The amount of light from a standard incandescent bulb depends upon the current g

Question

The amount of light from a standard incandescent bulb depends upon the current going through it. Assume the light-bulb is connected across a 120V voltage difference. (a) What is the resistance of a 60-W light-bulb? (2pt) (b) A old-style dimmer switch is really a variable resistor wired in series to the load, in this case a 60-W light bulb. To vary the light level, you adjust the resistance in the series resistor, thereby changing the current in the original. If the light level from the 60-W bulb decreases to 10% of the original light by changing the resistance of the variable resistor, what is the value of the variable resistor? Assume initially the variable resistor is set to a very low number (essentially zero) for full light output in the 60-W bulb. (2pt V.

Explanation / Answer

(A) P = V^2 / R

60 = 120^2 / R

R = 240 ohm

(B) P' = 0.10P = 6 watt

and P = I^2 R

6 = I^2 (240)

I = 0.158 A  

(R + R') I = V

R + 240 = 120 / 0.158

R = 519 ohm

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