FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician is analyzing a parallel circuit branch containing a 6-ohm resistor and a 12-ohm resistor connected across the same 24-volt bus. What is the total resistance of this parallel combination?
- A4 ohms
- B2 ohms
- C18 ohms
- D9 ohms
Show answer & explanationAnswer & explanation
Correct answer: A. 4 ohms
For two resistors in parallel, total resistance equals the product divided by the sum: (6 × 12) / (6 + 12) = 72 / 18 = 4 ohms.
Why the other options are wrong
- B. This does not match the product-over-sum formula result.
- C. 18 ohms is the sum of the resistors, which would apply only if they were in series.
- D. 9 ohms is simply the average of 6 and 12, which is not how parallel resistance works.
Parallel Resistance Calculation
Total resistance in a parallel circuit is always less than the smallest individual resistor and is found using the product-over-sum (for two resistors) or reciprocal method.
- Total R is always less than smallest branch resistor
- Two-resistor formula: Rt = (R1×R2)/(R1+R2)
- Multi-resistor formula: 1/Rt = 1/R1 + 1/R2 + ...
- Voltage is the same across all parallel branches
Memory trick: More lanes on the highway means less traffic resistance overall — adding parallel paths always lowers total resistance.