Any Physicists or Electricians here?

[Hellspell]

For years now I've not been able to understand how this works.

How can I touch one plate of a capacitor and not be shocked, but when I touch both of them I do get shocked? Say I touch the positive terminal and I am in one of those moods where I decide against floating in mid-air. Why don't electrons flow from the ground, through me and give me a shock?


Or perhaps imagine crocodile clips in a power supply. Why can't electricity shock you when you touch the one clip, and are earthed?

And how does the third rail manage to electrocute people who are earthed but not the crocodile clip, or charged plate. Because they all have a voltage, being connected to a switched on power supply, shouldn't they all shock me?

[TheNihilanth]

Well, you are in the wrong forum.
Try https://forum.allaboutcircuits.com, that is the forum that you are looking for. I've asked questions on the past there and I always had feedback, even to my surprise I got answers from one the gods of analog op-amps, Ron Mancini.

Anyway, I'll take the shot.
You are talking about an electrolytic capacitor, which has polarity, if the capacitor is charged and you only touch one terminal (positive or negative, doesn't matter) even if you you are touching ground bare feet, the other terminal is not tied to a reference, is there floating in the air, therefore the circuit is not closed, this is why you don't get shocked since is there is no current flowing across you as a result of a voltage difference, when you tie the other terminal to a ground reference the circuit is closed and therefore a voltage difference now exists and a current flows across your body and you experience a shock. Same thing with the power supply.

In the case of a 3 phase line, A, B and C lines carries a voltage, each one of said lines has a reference level (ground), and when you touch one of those lines (any of the 3) and you are touching ground you get electrocuted, fried, you hang in the sneakers, in other words you are dead. In case you wonder why birds are not fried when they are standing in a live line is because the voltage difference is zero, therefore no current flows across the birds.
Hope that helps a little bit.

[Ronald_Reagan]

Isn't this the fact that once electrons leave, there won't be anything to replace them? Sort of like holding a bottle of water upside down. Water doesn't flow unless air can get back into the bottle.

[TheNihilanth]

I didn't understand your question. But, as an active element a capacitor can stored a fixed amount of energy and once this energy is depleted the energy will remain zero on the capacitor until it is 'refiled'. How will get 'refilled' or how the capacitor operate under certain conditions will depend of the other circuit elements that interact with said capacitor(s). That is the circuit network configuration.

[Mousikos]

Don't dick around with capacitors. There's a reason you don't open CRTs unless you know what you're doing.

Vcurrent=Vintial(e^(-t/tConstant));

When you say "shocked", you mean that you can feel the current running through your body. I believe you can start feeling it in the 3mA range, but I'd have to look that up again.

Remember Ohm's law: V=iZ, or in this case, the one you're interested in can be rewritten as i=V/Z.

The resistance from your hand to the ground and from the ground back up through the air into the other terminal is very, very big. It can be modeled with an open circuit. Hence, it is "safe" to touch. On the other hand, if the energy stored in the capacitor is great enough so that even with that big resistance, there is NOT a negligible amount of current, you will get shocked.

I really don't want to answer your crocodile clip example because I don't have my EM textbook on me... I want to give the same example, but let's just say rubber or whatever it's made of doesn't conduct electricity as well.

Another piece of advice, if you're going to do something stupid (please don't), do it with one hand, and not two. Two hands puts the circuit through your heart. You die. One hand, you'll get injured but hopefully won't die.

Finally, capacitors explode. Electrolytic ones will send shrapnel EVERYWHERE, hurting you and everyone around you. Always double check polarity.

[TheNihilanth]

I wonder if OP did not get electrocuted and is still alive.

[Mousikos]

Jesus, TheNihilanth. Now I'm kind of worried..

[Waffles]

definitely a good idea to take a skill saw and chop through the main power line of a house.

[Krayce]

In the case of a 3 phase line, A, B and C lines carries a voltage, each one of said lines has a reference level (ground), and when you touch one of those lines (any of the 3) and you are touching ground you get electrocuted, fried, you hang in the sneakers, in other words you are dead.

You can even be electrocuted without touching anything if you are near to a high tension place (eg. air connection...).
We can illustrate what you said by this image below, instead of letters A/B/C I used numbers 1/2/3 (dedicated to Assault Cube FTW):

PS: Sorry for the image quality, I have again a prehistorical phone with 2.3MP!

In case you wonder why birds are not fried when they are standing in a live line is because the voltage difference is zero, therefore no current flows across the birds.
Hope that helps a little bit.

Yep, and because they touch only 1 live line, if they touch 2=dead.

Remember Ohm's law: V=iZ, or in this case, the one you're interested in can be rewritten as i=V/Z.

Yeah, and you can simplify this by;
i=V/R"omega"(live line)+R"p"(protection line)

When you say "shocked", you mean that you can feel the current running through your body. I believe you can start feeling it in the 3mA range, but I'd have to look that up again.

In fact, this start at 0.5mA in AC and 2mA in DC

Human body resistance is:
5000 ohm (no wet)
2500 ohm (wet)
1000 ohm (immersed)

Tension contact:
25V (wet)/ 50V (no wet)

[1Cap]

even a small shock can kill you or leave sequelae on you.

the resistance of the human body can vary greatly. depends to their feeding or blood type that you have or other factors.
dont tests. do not joke about it.
You can die in touching a neutral wire or a return wire, it because this wire may be touching on another wire.

capacitors are dangerous. batteries are dangerous.
very dry locations can produce static electricity and it can hurt you too.

you are curious about it? electricity ? make a basic, practical course about it. Internet content may be inaccurate, difficult to understand, could be wrong. do not risk your life on something you have read on the internet. be smart.

[TheNihilanth]

In the case of a 3 phase line, A, B and C lines carries a voltage, each one of said lines has a reference level (ground), and when you touch one of those lines (any of the 3) and you are touching ground you get electrocuted, fried, you hang in the sneakers, in other words you are dead.

You can even be electrocuted without touching anything if you are near to a high tension place (eg. air connection...).
We can illustrate what you said by this image below, instead of letters A/B/C I used numbers 1/2/3 (dedicated to Assault Cube FTW):

PS: Sorry for the image quality, I have again a prehistorical phone with 2.3MP!

Yes that's 100% correct.

And this is why one must not camp near the transformers in ac_power, kids.

[Ronald_Reagan]

Another piece of advice, if you're going to do something stupid (please don't), do it with one hand, and not two. Two hands puts the circuit through your heart. You die. One hand, you'll get injured but hopefully won't die.

Correction:

[...] if you're going to do something stupid (please don't), make your child do it. Remember, this is why you have multiple children.

Dave Barry anyone?

[Hellspell]

Dont worry. I dread to think of ever messing around. Not after I was tricked into being given a 5000 volt shock by my teacher. Thats what the power brick said, it sounds crazy high to me. How would I calculate the curent that flowed through me at that time? Would I use I = V/R, or do I need to know the power rating of the power source?

Also, if im not becoming (more of) a bore, why do we say "voltage across" and not 'voltage supplied to', or 'electric potential of'.

[Mousikos]

V=IZ. If the current drawn by load is greater than the rating that the source can supply, the voltage starts dropping.

It is voltage across because voltage is always given with respect to a reference point. So you are measuring the voltage from one side of the thing in the circuit. Generally, if you give a voltage without a reference point, it is understood that you are measuring with respect to ground.

[Krayce]

[...] if you're going to do something stupid (please don't), make your child do it. Remember, this is why you have multiple children.

Dave Barry anyone?

This guy can do something stupid i think:


And also this one:
http://www.youtube.com/watch?v=Q2__ODU_QA0

[Boomhauer]

Easy answer: Electricity always takes the path of least resistance.

[foa.trionkali]

For years now I've not been able to understand how this works.

How can I touch one plate of a capacitor and not be shocked, but when I touch both of them I do get shocked? Say I touch the positive terminal and I am in one of those moods where I decide against floating in mid-air. Why don't electrons flow from the ground, through me and give me a shock?


Or perhaps imagine crocodile clips in a power supply. Why can't electricity shock you when you touch the one clip, and are earthed?

And how does the third rail manage to electrocute people who are earthed but not the crocodile clip, or charged plate. Because they all have a voltage, being connected to a switched on power supply, shouldn't they all shock me?

I think you should not terry on such shocking revelations. ;P