An incandescent light bulb with an operating resistance of 95 Ω is labeled "150 W". Is this bulb designed for use in a 120 V circuit or a 220 V circuit? Ans. (It has been designed for 120 V)



Data Given:


Resistance of Bulb = R = 95 Ω

Power marked on the Bulb = P = 150 w


To Find:


For which Voltage (120 V 0r 220 V) the bulb is designed =V =?



Solution:


To find voltage V Using Ohm's Law

V = I R

The current is also unknown so, to find current I We have to use the formula for Power

P = I²R 

or

I² = `\frac{P}{R}`

by putting values

I² = `\frac{150w}{95Ω}`

I² = 1.578 A²

Taking square root on both sides 

`\sqrt I²` = `\sqrt {1.578 A²}`


I = 1.2565 A ----------------Ans. 1

Now using formula

V = I R

V = 1.2565 A ｘ 95 Ω

V = 119.37 V

or

V = 120 V -----------------Ans. 2

Thus the Bulb is designed for the voltage of 120 volts.

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