What determines the frequency of arc currents when arcs occur inside a microwave oven?

October 26, 2012 7:42 PM

What determines the frequency of the arc current when an arc occurs inside a microwave oven?

Earlier this evening, I put a 6 oz metal, mostly empty, tomato paste can (along with some other items) in my microwave oven as part of an experiment. When I turned the microwave on, an arc formed, seemingly across the diameter of the open top of the can.

Although I quickly shut the oven off once the arcing started, I noticed that the arc made a distinctive sound with a relatively low frequency; I would say well under 100 Hz.

This made me wonder what determines the frequency of the arc currents caused by a microwave. The microwave oven itself emits RF around 2.4 GHz, but the sound I heard was, obviously, well below that. I'm guessing perhaps it has something to do with the conductivity of the metal and the distance involved, but I would like to hear from someone who actually knows or has some sound ideas.

Also, for what it's worth, why did the can start arcing in the first place? I would have expected it not to, given the can's relatively rounded top edge. I would have expected current to be induced to flow throughout the can as opposed to arcing across its top (open) surface. Any explanations of that are welcome as well.

posted by Juffo-Wup to Technology (7 answers total)

Can't speak to the particular reason you saw an arc, but the most likely source of your low frequency sound is the mains electricity supply. The magnetron inside a microwave oven is fed from a high voltage derived from the mains via a high-turns-ratio transformer and a voltage multiplier, and there's very little smoothing involved because there's simply no need for it. The end result is that the microwave emissions are chopped at 2x the frequency of the AC mains supply.
posted by flabdablet at 7:59 PM on October 26, 2012

RC is a low-pass filter. The air that the arc is going through is a resistor (even after it turns to plasma). The opposite sides of the can implicitly have capacitance.
posted by Chocolate Pickle at 8:24 PM on October 26, 2012

The resonant frequency of the circuit made from the capacitance of the can and any reasonable arc would not be that low. I think flabdablet's explanation is more likely correct.
posted by hattifattener at 9:08 PM on October 26, 2012

Thanks for the explanation, flabdablet! Why didn't I think of that?? =) It makes perfect sense, in retrospect.

I wonder what the arc would sound like in one of these newfangled microwaves with an inverter power supply. Hmm.....
posted by Juffo-Wup at 10:37 AM on October 27, 2012

If you're sure that the sound you heard really was below 100Hz, that tells you that your particular oven has a half-wave voltage doubler.
posted by flabdablet at 6:50 PM on October 28, 2012

Interesting! Here I'd just talked myself into believing that it must have been 120 Hz, but now... Perhaps I will have to do it again and record it this time.
posted by Juffo-Wup at 10:08 AM on November 8, 2012

Even if it does have a 60Hz fundamental, the resulting waveform will be asymmetric enough to ensure that it includes a fairly hefty 120Hz second harmonic.
posted by flabdablet at 3:44 AM on November 9, 2012