Wikipedia:Reference desk/Archives/Science/2014 May 26

= May 26 =

Example of a distributed parameter system
Can someone give me an example of the equation for a distributed element model? For example, for a circuit with an inductor, $$V=L\frac{dI}{dt}$$ is the ODE for the lumped parameter system. Can someone give me an example of a distributed parameter system? 203.45.159.248 (talk) 02:23, 26 May 2014 (UTC)
 * Sure: the telegrapher's equations provide differential equations to define the voltage at each point in the signal path. You can do the same thing for the derivation of the radar equation, but that equation is conventionally simplified to solve for power received at a fixed location.
 * The key concept is that your lumped-element equation is an ordinary differential equation in one variable (time). When we model a distributed system, we must use a partial differential equation with variables for time and position.  The telegrapher's equation is of course simplified to one position-parameter: distance along the transmission line.  In full three-dimensional representation - like a full-form RADAR scatter solution - we need three position variables x,y,z; and before long, we are actually solving the general form of the wave equation, as expressed by Maxwell's equations, with electromagnetic parameters specified for every point in the model-space.  Nimur (talk) 03:50, 26 May 2014 (UTC)
 * OK. Thanks for your answer. I see the full telegrapher's equations under "Lossy transmission line" on the article. I am interested to know the three-dimensional equation, and how that reduces to Maxwell's equations. Also, are L, R, G, and C functions of position and time too? 203.45.159.248 (talk) 06:25, 26 May 2014 (UTC)
 * Those parameters might vary with position. That will make the solution more difficult - at a certain point, an analytic solution is impractical, and we would use a numerical method to solve.
 * For three dimensions, we have many options. I would solve the nonhomogeneous form of Maxwell's equations, where the nonhomogeneous term is a field function describing the resistance and inductance (in terms of permittivity and permissivity).  I would solve this using the staggered cell method, which is pretty stable but slow.  A friend of mine works for a small company and writes commercial software to solve this equation by the Galerkin method.  A few other options include solution by Fourier transform or solution by multiresolution methods.  All of these fall under the giant umbrella of finite element analysis, but they are simply techniques to deal with the many many terms in these equations.
 * Unless your simulation environment is mostly empty, a closed-form analytic solution is probably impractical. Nimur (talk) 15:19, 26 May 2014 (UTC)
 * Also, at the reference section in our article lists several of the standard textbooks on this topic. Nimur (talk) 17:07, 26 May 2014 (UTC)

What can be taken to heal mouth ulcer?
Zonex shrestha (talk) 04:42, 26 May 2014 (UTC)


 * We have an article on mouth ulcer which explains there are different kinds. If your interest is academic you might want to narrow down your question.  If you're looking for treatment for your mouth ulcer, people here can't readily diagnose which kind you have, and there are some people who are dogged about preventing people from even trying. Wnt (talk) 04:59, 26 May 2014 (UTC)


 * Zonex, the majority of your questions can be answered by googling or searching wikipedia. Try those first and come back if you have questions that weren't answered by your searches. Justin15w (talk) 15:11, 26 May 2014 (UTC)

Banana worms
I often hear of stories of people seeing worms or parasites in bananas but yet I've never seen one and can't even find a picture on the web. Are there really works which live inside bananas or is it just a figure of people's anxieties about parasites? — Preceding unsigned comment added by 82.40.46.182 (talk) 15:09, 26 May 2014 (UTC)


 * There are a few that attack the roots, and we even have some pictures. Please see List of banana and plantain diseases. Not sure whether any get to the fruit.--Shantavira|feed me 15:26, 26 May 2014 (UTC)


 * Our article on banana has a section on pests. Most of the pests that affect the fruit are fungi.  Nimur (talk) 15:31, 26 May 2014 (UTC)


 * Fruit fly larva are "worms". And, as you know, "Fruit flies like a banana, but time flies like an arrow". StuRat (talk) 15:35, 26 May 2014 (UTC)


 * I believe the OP is referencing a fairly recent urban legend making the rounds that the little smutzy bits at the end of each banana (i.e. the little chunks that usually stay in the peel or you otherwise pick off and throw in the trash) are parasites or discrete living things of some sort. Complete nonsense. There's a bit of background here. They're slightly fibrous and a bit squishy, but harmless. Matt Deres (talk) 01:32, 27 May 2014 (UTC)


 * Perhaps they are talking about soft/semi-dried bananas (like this) popular in S.E.Asia. Worms are pretty common in those. They're still delicious with the worms.  Sean.hoyland  - talk 07:20, 29 May 2014 (UTC)