A pair of 60-W and 100-W incandescent lamps are intended for 120-V operation. Which lamp has a larger resistance?
A)60-W lamp
B)100-W lamp
C)both the same

Question

Quizplus · Accepted Answer

The power of a lamp is given by the formula:

Power = (Voltage)^2 / Resistance

Therefore, the resistance of a lamp can be calculated as:

Resistance = (Voltage)^2 / Power

For both lamps to operate at 120 V, the resistance of each lamp can be calculated as follows:

Resistance(60-W lamp) = (120 V)^2 / 60 W = 240 ohms 
Resistance(100-W lamp) = (120 V)^2 / 100 W = 144 ohms

Since the 60-W lamp has a higher resistance than the 100-W lamp, that means the answer is A) 60-W lamp.

A Pair of 60-W and 100-W Incandescent Lamps Are Intended