# Ac and dc problem

1. May 8, 2006

### skywolf

when you have a ac-dc converter youre pulling charge out of nowhere arent you? or do you have a net charge buildup at the power plant?

i was thinking the other day, that the reason taht people liked ac over dc was that since you were only "shaking" the electrons, you didnt have a buildup of charge i assume would come from a dc source.

but then when you have a converter, i know that the electrons are just "tricked" into going the same direction through diodes, and that part makes sence. but i would assume that theres some law of nature that says you cant pull a charge in one direction without causing an opposite charge. i was wondering if anyone could tell me if, or where the charge would go, and how it gets there.

thanks
-Sw-

2. May 8, 2006

### Claude Bile

Electrons in a closed circuit are linked in a continuous loop, in AC the loop oscillates back and forth, in DC the loop flows in a single direction. Note that your question applies to all DC circuits, not just circuits with diodes (Since current is still only pulled (pushed?) in a single direction). For this reason, let us forget diodes for the moment because it potentially makes things more confusing.

There is no such law that says you can't just push a charge. While it is true that in a conductor if you displace an electron, you DO create a temporary charge imbalance, but the fact that metals are highly conductive means that these charge imbalances are rectified extremely quickly. Typically when current flows, everything is pushed SIMULTANEOUSLY so one never gets these charge imbalances in practice.

Now, diodes are a different kettle of fish because they are semi-conductors, but if you look at diodes from their functional point of view (What they do, not how they do it), diodes simply restrict this DC flow in a particular direction.

In closing, I will remark that in semi-conductors you can get cases where you generate positive charges (holes) by displacing electrons, though I suspect delving into such physics will be overcomplicating things.

Claude.