MadSci Network: Physics

Re: How come incandescent lamps operate normally on DC as well as AC?

Date: Sun May 3 11:00:33 2009
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Physics
ID: 1240936257.Ph

Hi Anbarasi,

That's a good question. Let's look at it your question the other way
around: why wouldn't incandescent lamps operate normally on AC vs DC? There
are two possible reasons:

1. Incandescent lamps are in some way asymmetric, meaning current flowing
in one direction produces a different effect than the other.

2. The frequency of the current somehow might cause a different effect.

In the first case, we know that incandescent lamps are electrically
symmetric. There is no difference electrically from one terminal to the
other. That means we could apply current in either direction and observe no
change in light output. This is easily verified by connecting a lamp to a
DC source, then reversing polarity. There's no difference. There are
asymmetric devices, of course. Light Emitting Diodes (LEDs) produce light
when current is applied one direction only.

In the second cases, there is a possibly an effect. If the frequency is
reduced very low, perhaps 1 Hz or so, we would likely see the lamp flash.
I'm sure you have seen flashing lamps at low frequency, so that's clearly
different from steady DC operation.

I suspect you are referring to standard line current (or mains as they
are sometimes called). In that case, the frequency is either 50 Hz or 60 Hz
depending on where you live. These frequencies are much higher than the
thermal time constant of filaments in incandescent lamps, so the filaments
don't have much time to cool off as current varies during a cycle. The
filaments tend average out at one temperature. There is some slight
variation, but even that is too small and too fast for our eyes to detect.

Using a photodiode with some filtering and amplification, you can measure
the variation in light output at the line frequency, but it will be small.

I hope that answers your question! -

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