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geekhack Community => Off Topic => Topic started by: geniekid on Mon, 17 November 2014, 14:36:20

Title: Confirm my understanding of static electricity
Post by: geniekid on Mon, 17 November 2014, 14:36:20: The basic principle is that your build up charge on some part of your body and then when you get sufficiently close to a conductive surface the electrons rapidly flow from you to the surface creating a shock and neutralizing the charge difference while doing so.

I've been told if you touch the door (presumably made of an insulator like wood or glass) before touching the door knob you are much less likely to be shocked and my experience supports this claim. Is that because wood/glass is still a little conductive and the same neutralizing effect is occurring between me and the doorknob at a much slower rate? Would that mean that I could hold my hand 1ft away from the doorknob for 30 seconds and let air (poor, but not zero conductivity) be the intermediate conductor?
Title: .
Post by: esoomenona on Mon, 17 November 2014, 14:44:38: .
Title: Re: Confirm my understanding of static electricity
Post by: dorkvader on Mon, 17 November 2014, 14:54:22: Quote from: esoomenona on Mon, 17 November 2014, 14:44:38
How many materials truly have an infinite resistivity?

none. You can even get electrons to flow in a vacuum via thermionic emission.

The reason there is a spark is because you have several thousand volts built up and the resistivity of air is like 40KV/cm so once you get close enough they jump the gap to the door which is usually pretty close to earth ground.

I'm not sure about touching a windowpane and if that would help. I think if the windowpane had a ground clip, then it could absorb the charge with less of a spark since the total resistance to ground is higher, but it would depend on things like: the resistivity of the window pane, the caulk, how it's mounted, etc.

I just take the shock. It's not dangerous.

So in the "doorknob: case, air is the intermediate conductor. If you ohms law it out, if you have 3KV charge on a hand flowing through a door to ground (lets say 10 ohms or so) then that's 300A (for very briefly). What actually happens is that when you are very close to the knob (roughly 0.75mm for a 3KV charge) then the electrons will arc through the air to the doorknob causing a shock you feel.

I'm thinking of the windowpane has a very high resistance to ground (more than air-to-knob-to-ground) but still some electrical pathway touching it will allow the charge to dissipate with no arc through air.

You can easily build up millions of volts with plastics and other insulators. Look at how DIY van de graaff generators are made. But I think mostly static electricity is in the range of a few KV to tens of KV max. Low current of course so it's pretty safe.
Title: Re: Confirm my understanding of static electricity
Post by: tp4tissue on Mon, 17 November 2014, 15:15:03: If only you could build up AMPS and not die instead of Volts.... sigh......(http://emoticoner.com/files/emoticons/onion-head/depressed2-onion-head-emoticon.gif?1292862499)
Title: Re: Confirm my understanding of static electricity
Post by: dorkvader on Mon, 17 November 2014, 15:27:18: Quote from: tp4tissue on Mon, 17 November 2014, 15:15:03
If only you could build up AMPS and not die instead of Volts.... sigh......
Show Image
(http://emoticoner.com/files/emoticons/onion-head/depressed2-onion-head-emoticon.gif?1292862499)

You can build up amps and not die. The short-circuit current of a lead-acid battery is many hundreds of amps, but if I bridge it with my hands I won't die.

You need 2 things.

1. voltage high enough to overcome your body's high resistivity.
2. a supply whose current limit is high enough to deliver.

So, for example, a van de graaff generator is safe because it can't push enough current, even though the voltage is certainly there. A lead acid battery won't work because there's not enough voltage to deliver the required current.

Some things will absolutely meet both requirements and are extremely dangerous (at which point, I like to mention tesla's laws of working with High Voltage) but the whole "voltage vs current killing you" is almost always an oversimplification.

Oh the final thing is that it has to run through the wrong part of your body. I've run almost-certainly-lethal amounts of power through parts of my body but nothing through the torso.
Title: Re: Confirm my understanding of static electricity
Post by: tp4tissue on Mon, 17 November 2014, 16:05:11: I kinda meant using ur body as the capacitor with m0re amps.. but.. yea. thx for the physics update .. (http://www.cute-factor.com/images/smilies/onion/069.gif)
Title: Re: Confirm my understanding of static electricity
Post by: intelli78 on Mon, 17 November 2014, 16:09:57: Quote from: dorkvader on Mon, 17 November 2014, 15:27:18
You need 2 things.

1. voltage high enough to overcome your body's high resistivity.
2. a supply whose current limit is high enough to deliver.

In other words it is the power, or the WATTS that kills you. Energy transferred to you over time.
Title: Re: Confirm my understanding of static electricity
Post by: tp4tissue on Mon, 17 November 2014, 16:28:59: Quote from: intelli78 on Mon, 17 November 2014, 16:09:57
Quote from: dorkvader on Mon, 17 November 2014, 15:27:18
You need 2 things.

1. voltage high enough to overcome your body's high resistivity.
2. a supply whose current limit is high enough to deliver.

In other words it is the power, or the WATTS that kills you. Energy transferred to you over time.

well the first story they tell you is 1 amp can kill you.. vs 1 volt ... the unit is what makes the exaggerated comparison possible
Title: Re: Confirm my understanding of static electricity
Post by: intelli78 on Mon, 17 November 2014, 16:36:25: Right... Power = Voltage * Current,

even with 1 volt or 1 amp, if the other is sufficiently high enough, the power delivered becomes huge and fatal (as dorkvader mentioned, assuming it goes through your brain or heart, or cooks enough of your flesh)