i sent this one to cecil but it was too detailed, so they recommended that i post here, plz feel free to communicate on this and related topics through email and messenger…here is the question in the original…Hi, Im a first year engg student from India with an amateurs interest in the pure sciences, especially theoretical physics. Ever since I learned about the atomic structure, quarks and quantum physics, I have always wondered this. It is stated that an isolated electron or any charged particle for that matter exerts a kind of electrical field around it that is experience by any other particle brought near it as a force. Since something is happening, there must be something to cause it. What is the source of energy for this field? I have learned a little about equation relating to work and energy and how if youre moving in a loop theres no work done etc. but as it relates to source of energy, I dont think its the correct explanation. Because if there is a source of energy then it must be infinite and so disproving the law of conservation. Also, wouldnt the same thing go for gravity? every particle with mass has a gravitational field around it. What is the source of energy to maintain such a field? Perhaps im taking a simplistic view, but i would like to state that im not one of those people trying to disprove any laws of physics or something, maybe i dont have enough knowledge, and i firmly believe in rational established scientific principles, but i have just always wondered this question and im really hoping you an answer this for me. Ofcourse if youre going to publish it online you will have to condense the question, sorry about the length :D.
Actually theres a little more. Relating to equations of work (path-based), we cant apply them to electrons right? because we dont know how an electron moves , how can we say it moves in a loop eliptically or whatever. doesnt heisenbergs uncertainty principle disallow that?
also…what is this spin of an electron? i dont understand it at all, apart from that,…how the heck is current transmitted? forget AC…i mean lets talk dc…the way its explained in the textbooks…i understand this…u have an electron depleted part on one side and many electrons on another…electrons flow to depleted part…now in the middle of the path we put a load…say a bulb or any device which takes up current…so what happens? is this quantity of charge sucked out of an electron? wht happens to the electron? what exactly is this current…and dont tell me flow of electrons please…
There is energy in an electric (or gravitational) field, but it’s just sitting there. It’s not going anywhere, so it doesn’t need to be replenished. The interaction between charged particles is mediated by virtual photons, if that’s what you’re asking, but being virtual particles, they don’t need energy to produce them. Virtual particles can have any energy and momentum at all, including zero or negative values.
You can also have energy in a field that’s moving somewhere, but you do need some energy source for that (which can be almost anything, depending on the situation). For instance, if you have energy moving in an electromagnetic field, that’s an electromagnetic wave, also known as light. The photons in an electromagnetic wave, unlike those in a static field, are real, and therefore do carry energy. That energy comes from some finite source, and when that source is used up, the light turns off.
What Chronos said. Moreover, if things move, for example if a proton approaches the electron, the field does work to it. So there was a need for energy to set up the field in the first place. You might have had a paired electron and proton, which are attracting each other, and then grab them and pull them a billion miles apart (so they’ve long forgotten each other). This would take a little effort, and it’s that effort that establishes this field, or from a practical viewpoint it’s that effort that establishes the separate fields around the now-distant proton and electron.
The energy in this field is not infinite. The electron can only do so much work as it draws a proton near. And meanwhile the proton is doing some of the work too, so both their fields contribute.
More precisely there’s only one field in the universe which contains both their effects and others, but it’s legitimate to calculate separate fields for them and add them linearly.
To expand on that, electromagnetic and gravitational fields are conservative–that is, any work done upon them is ultimately completely recoverable. This differs from work done on a fluid or a mechanical solid, in which at least a small proportion of work is lost as unrecoverable heat per the Second Law of Thermodynamics. There is a certain (and very precise) energy bound up and expressed as the mass or charge in a particle which is the focus of the field, but this doesn’t change; the only thing that changes are either the kinetic energy (from its motion), the binding energies (from chemical, nuclear, or gluonic bonds, the former involving electrons or other leptonic particles, the latter two involving quarks and particles that are composites of quarks, like protons and neutrons), and their potential energy relative to the “local” system. The field created by a charge or mass no more requires energy to sustain it than does a bowling ball require energy to remain at rest in the middle of a mattress. Olny when you decide to move these or break them apart is additional energy required, and it is accounted for in the increased kinetic energy (including heat) in the system.
Regarding what is meant when we speak of electric current, from [post=6687165]this thread[/post]:
I daresay that most people have a conception of electric current which bears a strong resemblence to Option 2, but in fact electricity is transmitted in solid state, i.e. from one electron to another, rather than progression of electrons down the wire like water through a hose.
Stranger
so chronos are you telling me that the way theoretical physicists approach this sort of a problem is by making up virtual particles as and when they please? or is there a decent experimental finding to back up such a hypotheses?
Im sorry if im sounding a little confused but thats how it is. Also, you told me the energy isnt going anywhere…everytime a charged particles comes near it, some energy goes? then how much energy is there? because the way that an electric field is explained is that it extends till infinity varying inversely as a power of the distance, so thats a lot of energy in one charge, isnt it? is there a way to quantify exactly how much it is?
also, the explanation that a field is setup when two charged particles are pulled away from each other to infinity, that is one way to explain it, agreed, but it doesnt ring true to me because it seems like it makes it necessary that there should be a pair everytime. Am I getting the wrong end of the stick?
Also relating to current, so basically electrons in DC stay in the same place but transmit energy due to difference in potential? is this energy the charge intrinsic to each electron? then the charge must get used up rite, even though the potential difference is there? we know an electron has mass, so wat happens? it becomes a neutral subatomic sized particle? I understand AC to an extent, the movement of electrons causing generation of electromagnetic waves.
Also, id like to understand the concept of electron spin.yes ive read up on it.
i still dont get it. umm…does that make me a dumbass?
I`m asking these questions because my degree is going to be in electronics and teleommunication and I want to do this right. Its going to be nagging folks 
I think you’re getting a little tied up in your shorts here. Lets say that we have an electron and a proton some distance away from each other. Will you agree that this system of two particles possesses potential energy? If you do agree then just think of this potential energy being converted to kinetic energy when the two particles start moving toward each other. No outside source of energy is required, all that’s happening is one type of energy is being converted into another type.
In your DC current question don’t think of the charge as being used up, instead think of the energy in the battery as being used up.
wow, thanks for “untangling my shorts”
Thank you, Stranger. Years ago I came up with an analogy and this is the first time I’ve been able to use it.
Imagine a half-inch water pipe a hundred feet long filled with just-under-a-half-inch marbles. You walk up, pick up a marble from a pile on the ground, and stuff it into the end of the pipe. Instantly --plonk-- a marble falls out the other. “Wow!” you think. “That marble sure traveled fast!” No, that marble is right at the end of the pipe, still touching your fingers. What instantly traveled was the force of the marble, passed along by 2,400 of its brothers to the last one in line.
I vaguely recall that the electrons actually travel at the rate of about 20cm per second – quite fast when you consider it in electron diameters – but I can’t find a cite right now.
That’ll depend on the amount of current, the size of the wires, and (to some extent) the material from which the wires are made. 20 cm/s is much faster than for most typical situations, but could be accurate for very high currents.
It is backed up by experiment, indirectly. Physicists construct a model involving virtual particles and their interactions, and make predictions based on those models. For electromagnetism, at least, the predictions made by the virtual-particle model (called Quantum Electrodynamics, or QED) match very, very closely with the experimental results, so we’re confident that in that case our model is good. For instance, there’s an experimentally-measurable quantity called the gyromagnetic ratio of the electron, which involves the ratio between the angular momentum of an electron and its magnetic field strength. Working from a naive classical theory, one would expect this ratio to be 1. If one brings in some relatively simple quantum mechanical results (without virtual particles), one would instead expect it to be 2. But if one introduces virtual particles into the theory, then the predicted value for the gyromagnetic ratio is approximately 2.0023193043737. When the value is experimentally measured, it agrees with this final figure, not with 1 or 2.
It doesn’t work as well for the Strong Force (there are very few cases where we can actually make predictions, in that model), so that model isn’t considered to be as good as QED, but we don’t have any other model which works better, so we still keep it.
The rate at which direct electron current flows in a wire is dependant upon the amperage of the current, the cross-sectional area of the wire, and the charge density of the medium (which changes somewhat with temperature), per f=I/(Qed^2*pi/4). A 1 amp direct current flowing through a 12 gauge wire moves at about 8 cm/hr; not so very fast, unless you’re a snail. The electron flow with an alternating current has no aggregate movement of course, though individual electrons will flow back and forth, and that rate depends on a number of factors that are difficult to calculate. Note that while the electron flow occurs entirely within the wire (unless you’re generating enough of a static field to ionize the atmosphere) most of the actually energy being transmitted is flowing outside the wire in the form of electromagnetic fields.
Stranger
Actually, Stranger, (this BTW is a real nitpick) I think all the energy is transmitted outside wire. The reason I say this is that the electric field vector inside the conductor would point in the direction of current flow which means the Poynting vector would be perpendicular and into the surface of the conductor.
My knowledge of this stuff is really rusty, but nonetheless it would seem this “inside the conductor wave” must be what causes the Ohmic heating.