# How many volts just went through my body?

I just opened up the inside of a disposable camera. BAD IDEA. I accidently touched the contacts to the capacitor, and it shocked the hell out of me. Why did this hurt so much? I know capacitors store up electrons but i thought they retained the same voltage as the initial battery. So exactly how many volts and amps just soared through my arm? The capacitor was a 120 micro Farads and 330 WV.

The capacitor stores the energy needed to power the flash. The flash is a xenon discharge tube that requires a high voltage (a couple of hundred volts) to get going, so the camera contains a circuit to generate a high voltage from battery power and store it in the capacitor. It’s usually around 300V. There’s a decent description here:

http://www.ee.washington.edu/conselec/CE/kuhn/labs/camera/fl.htm

Typically, these things fire at around 200-300 V with a 2 kV to 4 kV trigger. Since the cap is rated for 330 V, it can’t be any than that, so you got the firing voltage (which provides the current to make the flash bright), not the triggering voltage (this ionizes the xenon gas in the tube so the firing current can flow). Incidentally, volts don’t go through your body, they go across it. Current is what actually goes through you and hurts.

Yep, don’t be worried about the volts it’s them amps what do the killing.

I’m curious about this too. What’s the big deal about the voltage? I thought a plain old static shock from shuffling across the carpet and touching a light switch was ~1,000-2,000 volts. I remember playing with a Van de Graff generator in my high scholl physics class that the teacher said was around 50,000 volts. Not too bad there either…at least I’m still alive and it felt no worse than a decent sized static shock from touching a light switch.

I too thought it was amps that got you and you really needed to be more worried about.

This may be a hijack but real quick here is the analogy that I understand to explain volts versus amps…I’m sure someone here can tell me if it is accurate.

Compare electricity to a stream of water. Volts equates to how much water there is in the stream. So, say, you have the Mississippi river. LOTS of water (equating to high voltage) but low flow rate (current or amps). Jump in the Mississippi and no problems.

Now have someone turn a fire hose on you. Nowhere near the amount of water compared to the Mississippi but the current (amps) are far higher and it’ll knock you on your ass and hurt to be hit by it. Taken to an extreme I’ve seen water jets used to cut masonry but the stream of water looked like not much more than a Water-Pik. Pass your finger through the masonry cutter water stream though and you’ll lose that finger. In this case VERY high current (amps) but very low ‘mass’ (volts) of water.

It’s the amps that kill you, but it’s the voltage that creates those amps. In order to kill you, you need a source of electricity with both high voltage and the capacity to deliver high current. A low-voltage high-current source (e.g. automotive battery) is not dangerous because it can’t pass enough current through your body to kill you. Likewise a high-voltage low-current power source like a Van de Graff generator cannot deliver enough current to kill you. A 300-volt 120-uF capacitor in a camera does have enough voltage and current capacity to kill you, so be careful next time!

We’ve had this discussion before. That “it’s not the volts it’s the amps that kill you” is more or less BS. Yes, it takes a certain number of amperes flowing through sensitive organs like the heart or the brain to cause death, but the number of volts needed to drive this current varies wildy, depending on the circumstances under which the shock is delivered. 12 volts from a car battery can be lethal, if the conductors carrying it penetrate the skin to the salty, conductive fluids within in the right spots. meanwhile, a 120 V jolt from a wall socket might not kill you if your skin is dry and the contact is only on the outer surface. The impedance of the source is important too. Very high impedance sources, like Tesla coils deliver much less of a jolt volt per volt than a low-impedance source. A 7200-volt neon sign transformer has a fairly high impedance and delivers a nasty, but normally nonfatal shock. The low-impedance 7200-volt overheads serving your neighborhood’s pole transformers in the US is almost instantly fatal, on the other hand. So, it isn’t just amps doing the the killing, it’s amps, volts and ohms all working together.

Oops, I shouldn’t have said a car battery is completely safe. It’s usually safe, but under some circumstances (wet or broken skin) it can kill you. Best not to touch both terminals. I see on preview QED has already said this but I should emphasize just the same…

Whack-a-mole: I don’t think your analogy is quite correct. Volts would be the equivalent of water pressure, with amps being the equivalent of the flow rate - amount of water passing a given point per unit time.

Not exactly. Volts would be the pressure of the water, while current would be the rate of flow. The Mississipi River is a big fat pipe (low resistance) so a lot of water (currrent) can flow with a fairly low pressure (voltage). On the other hand, the water cutters use a very high pressure (voltage) to push a lot of water (current) through a very small opening (high resistance). The water/pressure thing isn’t a great analogy, but it’s a decent starting point.

So further to the water analogy:

High voltage and low current is like water dribbling over the edge of Angel Falls.

Low voltage and high current is like a lot of water pouring over a low wier across a big river.

For pain, muscle contractions, and heart problems, it’s not just the volts, amps, joules, etc. It’s also the frequency.

Your nerves only respond to low frequency electrical signals, and if the freq. is higher than a few kilohertz, sometimes you can’t even feel anything. Pass a few hundred milliamps through your body at 50KHz and you don’t even know it’s happening. But do it at DC or at 60Hz and you’re screaming in agony. The pain and muscle contractions are caused by nerve stimulation, not by the “electricity” itself.

On the other hand, if the total energy pumped into your body is large, then heat becomes a big issue. High power electric shock is painful because it’s cooking you internally. Low power electric shock is only painful because it’s interfering with your nervous system (and if the frequency is too high, then your nerves can’t “hear” it.)