Topic 3 of 5

Series circuits

In a series circuit every component sits on one single path, so the same current flows through all of them.

The rules

• Current is the same everywhere in the circuit.
• Total resistance is the resistances added up: R_T = R1 + R2 + R3.
• Voltage is shared between the components. The drops add up to the supply voltage, and the biggest resistance takes the biggest share.
• One break anywhere stops current flowing through the whole circuit.

Worked example

Three resistors of 10 Ω, 20 Ω and 16 Ω are in series across 230 V.

1. Total resistance: 10 + 20 + 16 = 46 Ω
2. Current: I = 230 ÷ 46 = 5 A
3. Voltage across each: 5 × 10 = 50 V, 5 × 20 = 100 V, 5 × 16 = 80 V
4. Check: 50 + 100 + 80 = 230 V

Exam tip: always do the check in step 4. If your voltage drops don't add up to the supply, there's a mistake somewhere.

Try it: Ohm's law calculator