Why Doesn't a 1.5V Battery Shock You?

[IMGOOD]

Why is it that when you put your two finger on the opposite terminals of a 1.5 V battery, you don't get an electric shock? (Or maybe you do, but you just don't feel it?). I am guessing that this is because our body has a high resistance, is that correct?

 

[chroot]

Yes, your body (more specifically, your skin) has a rather high resistance. A small amount of current still flows, but it is not sufficient to cause sensation.

A few years ago, the automotive industry decided upon 40V as the maximum safe voltage for human exposure; this is still well below the threshold of sensation. The minimum voltage required for sensation depends on specifics like skin moisture, but is usually above 60V.

Obviously, the mucosa presents a much lower resistance than does the skin; you can quite easily feel 10V placed across your tongue, for example.

- Warren

 

[IMGOOD]

Thanks a lot chroot.

 

[Number2Pencil]

can you measure static shock in volts? it smarts

 

[berkeman]

can you measure static shock in volts? it smarts

You mean like when you get up off your cloth-covered chair or walk across a dry rug? That shock is several kV.

There is actually an industry standard set of tests (EN 61000-4-2) that we use to test products to be sure that they can survive and continue operating normally when hit with an electrostatic discharge (ESD) transient. We test up to 15kV air discharges for most products.

 

[tehno]

A few years ago, the automotive industry decided upon 40V as the maximum safe voltage for human exposure; this is still well below the threshold of sensation. The minimum voltage required for sensation depends on specifics like skin moisture, but is usually above 60V.

Ho-Ho-Ho!
Ever tryed to turn off and on a 50V DC source while holding its electrodes?I highly recommend not trying it.
You might simply find yourself jumping up and down to a ceilling.

 

[triden]

Ho-Ho-Ho!
Ever tryed to turn off and on a 50V DC source while holding its electrodes?I highly recommend not trying it.
You might simply find yourself jumping up and down to a ceilling.

Yeah I remember back in Physics 12 where we were doing an electricity lab with some DC bench power supplies. Anyhow, we cranked it to 50v (its max output) and grabbed the electrodes. It gives a pretty good tingle, little too much for me to handle for more than a second.

 

[hover]

Ho-Ho-Ho!
Ever tryed to turn off and on a 50V DC source while holding its electrodes?I highly recommend not trying it.
You might simply find yourself jumping up and down to a ceilling.

Ever accidentally grounded yourself to an 120V AC outlet ? That really hurts ...

 

[foges]

if you place a 9V battery on your tounge you can actually feel the shock :D

 

[saad ahmed khan]

ya i grounded my self to a 220 V outlet quite a lot of times :
hurts like hell but i am still alive

some one tell me which is more dangerous
220V AC or 220V DC
for the same conditions

 

[chroot]

DC is usually considered more dangerous than AC. 60 Hz AC has instants of zero voltage 120 times a second. During the portions of the sine wave where the voltage is rather small, you might be able to let go.

- Warren

 

[saad ahmed khan]

DC

this i got to stay away from

once you are stuck there is no letting go...
am i right??

 

[saad ahmed khan]

thanks chroot by the way

 

[MedievalMan]

Mr. Eddison was so convinced AC was the devil (very very dangerous) compared to DC, he used the electric chair as his advocate. ;)

 

[tehno]

Ever accidentally grounded yourself to an 120V AC outlet ? That really hurts ...

Actually to 380V AC and lived to tell about it.
Not just that it really hurts but I was thrown across the room.
That incident when I was kid,thought me a lesson not to mess with electricity above 10 V (AC or DC whatsoever...)

 

[IMGOOD]

You mean like when you get up off your cloth-covered chair or walk across a dry rug? That shock is several kV.

Then how come that doesn't kill you?

 

[MedievalMan]

10-20kV "static electric" shock like when you get up from your desk isn't dangerous.

Sure, such a shock could ignite gasoline, but it's not going to hurt you otherwise.

Why?

It's CURRENT that kills you, not voltage, technically.

That 10-20KV static electricity is just a potential, very little to no current flows through you.

Now, hook yourself up to the terminals of a giant 10-20KV generator, and you're toast. That's because it will pump out tons of amps at 10-20kV (toasty!)

So when the electrical power areas say "Danger, High Voltage" it should really say "Danger, high voltage and current". Simply high voltage potentials aren't going to hurt you ;)



-Matt

 

[berkeman]

10-20kV "static electric" shock like when you get up from your desk isn't dangerous.

Mostly correct, but I'll add a little bit. The static shock you get on a dry day when you touch something would rarely get over 5kV. And as MedievalMan alludes to, the energy storage of your body is low. Also, the current flow is from the exterior of your body surface out your fingertip or elbow or whatever, and very little of it runs through the inside of your body where your heart and brain would be vulnerable.

Now having said that, I managed to accidentally shock myself arm-to-arm one day while doing 20kV ESD testing of a product (dumb error on my part). The EN 61000-4-2 testing that I was doing is designed to mimick the source impedance and energy storage of a human, while going to higher voltages than you will normally encounter (to ensure that your product is robust in the real world). I have to tell you, that arm-to-arm shot definitely went partially through my chest cavity, and my heart skipped a beat. Definitely not something I want to repeat (and I've been more careful about the error I made that resulted in the shot).

 

[tehno]

Mostly correct, but I'll add a little bit. The static shock you get on a dry day when you touch something would rarely get over 5kV. And as MedievalMan alludes to, the energy storage of your body is low.

Berkemen, do you know what's the threshold energy of ESD life dangeorous to adult humans?I bet medicine physicians working with defibs probably know...

 

[MedievalMan]

This is probably a bad analogy, but anyway:

Think of the amount of damage done to you by electricity in terms of power:

P=V*I

So, even if you have an extremely high V, if the I is negligble (as is the case of static shocks from your desk) it doesn't damage you.

Similarly, a car battery could kill you at 12V, since the battery can source a lot of amps.



(This might be a bad analogy here guys, correct me if I'm wrong.)

 

[berkeman]

Berkemen, do you know what's the threshold energy of ESD life dangeorous to adult humans?I bet medicine physicians working with defibs probably know...

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.

 

[hover]

Actually to 380V AC and lived to tell about it.
Not just that it really hurts but I was thrown across the room.
That incident when I was kid,thought me a lesson not to mess with electricity above 10 V (AC or DC whatsoever...)

ya i got hit with 120 AC when i was a kid too. I accidently grabbed the metal of a plug while it was in an outlet. I was a kid, what do you expect??

 

[light_bulb]

i'd expect it would have something to do with the path through your nervous system, if it's only two fingers on your hand it should have less impact then both hands. when i was little i stuck my finger and a screwdriver in a wall socket and came out with a black finger.

 

[Number2Pencil]

So let me get this straight (on the issue of static elect)...

since the body doesn't store very much energy to perform this

W = IVt.

So even though V is big, W is still small

W/V = It

so you have small divided by big which = smaller...and this is why the current is small and it doesn't last long (IE not dangerous)?

 

[chroot]

Static electricity as experienced by people doesn't typically involve very many electrons. Even if you only pull a few thousand electrons off a material, you can create enormous voltages. When discharged, there's a surprising shock, but only a very small amount of energy is really released.

- Warren

 

[light_bulb]

using the pipe and water analogy, if you visualize the potential difference (speed of the water) and current (width of pipe) you'll see that a lots of pressurized water through a small pipe will sting or cut you, through a large pipe it turns into a fire hose.

 

[Panda]

Fatal shocks are as much to do with current paths as intensity.
There is a nerve at the base of your heart that fires the heart muscle, defribrulators stimulate this nerve.
You can get big shocks through your body but if they don't cross your chest the nerve is not affected.
Static shocks are body discharges rather than current flows from point to point, so the current passing across the heart nerve is small.
When working with hazrdous voltages on a bench I work with my left leg off the floor and left arm off the bench, so any shocks would pass down one side. Of course that method doesn't counter the simpler rule of thumb about not sticking your head in a live circuit to start with.

 

[light_bulb]

using a foot pedal as a dead mans switch and a varaic with some type of current inrush protection would work wonders. that's a case where an esd mat and wrist strap could kill you

 

[Averagesupernova]

using a foot pedal as a dead mans switch and a varaic with some type of current inrush protection would work wonders. that's a case where an esd mat and wrist strap could kill you

Any wrist strap I've ever used has a built in 1 megohm resistor for just such protection.

 

[IMGOOD]

That 10-20KV static electricity is just a potential, very little to no current flows through you.

Now, hook yourself up to the terminals of a giant 10-20KV generator, and you're toast. That's because it will pump out tons of amps at 10-20kV (toasty!)

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?

 

[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)!