Friday, January 09, 2009

Me Eat Burger


  • Another sign I'm getting old: the average number of pills I take each day, which used to be close to zero, has gone up quite a bit.
  • I don't have much of an opinion on this upcoming or delayed transition to digital, but I can say that HDTV is a bit of a scam. Sure it looks great, but for many network broadcasts, the standard definition shows are just the middle section of the video. So what this means is that when you watch a show in high def, all you gain is some extra field of view on the sides. You know those annoying station identifiers and advertisements for news shows that appear in the corners of the screen? In high def shows, they don't appear in the corners because then the simple video processing to get a standard definition feed would cut them off. So they get even more annoying! And then the cable companies compress the signal, especially apparent during fast motion...
  • I'm more pedantic than the next guy, but this whole "fewer/less" thing is getting out of hand. I was watching TNT or TBS or some channel which kept advertising how it has "less commercials" thanks to the sponsorship of some company. And now there's a TV show by the title 10 Items or Less.
  • Lots o' physics bloggers actually take the time to explain things and answer actual questions. I much prefer asking questions but my readers are the absolute worst in terms of answering. So I guess I'll pose a question recently asked of me, and, when no one else provides an answer, I suppose I'll answer it. So the question is:

    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?

  • What does an Angry Physicist eat? Perhaps an angry whopper?

5 comments:

Paul A Houle said...

The polarized plug question is easy, but it's not typically taught in physics classes.

There are three prongs in a 2-phase 100 outlet: hot, neutral and ground

Under normal operation, neutral should be at the same potential as ground. However, neutral and ground are not connected and normally no current passes to the ground.

The ground, of course, plays a big role in safety: the metal shell of an electrical appliance is typically grounded, so that if anything at a potential touches the ground, the ground will short the circuit out and prevent the user from getting an electrical shock.

Bathroom outlets are equipped with a ground-fault interrupter that takes this one step better: typically these circuits contain a transformer the measures the difference in current going and and going out the hot and neutral leads. If the difference is more than 5ma, the GFI breaks the circuit.

http://ask.metafilter.com/24964/How-to-detect-the-neutralground-in-a-power-outlet
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gfi.html

CarlBrannen said...

Having wired more than my share of stuff in my blue collar incarnation, I can tell you the simple reason the plugs are polarized. It's cause one side will shock the urine out of you, the other is safe.

So all plugs should be polarized, the fact that some are not is because of custom. The newer outlets (sockets) have one side that is larger so they're also compatible with symmetric plugs.

As far as wiring them, there are three colors in the US, white, black, and green. Green is the ground, easy for me to remember. For the other two colors, remember that in the US, the custom is that the widow wears black; that is the hot side. The neutral side is white. Let's add a link, other countries have other customs.

Your AC volt meter should measure close to 0 between green and white.

Angry said...

All very well and good, but it doesn't quite get to the heart of the question. And I suppose I should say this came from someone not a scientist (though many professional physicists don't have any practical knowledge of electricity to speak of).

The key is that this person doesn't understand how one prong can be the hot one and the other neutral one (let's forget the ground for right now). They see that when the current is leftward, let's say, then the potential is high on the right. But when the current is negative/rightward, then the potential is higher on the left. This keeps oscillating and so how can one differentiate the two prongs?

PS--Not to nitpick, Paul, but the neutral and ground are connected back at the panel (the one with the main breaker) where the buses are.

PPS--And Carl, you say "all plugs should be polarized" but I think they *should* only when it provides a safety enhancement. For example, my notebook power supply has an ungrounded, unpolarized plug, I presume because there's no more safety with one wire being neutral than the other one (ie there's no shield).

Doug Natelson said...

Angry, I guess I'd argue that the direction of current flow between the poles isn't important. What's important is that you are probably going to be living at a roughly constant potential close to that of the neutral/ground. So, ground is "arbitrary", but it's not too arbitrary to you if it's your potential and it determines whether the current flows through you.

Angry said...

Doug: Good point. I think I have a good explanation that a layperson would get. Traveling now though...