FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsMedium
A technician is troubleshooting an aircraft circuit and needs to determine the total resistance of two resistors, 15 ohms and 30 ohms, connected in parallel. Using the product-over-sum method, what is the total resistance?
- A10 ohms
- B15 ohms
- C22.5 ohms
- D45 ohms
Show answer & explanationAnswer & explanation
Correct answer: A. 10 ohms
For two resistors in parallel, total resistance equals the product divided by the sum: (15 × 30)/(15 + 30) = 450/45 = 10 ohms. This confirms that parallel resistance is always less than the smallest individual resistor value.
Why the other options are wrong
- B. This equals one of the individual resistors, not the parallel combination.
- C. This is the average of the two resistors, not the correct parallel formula result.
- D. This is the sum of the resistors, which applies to series circuits, not parallel.
Product-Over-Sum Method
A shortcut formula for calculating total resistance of exactly two parallel resistors: Rt = (R1 x R2)/(R1 + R2).
- Only valid for exactly two resistors at a time
- Result is always less than the smallest resistor
- For more resistors, use the reciprocal formula instead
Memory trick: Product over sum — multiply then divide, and the answer always shrinks below the smallest value.