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/sci/ - Science & Math

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8109290 No.8109290 [Reply] [Original]

I am in no way an expert on electricity so that means it's confusing af! I just saw a video on how a variac functions and he uses a lightbulb to demonstrate. Transformers output the same power but at different voltages/current (assuming there's no loss), so if thats true how can if he's outputting 5 volts, the variac is stepping it down, the lightbulb has a constant resistance so that means it should be really bright? Since lightbulbs at 120v operate normally with idk how much current going through, shouldn't decreasing the voltage with the same power increase the bulb's luminosity because of more current going through? Same power coming from the mains? This is so confusing.

>> No.8109300

>>8109290
V=IR
V decreases and R stays the same so I ...

>> No.8109304

>>8109300
yes the current either decreases or increases, so if it's more current at 5 volts mark, why is the lightbulb off?

>> No.8109319

>>8109290
bump

>> No.8109337

>>8109304
The bulb might be designed to operate at higher voltages and won't work with 5 volts or less... the current simply passes through or more likely, blows the filament

>> No.8109348

>>8109290
>the lightbulb has a constant resistance
you couldn't be more wrong

resistance increases a lot with temperature


power for constant voltage = V^2/R

even though the resistance gets bigger when the bulb is hotter, it's not enough to compensate a decrease of power with the SQUARE of the voltage