They're like that in this apartment we're renting and I keep seeing them elsewhere. I don't get it.
this post was submitted on 25 Jan 2025
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Oh I've never seen that in the US, maybe I just didn't notice
Laptop power bricks is probably where I see it most. Or if you plug in something with a motor already switched on. Listen for a soft popping noise if you plug in a big power brick.
You're right, now that I think about it. Laptop power does it more than anything else.
Many, many big power-smoothing capacitors inside those jumping from 0 to 120V in a microsecond, that's why. The better-smoothed the power supply, the more capacitors and the bigger the sparks tend to be, although some really high quality ones put most of them behind inrush-current limiters to reduce the sparking, but that can also marginally reduce efficiency. High power electronics are always a bit of a tradeoff. The problem is that capacitors charge and discharge almost instantly in most cases, and when empty they act like a short circuit until they're filled, so they can create some pretty big sparks, even though the actual energy going in is minuscule by any reasonable measurement. It's almost like a static shock, huge spark, tiny energy.
Some motors will also spark badly when disconnected, but the reason is slightly different. They have a huge electromagnetic field which suddenly fills or collapses and that inductance in the coils can draw a lot of amps on startup and generate some pretty high voltages, more than enough to spark across the gap. Like the capacitors, they are very nearly a short circuit until they start moving.
Ah, this is fantastic. I learned some stuff, thank you.
Home electricity in North America has roughly half the voltage as elsewhere in the world, and double the voltage is double the arcing potential, so that figures.