IV. he amount of light from a standard incandescent bulb depends upon the curren

Question

IV. he 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 dimmer switch is really a variable resistor wired in parallel to the load, in this case a 60-W light-bulb. To vary the light level, you adjust the resistance in the parallel resistor, thereby shunting some of the current away from the original light-bulb. 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 (parallel) resistor? Assume initially the variable resistor is set to a very high number (essentially infinity) for full light output in the 60-W bulb. (3pt)

Explanation / Answer

a) Power = V^2 / R

60 = 120^2 / R

R = 240 ohm

b) power now = 0.1*60 = 6 W

6 = 120^2 / R

R = 2400 ohm

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