IGCSE Physics / Electrical resistance
Calculating current, voltage and resistance in a series circuit revision notes
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.19. Free lesson.
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In a series circuit, resistances add and the same current flows through every component. These rules let you calculate the circuit current, the voltage across each resistor and an unknown resistance.
How to find total resistance in series
A series circuit has one path for the current. Because there is only one path, the current is the same through every resistor. The resistors add to make the total resistance, so use the total resistance when finding the current from the supply voltage. Total resistance: R = R₁ + R₂, measured in ohms (Ω). R is the total resistance; R₁ and R₂ are the resistances of the individual resistors.
How to calculate the current
Use the supply voltage and the total resistance in V = I × R. The equation can be rearranged to I = V/R. The current you find is the same everywhere in the loop.
How to find the voltage across each resistor
Use V = I × R for one resistor at a time, using that resistor's resistance and the circuit current. The voltages across the resistors add up to the supply voltage, which gives you a useful check. For each resistor, multiply the current by its resistance. If the individual voltages do not add up to the supply voltage, check your working.
How to work back to an unknown resistor
You can use the voltage and resistance of a known resistor to find the current first. In series, the same current flows through the unknown resistor, and the voltages across both resistors add to the supply.