Wikipedia:Reference desk/Archives/Science/2014 August 24

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August 24

Maxwell's equations

The article Computational electromagnetics states Maxwell's equations as:

${\frac {\partial }{\partial t}}{\bar {u}}+A{\frac {\partial }{\partial x}}{\bar {u}}+B{\frac {\partial }{\partial y}}{\bar {u}}+C{\bar {u}}=g$

Someone please define $g$ in the above equation (this question is from the article Talk page). 84.209.89.214 (talk) 17:11, 24 August 2014 (UTC)

In this case, g is the generalized forcing function. Are you familiar with general solution methods for inhomogeneous differential equations? If not, that is where you ought to start reading. By extension to the 1-D case, g is the nonhomogeneous part, and it is a vector in the same space as u (as formulated above).

In physical terms (if you're one of those pesky "touchy-feely" computational electromagneticists who want to intuitively understand the equations), g is the representation of any "externally-applied" field.

The reason I express caution (rather, when I use wikt:touchy-feely in usage (2), ergo, with derogatory connotation), is that in practice this term is a sort of "garbage dump" for any other term in the equation. It can be an externally applied field. But it can also be a minimization constraint, or a nonlinear function in u, or anything else. If you really want to make use of it, you must understand that g is simply "the nonhomogeneous part of the equation": nothing more, nothing less.

Nimur (talk) 21:25, 24 August 2014 (UTC)

Thank you for your good answer and edits to the article. I suggest $g$ can be written ${\bar {g}}$ for notation consistency. 84.209.89.214 (talk) 12:39, 25 August 2014 (UTC)

I think you are correct, ${\bar {g}}$ makes more sense for consistency. When I first read that article section, I interpreted the overbar to mean that u was a vector of vectors, $({\vec {E}},{\vec {B}})$, but in fact whoever defined it actually wrote individual components and reduced to a simplified 2-D case. (This allows explicit specification of the elements of matrices A, B, C, which appears to be the actual purpose of the section). So, it seems we should use an overbar on ${\bar {g}}$. Notational consistency is a sore point on Wikipedia; this is one of the chief advantages that a conventional textbook source can provide. Personally, I've seen so many variations that I've totally given up on notation: my philosophy is, if it's not written in compile-able program code, I consider it "approximately notated, designed for human consumption (hand-waving implicit and expected)." Nimur (talk) 15:44, 25 August 2014 (UTC)

Electricity

list safety precautions required for series and parallel connections — Preceding unsigned comment added by 197.111.223.245 (talk) 20:02, 24 August 2014‎

We're not going to do your homework for you. Jc3s5h (talk) 20:26, 24 August 2014 (UTC)

First, be sure you're wearing insulated gloves. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:13, 24 August 2014 (UTC)

And learn how to spell the word. HiLo48 (talk) 21:26, 24 August 2014 (UTC)

Which one? —Tamfang (talk) 22:39, 25 August 2014 (UTC)

This section was headed "electrity"[1] until an alert editor cried out, "I C!" ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:42, 27 August 2014 (UTC)

The safety precautions you need depends on the voltage and currents you will be working with. Some information is given by High_voltage#Safety. Ulflund (talk) 21:48, 24 August 2014 (UTC)

For me, it's a short list. Step 1: Turn off the electricity. --DHeyward (talk) 05:36, 25 August 2014 (UTC)

No "Step 2: take whatever actions are necessary to make sure nobody undoes Step 1."? DMacks (talk) 05:43, 25 August 2014 (UTC)

People: This is a question about safety. A rather vague question in my opinion. Does anyone understand the question well enough to give a proper safety-conscious answer? If not, wouldn't the wisest course be to NOT give answers? CBHA (talk) 06:07, 25 August 2014 (UTC)

Well, the question requires the OP to regurgitate whatever was in the course notes, whether or not that's true or accurate. The actual safety precautions required for electrical work will depend on jurisdiction and the voltages involved (and, if we knew those details, we could give a proper answer), but this question just a "Wakalixes makes it go" test of the OP's memory. Tevildo (talk) 08:32, 25 August 2014 (UTC)

The main thing I think of where parallel connections are concerned is that each component can, potentially, create a short in the system or something near to one that might start a fire. But yes, you need to give more information. Also, I'm thinking if you're asking for safety tips on the Internet maybe you need an electrician. :) Wnt (talk) 10:26, 25 August 2014 (UTC)

Too vague a question. Safety precautions vary from designed in safety precautions such as current limiting devices as well as actions such as "how to work on a solar array without dying." Solar for example has both parallel and series wired panels and the short circuit problem is not nearly as problematic as the open circuit problem on the DC side. There's no mechanical off switch that will de-energize the array. --DHeyward (talk) 23:31, 25 August 2014 (UTC)

Nyuk, nyuk, nyuk! I'll fix that! (Shakes can of black spray paint) :) Wnt (talk) 13:00, 26 August 2014 (UTC)

Sliding a cover over it or pointing the panels at the ground ought to do it. StuRat (talk) 15:17, 26 August 2014 (UTC)

But that wouldn't be funny! Wnt (talk) 17:22, 26 August 2014 (UTC)

Covers are usually used. Still there is a separate section on solar array installations in the U.S. NEC that are safety related and constantly evolving. The differences are large enough that we will eventually see deaths and fires. This has already happened on small scales. The deratings required for main circuit breakers for fault tolerance, for example are not intuitive to average electricians (not even a factor for home systems ~5kW). Breaker location for grid-tie inverters is based on fault tolerance, too. I saw a backfeed design that was 250 amps into a 2000 amp service. The kicker was that the 2000A main breaker was to protect the switchgear. Adding 250 amps of potential fault current means the mains had to be derated to 1600 amps (the next lowest setting available). Kind of a big deal if they needed the 2000 amps. They were fortunately at the load limit at 1600 amps and could do it without upping the switchgear. Now, the instruction to reset the main breaker to 1600A is engineering instruction. The installer didn't do it because they see the solar array as source that offsets the incoming power, not as a source of fault current in addition to the 2000A of fault current. Kind of a bummer to add 250A of solar but reduce the site by a net 150A because of fault current limit in the switchgear. The Solar marketing guy did not like having that conversation with his customer. It's a mindset that is part of the code for solar but very counter-intuitive to electricians and contractors and Marketing. --DHeyward (talk) 06:53, 27 August 2014 (UTC)