2 · IGCSE Physics
Series and parallel circuits
Connect one, two or three lamps or resistors to a battery in series or in parallel, then read the ammeters and voltmeters and compare lamp brightness. Break a component to see what happens to the rest of the circuit.
Gold dots show conventional current (from + round to −); their speed is proportional to the current.
Series: R = R₁ + R₂ + R₃ Parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ I = V / R P = I V
Try this
- In series with three lamps, add up the three voltmeter readings and compare the total with ε. Then break lamp 2: what happens to the other lamps, and what does the voltmeter across the broken lamp read?
- Switch to parallel and go from one to three lamps. Each lamp keeps the full battery voltage, but watch the battery current and the total resistance as each branch is added.
- In parallel set R₁ = 10 Ω and R₂ = 20 Ω. Which branch carries more current, and do the branch currents add up to the reading on the main ammeter?
Exam tip: the combined resistance of resistors in parallel is always smaller than the smallest single resistor, so use this as a quick check on any parallel calculation.