CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A pilot light in a control panel consumes 15 watts of power when connected to a 24V DC supply. What is the approximate resistance of the pilot light filament?
- A1.6 ohms
- B0.625 ohms
- C38.4 ohms
- D360 ohms
Show answer & explanationAnswer & explanation
Correct answer: C. 38.4 ohms
First, calculate the current (I) using the power formula: I = P / V = 15W / 24V = 0.625A. Then, use Ohm's Law to find the resistance (R): R = V / I = 24V / 0.625A = 38.4 ohms. Alternatively, R = V² / P = (24V)² / 15W = 576 / 15 = 38.4 ohms.
Why the other options are wrong
- A. This would be the current (I = P/V) if the question asked for current.
- B. This is the current (0.625A), not the resistance.
- D. This would be R = P * V, an incorrect formula.
Power Formula (Resistance)
Resistance can be calculated from power and voltage using R = V² / P, or by first calculating current (I = P/V) and then applying Ohm's Law (R = V/I).
- P = V × I, P = I² × R, P = V² / R
- Can derive resistance if power and voltage are known.
- Applies to DC circuits and resistive AC circuits.
Memory trick: Power's Voltage-Squared over Resistance, or first find Current.