring

FeatureSeries circuitParallel circuit
Controlling all components togetherYes, one switch acts on every componentPossible, but each branch usually has its own switch as well
Controlling components separatelyNo, they share one current pathYes, each branch has its own switch
Effect of one broken componentAll others stop workingOnly that branch stops; the others carry on
Wiring complexitySimple, fewer wires neededMore complex, more wires and junctions
Voltage across each componentShares the supply voltage; depends on resistanceEach branch gets the full supply voltage
Exam tip

Voltage across parallel branches

What comes up: explain why parallel components each get the full supply voltage, or state the voltage across one branch.

Watch out: each parallel branch gets the full supply voltage — do not "share" it between branches the way voltage is shared in a series circuit (sharing is the series rule).