Voltage, Power, and a brown out?

Question

Say the voltage of the house went from 120volts to 108 volts. What would be the power consumed by a 100 watt bulb (that is, a lightbulb that consumes 100watts when connected to 120volts).

Answer 1

Houses may vary from 110 to 120 volts, but based on your question, we'll say it was originally 120 v, and then dropped to 108.

Two basic equations are used here: I = V/R and P=IV

P = IV will tell you how much current goes through the bulb when plugged into 120 V.

100 W = I (120V)
I = .83 Amps

Next use I = V / R to find the resistance of the bulb.

R = V / I = 120 V / .83 Amps = 144 Ohms

Now repeat the process in reverse for the new voltage of 108, but keep the resistance you just found.

I = V / R = 108 V / 144 Ohms = .75 Amps

P = I V = .75 Amps x 108 Volts = 81 Watts

By the way, it's not a perfectly realistic answer (even though it's what you're expected to get) because a real light bulb's resistance will be less when run on 108 volts and as a result it would actually have more current and power than shown here.

Answer 2

Power = volts x amps
Assuming the same amperage (actually it would be slightly less) and also assuming that the bulb used 100watts at 120 volts, then at 108 volts the power consumption would be (108/120)*100 = 90 watts.

Answer 3

Assuming that it is an incandescent bulb and that its resistance does not change(that is not exactly true because as the current lowers the temperature of the filament decreases and so the the
resistance),as power is V*I and I = V/R The new power would be
W = 100*(108/120)^2 =81Watt

Answer 4

at 120v the bulb would dissipate about 100 W with a current of about 833 mA and a resistance of about 100Ω
at 108v the bulb would dissipate about 90 W with a current of about 750mA assuming a resistance of about 100Ω

Answer 5

Houses are actually 110v. So it would consume about the same.