Why Doesn't a 1.5V Battery Shock You?

[berkeman]

That's what I don't get. How can a 10kV generator pass more current through your body than a 10kV static shock? Shouldn't the current be the same in both cases? By Ohm's law, I = V/R and if you assume that your body has some finite resistance, then at a particular voltage (10kV in this case) the same amount of current should be passing through body, right?

Think in terms of source impedance and energy storage. With your body, it's not a constant 10kV source voltage with a low output impedance. Your body is only a handfull of pF worth of energy storage, and the shock event depletes the charge to almost zero very quickly.

 

[MedievalMan]

IMGOOD, I see what you're saying.

However, I think everything isn't always as simple as Ohm's law, or we are viewing the problem the wrong way.

Ultimately, the static potential is just that - a potential, with little to no capability to "pump" a lot of current.

However, a 10kV generator, spun by a large mechanical prime mover, can pump out lots and lots of amps (and thus V*I=power).

I don't know if these are exactly realistic numbers (atleast for the path resistance), but let's assume the following:

Let's assume you are holding your hands across the armature windings of a giant, spinning,dc generator ( I wouldn't recommend this), and compare to a static discharge when moving from your desk:

Vstatic=10 KV
Istatic= 0.0001 A (negligble amount of amps the static discharge can source)
Vgenerator=10 KV
Igenerator(max)(due to winding resistance limitations, prime mover limitations etc etc)= 500 A.
Rgenerator(armature)=10 Ohm

Rpathofhuman= 990 Ohm, Rpath_effective=Rgenerator+Rpathhuman=1000 Ohm

So, the static electricity can only source 0.0001 A. So, the "power" dissipated by you when you move from the chair is:

P=V*I= 0.0001 A * 10 kV = 1 w (it doesn't matter that I=Vstatic/Rpath=10A, there's no way a static discharge is going to source 10A @ 10,000 kV , else everyone would be fried by standing up from their desk.. :) )

The generator, could source up to 500A. However, since Reffective=1000 Ohm, it will only source, depending on the speed (back emf) (assume low):

I=V/R= 10kV/1000=10A
So, P=V*I = 10,000 V * 10A = 100,000 W (i.e. you are toast)

I'm not sure if this is a good analogy, someone correct me if I'm way off base.
I'm not really willing to do a real experiment on it :)

 

[tehno]

The static shock would have to get into your heart or brain, which doesn't usually happen with real-world zapping yourself accidentally. Defibs use conductive gel to help get the shock into your chest (at two points to cross the chest cavity with the current), and the heart is not beating anyway.

Bottom line is that low-energy ESD shocks shouldn't normally be dangerous. But as you say, shocks from other sources with more energy can be dangerous.

I mean data relating typical path (like arm-chest-arm-ground or head-legs-ground).
What's the lower limit considered dangeorous for adults when it comes to ESDs (1 Joule,10 Joules,100 Joules..)?
Here are two examples:
1)http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=10060
Small shock from a small electrostatic generator with 150 KV output

and...

2) http://www.mos.org/sln/toe/cage.html
Without Faraday cage protection ,produced would be a very strong ,and potentialy leathal shock, from a giant electrostatic generator of 1500 kV output.

 

[tehno]

Hey berkeman,

I just got a response that 50 Joules from 100pF capacitance,if discharged directly,can knock you down with no problem.
Well well how about that...

 

[chroot]

Hey berkeman,

I just got a response that 50 Joules from 100pF capacitance,if discharged directly,can knock you down with no problem.
Well well how about that...

That's kinda stupid. A 100 pF capacitor would have to be charged to a million volts to store 50 J. Good luck finding such a capacitor.

50 J is also the kinetic energy liberated when a 0.3 lb baseball hits you at 60 mph. That'll knock you down, too.

- Warren

 

[seang]

What are the biggest outlets around? There's a 600V in our school's lab.

 

[tehno]

Easier than you expect

That's kinda stupid. A 100 pF capacitor would have to be charged to a million volts to store 50 J. Good luck finding such a capacitor.

Not that I'm looking for it ,but a 5 feet diameter smooth metal ball elevated 5 feet above ground plane is kinda construction plan for such a capacitor.
For starter,not very complicated I think .
BTW,see the link above I found about giant Van de Graaf in Boston ( ~8' spheroids)!