You get a shock when there is a big protentional energy difference between your body and something else. Either high or low.
So the "let go before its safe" means they want to stop the generator, and ground it, so the electrons leave though a path that isn't your body, to avoid the shocks
Why wouldn't the electrons go to ground through your body while you're touching it?
They can and do, but you feel the zap when there's an arc. If there's current moving though your body without an arc, it feels like your muscles vibrating or cramping, depending on whether it's AC or DC, mostly. After all, your muscles are activated with electrical signals.
Usually for a Van de Graf demonstration The person touching it has some level of electrical isolation from the ground. Be it their shoes or something else. If they were touching the ground directly or a piece of metal they would constantly be passing the electrons through and they wouldn't get that whole hair raising effect.
Okay, so you're insulated from ground. The generator charges you up. You are at the same charge as the generator. You let go of the generator. Why is there a potential difference?
If the generator is running, it's still adding more charge, so you will be instantly at different potentials
I see. Would there also be an arc if you put your hand near the generator while it was running, then?
As long as the potential is great enough to break down the dielectric of air yes.
I am not expert, but seems plausable. Shock comes from high voltage electric charge jumping from metal to skin. If you press it, you are part of the electric charge. If you are far away, charge cannot jump. Problem is only when you are couple of centimeters close to it. AFAIK, this is not current, but electric discharge, I think it cannot kill you (it is just very unpleasant), but maybe someone else knows better?
Thing that confuses me is that when you let go, you should have the same charge as the generator. No charge difference, no arc. Unless I'm wrong about something, which I probably am (hence my confusion).
The generator is generating a difference. Even if you have the same potential when you're holding it, as soon as you let go, that ends.
Does the human body rapidly discharge into air or something?
Enough for a change in potential to cause arcing, as we can see. I'm sure you could find relevant experimental studies, or even conduct them on yourself with a proper transformer and voltmeter.
Yeah, if you move your hand around on those things you'll get a static shock, it's going to hurt, but it won't kill you. If you watch those demonstrations they have a pole they ground the generator out with.
If you're in some kind of science experiment where you're being conditioned to not stop touching something: Yes.
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