The current and voltage (potential difference) rules for series and parallel circuits, including the conservation of current at a junction and the sharing of voltage around a loop, and using them to analyse simple circuits.

What this topic is asking

WJEC Eduqas wants you to know the current and voltage rules for series and parallel circuits and to use them to analyse a circuit: how current splits and recombines at junctions, and how voltage is shared around a loop. These rules underpin potential dividers, resistor networks and almost every circuit calculation in Component 1.

The series current rule

Because there is only one path, every electron that passes one component must pass them all, so the ammeter reads the same wherever you put it in the loop. This is why a single break anywhere in a series circuit (a blown lamp, an open switch) stops the current everywhere. The shared current is the key feature of a series circuit.

The series voltage rule

Each coulomb gains a fixed amount of energy from the supply and gives it all up as it passes through the components, so the voltages across the components must add to the supply voltage. If two equal resistors are in series across a supply, each takes half the voltage; if one resistor is larger, it takes the bigger share. This sharing of voltage is exactly what a potential divider exploits.

The parallel voltage rule

Both ends of every branch are joined to the same two nodes, so each branch experiences the same potential difference, equal to the supply (ignoring the resistance of the connecting wires). This is why household appliances are wired in parallel: each gets the full mains voltage and can be switched on or off without changing the voltage across the others.

The parallel current rule

At a junction, charge cannot build up or disappear, so the current arriving must equal the current leaving. The supply current divides between the branches in inverse proportion to their resistance (the easier path takes more current) and the branch currents add back together when they recombine. Adding another branch in parallel gives the charge an extra path, so the total current from the supply increases and the total resistance falls.

Try this

Q1. Three equal resistors are in series across a $9.0\,\text{V}$ supply. State the voltage across each. [1 mark]

• Cue. The voltage is shared equally: $\dfrac{9.0}{3} = 3.0\,\text{V}$ each.

Q2. A supply delivers $0.60\,\text{A}$ to two parallel branches. One carries $0.25\,\text{A}$. State the current in the other branch. [1 mark]

• Cue. Current is conserved: $0.60 - 0.25 = 0.35\,\text{A}$.