Friday, December 28, 2018

Hear the Roar of the Grid in the High Voltage Explosion that Lit Up NYC's Sky Last Night

The world is ending. The aliens are here. That's what some New York City residents thought last night when the sky over Queens lit up with a bizarre blue glow. For a few minutes, the eerie, flickering light was bright as day.

The electric blue hue of the light might give you a clue what really happened.
As usual, it wasn't the apocalypse or an alien invasion: a high-voltage transformer at the Con Edison plant in Astoria broke down, unleashing a violent electrical arc.

A resident named Chris Okada got video of the event from nearby, which features some interesting physics, that you can see (and hear!) here:


If you turn your sound on for the video, you can hear a roaring buzz—that's the sound of alternating current!

Sixty times a second, the wires of the power grid switch from positive to negative voltage (and vice versa), and then switch back again. As long as the current stays in the wires, that happens without much fanfare—although some fluorescent and LED lightbulbs dim for an instant each time that transition happens, it's such a quick thing that our eyes and brains don't really notice the brief dark spell. And since resistance is a measure of how well a material converts electrical current into other kinds of energy (like heat, light, and sound), the low-resistance wires that make up the grid don't make a fuss.

But when high-voltage components break down, the electricity has to travel through the air. While air has high resistance ordinarily, extreme voltages can strip electrons free from the air molecules and turn the gas into a plasma, which is much more electrically conductive. This plasma arc carries the current, and the changes in the electricity's flow create changes in the behavior of the plasma—and in the air around it. Since sound waves are just vibration in the air, the 60 Hz alternating current of the grid effectively turns that arc into a giant plasma speaker playing a 120 Hz tone.

Check out the video below, and compare the test-tone to the sound you hear in Okada's video above!


If you want to harness that principle in a more controlled fashion, you can even make a plasma speaker of your own at home; they can have surprisingly good fidelity, even if they require a high-voltage transformer to run.


Another fun bit of physics here is the color of the glow: there's a reason they call it "electric blue"! If you take a look at the emission spectrum of nitrogen (the main gas in our atmosphere) you can see a bright band right in the blue-green region. To do a little bit of amateur spectroscopy, compare that to the image at the top of this article.
At lower voltages, like in the plasma speaker video above, the red emission lines contribute a fair amount. At higher voltages, the blue band drowns it out.
So if you ever see the sky light up that color, you'll have a pretty good idea what's going on. You can turn to your friends and say "that's nitrogen plasma!"—and hopefully quell their fears, or at least come off like someone who runs into this kind of thing all the time. And while we might well expect the alien invasion to be heralded by massive plasma discharges, keep an ear out: if you can hear the 60Hz hum, you're probably safe...although you might be going without power for a while.

—Stephen Skolnick

1 comment:

  1. Interesting, except that the emission spectrum was probably (mostly) from molecular nitrogen rather than atomic nitrogen. When you put nitrogen in an electric discharge you see an emission spectrum of N2 (bands rather than discrete lines) extending from the green through the blue and into the UV. I had to analyze this spectrum for a class in grad school!

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