4 ms·

Ignoring 'complex power' concerns, the AC voltage driving a tungsten filament bulb is a sine wave at line frequency (60hz for the US). Because it is a sine wav

by pwg 7y ago

Ignoring 'complex power' concerns, the AC voltage driving a tungsten filament bulb is a sine wave at line frequency (60hz for the US). Because it is a sine wave, and because the tungsten filament is a resistor, the current through that resistor rises and falls in sync. with the AC sine wave. The current will be zero at the zero volt crossing point of the wave, and maximum at the peaks of the sinewave. The varying current through the tungsten will result in a varying power consumption (varying at a 60hz rate). The varying power consumption will result in a small amount of varying temperature on the filament. Now, the filament does not cool down instantly, so the result is its temperature will vary by a few degrees, but likely not enough to ever be perceptible to human eyes. But with a sensitive enough (and fast reacting enough) temperature probe, one could likely measure the temperature rise/fall of the filament that is synchronized with the line frequency.

SamBam 7y ago
Right, the wave filter effect caused by the thermal mass of the filament was my point. I'm not sure you've shown that this would be visible to a housefly as a "slow pulse from light to dark," as the post I was replying to claimed. First of all, you said the power consumption will vary at a 60hz rate, but it seems to me it will vary at a 120hz rate, since the current will be at its max magnitude twice per cycle (as you say in the first paragraph). But more to the point, I don't know whether the temperature/lumens will decrease enough in 1/120th of a second to be visible to houseflies or anything else. Trying to research the answer, I found this physics lab worksheet [1] from Pasco (the makers of sensors), for a high school lab measuring the output frequency of incandescent bulbs vs fluorescent bulbs, but without doing the lab I don't know the results. 1. http://phylab.yonsei.ac.kr/exp_ref/pasco/P54_BULB.pdf http://phylab.yonsei.ac.kr/exp_ref/pasco/P54_BULB.pdf
- Dylan16807 7y ago
  https://www.quora.com/Would-a-120-volt-incandescent-bulb-operate-at-the-same-brightness-on-both-AC-and-DC-at-120-volts https://www.quora.com/Would-a-120-volt-incandescent-bulb-ope... The numbers here show a >25% difference for an example bulb. Also, even ignoring how the cooling starts off at its fastest, with radiation scaled to the fourth power of temperature... incandescent lights switch off in about a tenth of a second. Even if that was completely linear, it would be very visible. Even a 1% flicker would be visible.
  - SamBam 7y ago
    Huh, good find. I wish the poster had some sources, especially for that initial temperature graph, but there's no reason to doubt it. Thanks for the research.

by pwg 7y ago

SamBam 7y ago

Right, the wave filter effect caused by the thermal mass of the filament was my point. I'm not sure you've shown that this would be visible to a housefly as a "slow pulse from light to dark," as the post I was replying to claimed. First of all, you said the power consumption will vary at a 60hz rate, but it seems to me it will vary at a 120hz rate, since the current will be at its max magnitude twice per cycle (as you say in the first paragraph). But more to the point, I don't know whether the temperature/lumens will decrease enough in 1/120th of a second to be visible to houseflies or anything else. Trying to research the answer, I found this physics lab worksheet [1] from Pasco (the makers of sensors), for a high school lab measuring the output frequency of incandescent bulbs vs fluorescent bulbs, but without doing the lab I don't know the results. 1. http://phylab.yonsei.ac.kr/exp_ref/pasco/P54_BULB.pdf http://phylab.yonsei.ac.kr/exp_ref/pasco/P54_BULB.pdf

Dylan16807 7y ago
https://www.quora.com/Would-a-120-volt-incandescent-bulb-operate-at-the-same-brightness-on-both-AC-and-DC-at-120-volts https://www.quora.com/Would-a-120-volt-incandescent-bulb-ope... The numbers here show a >25% difference for an example bulb. Also, even ignoring how the cooling starts off at its fastest, with radiation scaled to the fourth power of temperature... incandescent lights switch off in about a tenth of a second. Even if that was completely linear, it would be very visible. Even a 1% flicker would be visible.
- SamBam 7y ago
  Huh, good find. I wish the poster had some sources, especially for that initial temperature graph, but there's no reason to doubt it. Thanks for the research.