Part 08 · Circuits and magnetism · Card 3 of 12
Series and parallel resistors
In series, resistances add. In parallel, reciprocals add: 1/R = 1/R1 + 1/R2.
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Analogy
One lane road with many tolls versus several lanes side by side.
Example
Combining
Series: 6 + 3 = 9 ohmParallel: 1/R = 1/6 + 1/3 = 1/2R = 2 ohm
In practice
6 ohm and 3 ohm resistors give 9 ohm in series and 2 ohm in parallel.
Practice
Flashcards
- What is the combined resistance of resistors in series?The sum of the resistances.
Interview questions
- A 12 V battery is connected to a 6 ohm resistor in series with a parallel pair of 6 ohm and 3 ohm resistors. Find the current from the battery and the power used.
- Parallel pair is 2 ohm
- Total is 8 ohm
- I = 1.5 A, P = 18 W
Questions people ask
Why is total resistance in parallel always smaller?
Adding a parallel branch gives current an extra path, like adding a lane to a road. More paths mean more current for the same voltage, so the total resistance falls. The combined value is always less than the smallest branch.
Why are home sockets wired in parallel?
Each device then gets the full mains voltage, and one switched off or broken does not stop the others. In series, every device would share the voltage and one failure would break the whole circuit, like old strings of fairy lights.