Wednesday, January 21, 2009

Assymmetry in an Outlet (Part I)

I had mentioned the question posed to me by a layperson (nonscientist) recently:

In a normal electrical outlet, the current oscillates between positive and negative, and hence left and right between the two prongs of some device. So then, why are some plugs polarized? Shouldn't things be symmetric?

There are some helpful comments there, but I think they're perhaps not sufficient for someone with little understanding of electricity. So I figured I'd give it my best shot. This is perhaps my first posting trying to explain physics. There's so much good stuff on the web, but I couldn't quite find what I wanted for this person, so I'll write it myself.

So I'll start with an analogy and restrict myself to the case of DC first. There'll be a quiz at the end so pay attention.

You know the "log ride" at an amusement park (as always, there's a wiki when you need one)? It's an artificial "canal"-type conduit in which you float on an artificial "dug-out" log. Basically a mild roller coaster on water.

So imagine such a ride that is never go up or down. Assuming there's nothing pushing on the water, your log would just sit there. So forget the log, and we'll just consider the water. The water seeks its own level. Here level is the potential energy of the water and it's all at the same potential.

To make the ride interesting, the ride is instead constructed with a high starting point and the water is pumped up to that point. This high water is now at a different potential and it falls because of gravity.

The analogy is that the water is similar to charges and the water's height is analogous to electric potential (measured in the familiar units of volts). The water pump is like a power supply, or, for the case of an outlet in your home, the power company. In either case, you get a potential difference. With the ride, the water is higher than the rest of the ride. With the outlet, one prong is at a higher potential than the other and therefore charges want to make it to the other prong.

Quiz time:

  • If you plug in a simple light bulb to an outlet, you're putting a load across the prongs. To what property of the ride does this correspond?
  • Electrical resistance is essentially a measure of the difficulty the charges face in going to lower potential. What property of the ride is analogous to resistance?

Next time I'll discuss grounding and AC circuits within this analogy.


Vanilla Gorilla said...

the (who you arrogantly referred to as) "layman" asked a question that didn't require the moronic explanation you went through. let me rephrase his question so you can understand it, dummy.

you have a logflume, where, at one point, the beginning of the ride is 100 ft above the end of the ride. at another point (about .03 seconds later) the end of the ride is now 100ft above the beginning. what is then the CHARACTERISTIC DISTINCTION between the end of the ride and the beginning? why bother calling one the beginning and one the end?

Cole Petersburg said...

The wide blade is grounded so that if you stick your finger in a light bulb socket, you won't get a shock. The narrow blade oscillates above and below zero volts. Ultimately, the round ground pin is connected the wide plug and to the earth outside your house, but the round ground pin is connected to the metal case for safety while the wide blade is not. Thusly, current doesn't flow through the case. From what I've read, grounding the wide blade at each house prevents problems for the electric grid.

Europeans don't always use ground pins because their electronics are double-insulated for safety. Many of their plugs are completely symmetric.

Stereo amplifiers, depending on the kinds of magnetic windings they have inside, may sound better with the correct polarity, according to audiophiles.