When you shock your self due to build up of charge

Molondo

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Sep 6, 2005
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Just saw on tv, they claimed its about 30k Volts. How much current would there be?
 

PottedMeat

Lifer
Apr 17, 2002
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Not very much - maybe nanoamps/microamps or something?

Like discharging a capacitor I guess - current peak at the start and decrease with time and stop flowing when the voltage is less than the dielectric strength of the air or when the sides are equalized.

 

wwswimming

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Jan 21, 2006
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since current is defined as

1 amp = 1 joule/ 1 second

(i think), the peak transient current may be higher.

it would be interesting to put a low impedance current analyzer in the loop & measure.

 

Modelworks

Lifer
Feb 22, 2007
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yeah its very very low current.
Like the jacobs ladders you see in movies where the electricity travels up the wires.
Some of those use voltages in the 50-75kv range.
The current though is extremely low.
 

Mark R

Diamond Member
Oct 9, 1999
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Depends on the conditions and resistance, etc.

I suppose the current could approach 100 mA depending on the resistances/capacitances of your body/skin/touched object. However, at this sort of current, the discharge will be extremely brief (a few dozens of nanoseconds).
 

DrPizza

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Mar 5, 2001
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Originally posted by: wwswimming
since current is defined as

1 amp = 1 joule/ 1 second

(i think), the peak transient current may be higher.

it would be interesting to put a low impedance current analyzer in the loop & measure.

Weird. How is someone able to spit out the words "low impedance current analyzer" and "peak transient current" but have 1joule/sec for an amp? It's 1 coulomb per second, or 6.25 * 10^18 electrons per second. Joules are energy. Joules per second would be a rate of using energy - aka power. 1 Watt = 1 joule/sec

OP: If you've ever seen a Van De Graaff (the shiny ball that people put their hands on and their hair stands up), a typical one used in a classroom or science museum generates a potential difference (voltage) of 100,000 V or more; 200,000 volts not being out of the question. I've actually used a voltmeter to measure the current for a Van De Graaff, or rather the rate that charge is transferred from the belt to the dome on top; I always do so when I'm fine tuning it to maximize the rate at which it charges. Darn it.. I can remember that the best I got was 2.1 x 10^-# amps, but I can't remember the exponent. I typically got 1.8 times 10^ something amps... If I had my multimeter with me, I could just look at the range I use... It may be 0.18 microamps, but don't quote me on that.
 

BrownTown

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Dec 1, 2005
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remember that this is an electrostatics problem, the current will decay exponentially with time (sorta). I'd image more realistically that initially upon dielectric breakdown of the air the current will rapidly increase due to the decrease in resistance as the air is ionized, however this action will be counterbalanced by the fact that the charge on the finger and therefore the electric field across the air gap will be decreasing rapidly. At some point after the current has risen to a peak value the second action will begin to dominate and the current will begin to go down, as this happens the energy going into ionizing the air is also reduced meaning the resistance will increase again further limiting current. As a result the ionized particles will recombine much faster than they are formed and eventually the dielectric strength of the air will return to a high enough point such that spark will be completely extinguished.

In summary, as you suspected the current is clearly very small and further the time duration is very small so the total energy released is not enough to cause any harm (although it sure can hurt sometimes).
 

Mark R

Diamond Member
Oct 9, 1999
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I was curious so I had a look through some of the literature out there. Most of it is to do with protection of electronic devices against ESD.

However, it does suggest that currents of over 1A (highest measurement I've seen is 1.2 A) can flow during discharge from a 'human body model' - basically a capacitor/resistor combination that are supposed to be roughly equal to the net equivalent of a human body. Similar currents are reported by teams working on hardening computer I/O connections (e.g. ethernet) against static (e.g. if a user plugs in a cable after having dragged it across carpet).

Time at peak current is short, just a few ns - with the overall pulse being 30-100 ns.
 

Navid

Diamond Member
Jul 26, 2004
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Originally posted by: Molondo
Just saw on tv, they claimed its about 30k Volts. How much current would there be?

http://www.esda.org/basics/part5.cfm

The human body model is a 100pF capacitor charged to the ESD voltage in series with a 1500-Ohm resistor.

ESD voltage varies. It depends on many factors like how dry the air is, what material is touched to cause it, .....


If the ESD voltage is 30kV, the starting current will be 30000V/1500 Ohms = 20A!
The time constant of such circuit is 1500 Ohms x 100pF = 150ns. The current will reach 0 in 5 time constants. In this case, 5 x 150ns = 750ns.
 

wwswimming

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Jan 21, 2006
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Originally posted by: DrPizzaWeird. How is someone able to spit out the words "low impedance current analyzer" and "peak transient current" but have 1joule/sec for an amp? It's 1 coulomb per second, or 6.25 * 10^18 electrons per second. Joules are energy. Joules per second would be a rate of using energy - aka power. 1 Watt = 1 joule/sec

good point. a combination of dendritic growth in my brain and an under-supply of caffeine in my blood, perhaps.

looking forward to telling one of the guys at the gym who works at Agilent. if it makes him laugh, my little mistake is worth it.

without going to a reference, i'm wondering if 1 coulomb sitting at a potential of 1 volt has an energy of 1 joule.
 

Navid

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Jul 26, 2004
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You will need 1 Joule of energy to move 1 Coulomb of charge from point A to point B if the potential at B is +1 Volt higher than potential at A.