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When two resistors are connected in parallel, the same voltage appears across each of them and the effective resistance of the circuit will always be less than any one of the two resistors taken alone. The effective resistance, R, of a circuit with two resistors, R1 and R2, connected in parallel is given by:
1 / R = 1 / R1 + 1 / R2
Similarly, where three resistors (R1, R2, and R3) are connected in parallel, the effective resistance of the combined circuit will be given by:
1 / R = 1 / R1 + 1 / R2 + 1 / R3
In the case of two resistors, the formula can be more conveniently re-arranged as follows:
R = (R1 x R2) / (R1 + R2)
This can be remembered as "product over sum".
<^< Worksheet: Resistors in Series | Course index | Worksheet: Resistors in Parallel >^>
