California Electrical Contractor (C-10) License Practice Exam

To determine the resistance of a lamp using its wattage and the voltage of the line, you can use the formula derived from Ohm's law and the power equation. The power (P) in watts is equal to the voltage (V) in volts multiplied by the current (I) in amps (P = V × I). Ohm's law states that V = I × R, where R is resistance.

By rearranging the power equation, we can express resistance as R = V² / P. In this scenario, the lamp consumes 110 watts and is connected to a 220-volt line.

Substituting the given values into the equation, you have:

R = (220 V)² / (110 W)

R = 48400 V² / 110 W

R = 440 ohms.

This calculation shows that the correct resistance of the lamp is 440 ohms. Thus, the answer is B, as it reflects the accurate calculation based on established electrical principles.