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A few circuits questions...

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Unread postAuthor: Mr.Tallahassee » Wed Oct 12, 2011 9:14 pm

From what I understand, capacitors have no actual net charge no matter what. It seems that applied voltage moves electrons from one side of the capacitor to the other so one side is exactly opposite in charge to the other. When you remove the voltage the electrons can't go trough the dielectric or through the air(due to resistance) so it maintains a charge for some time. The theoretical reason that this happens is because of the permittivity of dielectric. The charge can't cross the dielectric but the field induced between the plates, which is much like that of a transformer, is built up until you make contact between the terminals of the capacitor.
On a side note, I built a massive three plate capacitor from the plastic film on three ring binders and aluminum plates. It was large enough that the arcs from my flyback HV generator were as big around as a pencil lead. It was 6.5'' by 8.5'' and was a scary piece of equipment.
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Unread postAuthor: Technician1002 » Thu Oct 13, 2011 9:37 am

Zeus wrote:What do you consider "ground"? I see it as the circuit return, but I think you're seeing it as earth?

If you earth one side of a capacitor, and the target is in contact with earth, then they will be shocked. If I were you though, I'd just have electrodes running parallel to the baton, alternating between each lead of the capacitor.



Remember anything in the current flow path will be shocked when current flows. If this is a hand held batton, and you are the ground path, shocking someone with it will zap both of you. Tasers and stun guns work by using two electrodes. What ever bridges the gap gets zapped. The operater is not part of the current flow path so he does not get zapped even if the operator and victim are standing in the same puddle of water of on the same metal deck.

As mentioned earlier, only a complete circuit carries the current. There are exceptions with higher frequencies and object capacitance such as tesla coil circuits and RF but that is another subject. You can get an RF arc burn even on an insulated ladder. Large objects have enough capacitance to draw arcs from AC lines. Example is in the video below.

A shunt connection to bypass an arc path is used to protect the person from the arc current. Once the two objects are at the same voltage the current path through the person is no longer a hazard.

I like the end of the video. There are only three things he is afraid of. Electricity, Heights, and women.

[youtube]http://www.youtube.com/watch?v=LIjC7DjoVe8[/youtube]
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Unread postAuthor: Hotwired » Thu Oct 13, 2011 11:29 am

Wonder what kind of danger pay you get for that lark :shock:
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Unread postAuthor: Technician1002 » Thu Oct 13, 2011 4:47 pm

It actually pays pretty good. Not as good as a surgeon, but more than my current job. There were openings a few years ago, but due to affirmative action, the entrance score for white guys is quite high. I went into private sector instead.

Safety on that job is serious business. There are very few injuries.

Disclaimer My dad is a retired substation operator. I would have been too except for reverse discrimination called Affirmative Action.

Affirmative action refers to policies that take factors including "race, color, religion, gender, sexual orientation or national origin"[1] into consideration in order to benefit an underrepresented group, usually as a means to counter the effects of a history of discrimination.


http://en.wikipedia.org/wiki/Affirmative_action
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Unread postAuthor: jimmy101 » Thu Oct 13, 2011 7:54 pm

saefroch wrote:When charged, electrons migrate from one side of a capacitor to another, according to a logarithmic function (IIRC), which tapers off dramatically. So they charge almost to a limited voltage.

The earth is normally considered an infinite source of positive and negative charges. So I conclude that I can use electrons from the earth and cram them onto one side of a capacitor, leaving zero charge on the other side, since it's connected to the earth, and as soon as electrons are taken off, they're replaced.

Nope. When you charge a cap you push one electron in one of it's wires and one single electron flows out the other wire. In this mode a cap looks like a short circuit, that is it does as long as an infinitly small current is flowing and the cap has zero charge across it. But electrons never cross between the two sides of the cap (if they do the cap has failed or is leaking).

The "logarithmic" function comes from the difficulty in shoving the second electron into the cap when there is already an excess electron on that plate of the cap. Electrons repel each other so it takes more work to push the second electron in than it did for the first. The second electron also ejects an electron from the other plate of the cap and out the other wire. Repeat ad naseum since a typical cap holds zillions of electrons when charged.

Can a cap be discharged to ground? Depends on what you mean. If you take a cap and hook it up to a 1.5 V battery, then disconnect the battery and measure the voltage across the cap it'll read as 1.5V. (Depending on the internal resistance of the meter and the value of the cap the cap may discharge through the meter pretty quickly.)

Now if you connect one of the cap's two wires to ground and don't connect the other wire to anything ... nothing much happens. The cap is still charged. If the wire you touched to ground happened to have been the negatively charged wire you might think the excess electrons on that plate would flow to ground, ... but they can't because that would require that the electrons move away from the positive charge on the other plate of the cap.

Here is what a charged cap might look like, negative on one plate and positive on the other. Push one electron into the negative plate from the battery negative terminal (-battery is on the left) and one electron from the positive plate flows into the batteries positive terminal (+battery is on the right) .
<pre>uh oh ascii art
-| |+
-| |+
-------| |---------
-| |+
-| |+
</pre>

Electrical circuits always need a complete circuit (though sometimes that circuit isn't obvious) and electronic devices (almost) always have at least two electrical connections (antennas only have one but the universe is the other connection). One connection, or the absence of a complete circuit, means no current flows, hence the cap can't discharged. (The cap will slowly "leak", either electrons cross the internal insulator or current is carried by the air between the cap's two leads.)
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