The circuit rules you've learned are based on two fundamental conservation laws:
Current at a junction: The total current entering any junction equals the total current leaving it. This is just conservation of charge — charge cannot be created or destroyed. In parallel circuits, the current splits but the total is conserved.
Potential difference around a circuit: The energy given by the supply equals the energy transferred by all the components. This is just conservation of energy. In series circuits, the potential differences across the components add up to the potential difference of the supply.
These rules apply to all circuits — simple or complex. When you solve a tricky mixed circuit in your GCSE exam, you're using them whether you know it or not.
Quick Check: In a parallel circuit, the supply is 6 V. What is the voltage across each branch?
6 V across every branch. In a parallel circuit, voltage is the same across each branch and equals the supply voltage.
This how it works covers How It Works: The Rules Behind the Rules within Series & Parallel Circuits for GCSE Physics. Revise Series & Parallel Circuits in Electricity for GCSE Physics with 20 exam-style questions and 30 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 8 of 16 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Series & Parallel Circuits
In a series circuit, what is true about the current at all points?
A student adds an extra lamp to a parallel circuit. Explain how this affects the total current from the supply and the brightness of the original lamps.