AC current and incandescent bulbs

[KozakDave]

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 don't know why?



Please help

 

[milesyoung]

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?)

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.

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 don't know why?

Have a look at its sinusoidal current waveform. How many peaks are there in a period?

 

[KozakDave]

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?

 

[milesyoung]

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?

I think you have the right idea. Try to formulate it in terms of filament temperature instead.

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?

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.

 

[HalfLostAndSearching]

1. Yes, the DC offset in the incandescent bulb is due to the filament heating up. As the filament heats up, it becomes a better conductor and reduces the resistance of the circuit, allowing more current to flow. This results in a DC offset in the sinusoidal curve as the current is not constant but fluctuating due to the heating and cooling of the filament.

2. No, the flicker frequency for each of them is not the same as the 60hz wall plug frequency. The 60hz frequency is the frequency of the AC current that is supplied to the bulb. However, the flicker frequency of the bulb is determined by the rate at which the filament heats up and cools down, which is influenced by the resistance and capacitance of the bulb. In the case of the incandescent bulb, the filament takes longer to heat up and cool down compared to the CFL bulb, resulting in a flicker frequency of 120hz. This is because the incandescent bulb has a higher resistance and lower capacitance compared to the CFL bulb.