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Homework Help: AC current and incandescent bulbs

  1. Mar 30, 2014 #1
    In my class we did an experiment where we measured the flicker frequency of an incandescent bulb and a CFL bulb (powered by a 60hz wall plug) using a photodiode and a oscilloscope. We were then asked some questions about the sinusoidal curves that were produced.

    1. The incandescent bulb probably has a large "DC offset". Why do you think this is?
    (im struggling with this question. Does it have something to do with the filament heating up?)

    2. Is the flicker frequency for each of them the same as the 60hz wall plug frequency? why or why not?

    From the experiment I found out that the frequencies were 120hz but I dont know why?

    Please help
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 30, 2014 #2
    You're definitely on the right track. If you're in a room that's only illuminated by an incandescent bulb, you can tell that it takes a noticeable amount of time for it to completely turn off.

    Try calculating how much time passes between each peak of AC that flows through the filament of the bulb.

    Have a look at its sinusoidal current waveform. How many peaks are there in a period?
  4. Mar 30, 2014 #3
    1. So maybe because the energy doesn't totally dissipate before the next peak of AC current? The wave is just a big higher because its starting at a different point?

    2. So there is two peaks in each period. One in the positive and the other in the negative y axis. So are you trying to say that each peak equates to 60 hz? so why isn't the ac current considered 120hz?
  5. Mar 30, 2014 #4
    I think you have the right idea. Try to formulate it in terms of filament temperature instead.

    No, but since you have two peaks in each period of AC, the bulb "powers up" twice in a period (it doesn't care about current direction). That means something for the frequency of the power waveform and, as a consequence, the frequency of the flickering.
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