Formulas
Series: R_total = R1 + R2 + R3 + … Simple sum of all resistors. Parallel: 1/R_total = 1/R1 + 1/R2 + … Invert the sum of reciprocals. The parallel result is always less than the smallest resistor in the group. Current at a given voltage = V / R_total (Ohm's Law).
Frequently Asked Questions
Series total resistance = R1 + R2 + R3 + … The total is always greater than the largest individual resistor. Current is the same through all resistors in series; voltage divides proportionally.
1/R_total = 1/R1 + 1/R2 + 1/R3 + … The total resistance is always less than the smallest individual resistor. For two resistors: R_total = (R1 × R2) / (R1 + R2). Voltage is the same across all parallel resistors; current divides proportionally.
In series, all current flows through each resistor in sequence — higher total resistance. In parallel, current splits across resistors — lower total resistance. Real circuits often combine both: resistor groups in parallel connected in series with other components.
Common reasons: creating a resistance value not available as a standard component, increasing power handling capacity (parallel resistors share the power load), or adjusting voltage divider ratios to non-standard values.
Power (W) = I² × R = V²/R. Resistors in parallel each dissipate less power than a single resistor carrying the full current. Resistors in series each dissipate power proportional to their share of the total resistance.
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