User:Light current/My Sandbox

Sandbox 1

Displacement current
Just come across another problem quoted by Nigel Cook. When a capacitor is in series with a load, how exactly does the energy get from one plate to the other. Does this need displacement current?. If so, how is this didsplacement current generated? --Light current 20:37, 19 December 2005 (UTC)


 * The energy is transmitted when the field goes across the places. And, the energy goes through the wires on either side of the plate.  Pfalstad 20:43, 19 December 2005 (UTC)

Ahh but there can be no energy flow across (perpendicular to) the plates beacuse the Poynting vector points in a direction parallel to the plates -- no?. --Light current 20:52, 19 December 2005 (UTC)


 * Right you are! Interesting.  Well, then the energy goes down the wires.  It doesn't go across the plate.  Pfalstad 20:58, 19 December 2005 (UTC)

What, and by passes the capacitors as if they werent there?--Light current 21:17, 19 December 2005 (UTC)


 * No, I meant it stops at the capacitors as if they were an open circuit. Pfalstad 21:53, 19 December 2005 (UTC)

If that's the case, how does energy get to the load?

capacitor | |   -| ||                     | |               |--|  ac source                            |  | Load (resistor) | |                                       |--|    |

--Light current 21:58, 19 December 2005 (UTC)

The energy gets to the load by the bottom wire. And to anticipate your next question, if you put another cap at the bottom, then I don't know what happens. There must be some energy flow across the cap... I can't simulate the fields with the resistor present, so I don't know what the poynting vector looks like in that case. Pfalstad 22:12, 19 December 2005 (UTC)

Ok, consider the case where there is a capacitor on either side of the load. Straighten everything out so the wire, caps, and resistor are in a line. Consider the time when current is maximum (voltages across each cap = 0). There is a magnetic field around the wire/caps/resistor. The electric field lines lead from the top wire to the bottom wire; near the resistor, they are parallel to the wire. If you work out the poynting vector, you'll find that it's pointing to the resistor. Energy is flowing from the wires into the resistor. It's not flowing across the caps, though; it just goes around them. Weird. Pfalstad 22:24, 19 December 2005 (UTC)


 * Interesting! Im going to think about that for a while. Needless to say, I dont know the answer.--Light current 22:30, 19 December 2005 (UTC)


 * Do you mean arrange cct as below: ?

|                                                                 |___                                    ____  C (Zo is very low) |                   Energy flow   |--| ->      |  |                                  |  | R Load ac source(Zout=R)      |  | |--|                                   |___                                    ____  C (Zo is very low) |       | yes.

I've drawn the Cs as little transmission lines to help us in thinking about this cct. This arrangement of components would indicate that the energy gets shared between the 3 components initially but we know that the Cs do not charge up and can therefore hold no energy. If they do not hold energy, they must reflect it.

Ah ha! Maybe the energy stolen by the capacitors in the first few ns (2 way transit time of TL) is given back to the load or source after reflection. Probably given back to the source actually (assuming matched source and load resistances). (ie the capacitors introduce a small but positive mismatch to the source) Any reaction to my thoughts?--Light current 23:07, 19 December 2005 (UTC)


 * Well the capacitors definitely charge, and they hold energy when charged. I'm not sure I understand what you are saying. Pfalstad 23:16, 19 December 2005 (UTC)

No. The capacitors do not charge up because you have an ac source! (the voltage on each side of each capacitor is the same--roughly). Charging means holding separated charges on each plate and having a steady voltage difference between the plates (Q=CV)-- we dont have that here. In normal circuit theory, large capacitors act like a low impedance to the ac and so dont drop any voltage. In the em field representation, they dont drop any voltage because their characteristic impedance is so low compared with the resistance of the load. Understand so far?--Light current 23:43, 19 December 2005 (UTC)


 * There you go again, assuming the high-frequency limit without saying it. Ok, in that case the caps charge very little.  I don't think I understand enough about your cap-as-TL model to say whether the energy is reflected or what happens to it.  Pfalstad 23:52, 19 December 2005 (UTC)

Sorry. Yes I am assuming the capacitors heve low impedance compared to the resistor. But thats what you expect for coupling capacitors isnt it? OOPs looks tho I didnt mention that. But the argument still holds for any size capacitor. Anyway so far so good. What Im trying to show here is that energy can get to the resistor without the need for displacement current going from one plate to the other in the capacitors. EM energy can flow up and down the transmission line capacitors which are o/c at the far end of course. My propostion is that it all must happen by EM fields.


 * I don't think anyone disputes that the transmission of energy across the capacitor all happens by EM fields. I certainly don't.  I don't understand how the TL representation helps.  Of course I mentally picture a TL as a chain of caps and inductors.  I suppose you have lots of experience with TL's, so perhaps they are more comfortable for you to work with. User:Pfalstad

Now in one limit, if we make the capacitors smaller and smaller the energy transferred to the resistor will tend to zero. In this case all the energy must be reflected back to the source by the capacitors. In the other limit, where C-->oo, none on the energy is reflected back to source. So the only effect the capacitors have is to decide how much of the incident radiation is reflected! Current does NOT need to pass from one plate to another. Energy enters the resistor sideways. Energy enters the capacitors sideways and is reflected (to a greater or lesser degree). Nigel Cook is WRONG!! HA HA!! (See : Talk:displacement current to see what Nigel said).--Light current 00:19, 20 December 2005 (UTC)

In preparation
Im not sure that a single logic pulse can exist in isolation. Although it may be able to as in the soliton wave. If we were able to use Fourier analysis for the analysing of this hypothetical single logic step, then I feel all would be qute simple. However it appears at the moment that we cannot do analysis by Fourier.

What is generally accepted though, is that as a pulse (not a step) is progressively reduced in width so it approaches a Dirac delta function of zero width infinite height but finite strength., the band of frequencies it generates spreads upward to infinity.

For instance in the case of the TL charging, if we assume thet the em wave does not stop when it has finished charging the line but continues to bounce back and fore (undetectable of course), two repetitive counter propagating waveforms exist, each of which separately is amenable to Fourier analysis. Now we know that the sum of all the Fourier components of each wave added together gives a constant value (dc on the line), but what can we say about the indiidual waves? Well of course they could be any shape as long as they were complementary so they add to dc. But what is the reason to assume that they would change shape from the nice square waves which originally existed whilst charging the line. Let us therefore assume that the waves do not change shape, they continue propagating in opposite directions always adding to dc. In this secenario, it is quite in order to assume that the waves are TEM because that is how they started out whlst being launched from the source into the line.

Introduction to differing terminology
Electrical and electronics engineering is a very broad field that encompasses many subfields including those that deal with power, instrumentation engineering, telecommunications, and semiconductor circuit design amongst many others. Some common usages of the term 'electrical engineering', particularly in North America and in other parts of the world, include the field of electronics engineering.

Electrical engineering is considered distinct from electronics engineering especially in the United Kingdom and parts of Europe where the strict definition of electrical engineering (whose practitioners are called electrical engineers), is: an engineering discipline that deals with the study and application of electricity and electromagnetism. Where this distinction is made, electrical engineering is considered to deal with the problems associated with large scale electrical systems such as power transmission and motor control where as electronics engineering is considered to deal with the problems associated with small scale electronic systems such as semiconductor circuitry very-large-scale integration.

However, there is considerable overlap between electrical and electronics engineering fields in practice, and in some countries the two phrases may be used interchangeably. Whether or not electronics engineering is distinguished from electrical engineering must be interpreted from the context in which the term is used. Some have suggested that in places such as the United States the distinction is less common than in places such as the United Kingdom. However both usages can be found throughout the world. For example, the Institution of Electrical Engineers (which also includes electronics engineers) is a U.K. based organization but the Institute of Electrical and Electronics Engineers is a U.S. based organization. Conversely the Massachusetts Institute of Technology names its electrical and electronics engineering department as the "Department of Computer Science and Electrical Engineering" where as the University of Sheffield refers to its departments as the "Department of Electronic and Electrical Engineering".

Education in electrical or electronics engineering
The university departments offering these courses are sometimes called 'departments of electrical and electronic engineering' although recently in Europe, the tendency has been for electronic engineering (representing the lighter current, telecomms and computer aspects) to have been made independent of electrical engineering and to have their own departments.

Electrical engineering
Usually, in Europe, the degree courses are separately defined as electrical OR electronic although there are a number of common subjects studied in both especially in the first year. The subject is sometimes, (but not usually) taught as a combined degree 'electrical & electronic' course that is offered at some universities.

Electronics engineering
Electronics engineering represents the lighter current, telecomms and computer aspects.

Electrical and electronics engineering
The combined subject Electrical and electronics engineering necessitates the study of both electrical engineering and electronics engineering but usually without a great amount of specialisation. The course focus, however, does change in the final year to concentrate on chosen specialisms.