How to Calculate Circuit Resistance
What is Circuit Resistance?
Resistors in series simply add together. Resistors in parallel have a combined resistance lower than any individual resistor — current has multiple paths, reducing overall opposition.
Formula
Series: R_total = R₁+R₂+... | Parallel: 1/R_total = 1/R₁+1/R₂+... | Ohm's Law: V = I × R
- R
- Resistance (Ohms (Ω))
- V
- Voltage (Volts (V))
- I
- Current (Amperes (A))
Step-by-Step Guide
- 1Series: R_total = R1 + R2 + R3 + ...
- 2Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
- 3For two parallel resistors: R = (R1×R2)/(R1+R2)
- 4Parallel resistance always less than smallest resistor
Worked Examples
Input
R1=10Ω, R2=20Ω in series
Result
30Ω total
Input
R1=10Ω, R2=20Ω in parallel
Result
(10×20)/(10+20) = 6.67Ω — less than either resistor alone
Frequently Asked Questions
Why is total resistance different in series vs parallel?
Series adds resistance (longer path). Parallel reduces resistance (multiple paths for current). Parallel resistance is always less than the smallest individual resistor.
What does Ohm's Law tell us?
V=IR shows voltage, current, and resistance are proportional. Double the voltage and current doubles (if resistance stays same).
What is the practical difference between series and parallel in household circuits?
Household outlets are parallel (each has full voltage). If one fails, others work. Christmas lights were historically series, so one dead bulb broke the whole string.
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