Discussion in 'Physics' started by alphaone, Dec 21, 2005.

  1. alphaone

    Thread Starter New Member

    Dec 17, 2005
    can someone please help with an explaination of electrical and magnetic fields...

    I want to understand how inductance works but im having troubles because i cant quite grasp what a field is.

    The assertion that electrical fields have no mass is a tad confusing, If energy is propagated by a field surely the field must have mass of some sort:

    Its my understanding that electrons can not flow between insulated conductors and i currently perceive fields to be almost like sound waves, in the sense that an excited electron produces a "wave" that may propagate through a medium of some sort, the wave may then have an effect on another electrically insulated electron.

    This doesnt figure because as far as i know fields dont behave like waves and they can travel through vacuums, in which case some sort of mass must be involved in the transfer of the electrical and magnetic energy contained in the field, something must be lost by inductance even if it's not electrons! Also with shielding; something tangible must be reflected by the shield.

    and just when i thought it was starting to make some sense!

    Do 0Ω superconductors also generate electromagnetic fields in the presence of a current?
  2. peajay

    Well-Known Member

    Dec 10, 2005
    > I want to understand how inductance works but im having troubles because i cant quite grasp what a field is.

    Well, I can't claim to know either, but lets see what I can do...

    > The assertion that electrical fields have no mass is a tad confusing, If energy is propagated by a field surely the field must have mass of some sort:

    No really. Imagine, for instance, that I had a button on my computer that, if I pressed it, the sun began flashing on and off at a rate of 5 flashes per second. Certainly we'd all wonder how it worked. We'd look, but not find any connection between the button and the sun. We wouldn't even begin to be able to guess how it worked. Yet none of that would stop the sun from flashing when it was pressed.

    Physics is the same way. It doesn't have to make any sense, that's just how it is. It'd certainly be easier to understand if it made sense, but I don't think the laws of the universe are out to please us. Overall, I guess you've just got to look at it as if you're learning the laws of magic in a fairy tale universe, because things just are the way they are, wether it makes any sense or not. If people disappear when someone says abracadabra, then they disappear, and it may not ever make any sense.

    But, like electrons and photons and all that goo, electrical fields don't have any mass either.

    As for coming up with non-physics examples of how things work, I try to imagine individual particles as simply being three coordinates in a computer program. As the particle moves, the numbers change. Does the particle have mass? That depends on wether the program takes it into consideration when it's doing calculations involving gravity or when it's doing calculations involving momentum. As for things like electrical force, I just imagine the program going around and seeing where all of the particles are and calculating what forces are being imparted on each using whatever formula it uses. These fields I guess are just some other fancy construction of programming, and all we can really do is look at what happens with them and formulate theories as to how they work, and so far, as best we can tell, they aren't involved in any mass calculations, and so we say they don't have mass.

    (hopefully that was helpful for something)

    Anyway, as for those inductors... Damn, I need a chair with a back... Uhg...

    Well, you put a voltage across one. At first it has a voltage drop nearly equal to the voltage you apply to it. This is becase as soon as a little bit of current begins to flow, it creates a magnetic field that opposes that current flow by creating a voltage going against it. So it's kind of like connecting two batteries, negative to negative and positive to positive, no current flows because both voltages are the same.

    However, it's only a nearly equal voltage, and some small amount of current continues to flow anyway. Also, that voltage generated by the magnetic field is only generated by a changing field. A static field doesn't create voltage. So the field created by that initial bit of current only stops further current flow for a moment.

    So then more current flows. This creates more change in the magnetic field, which creates more voltage to oppose further current flow. Again, this field ceases to change, and so the current again ceases to be opposed, and so even more flows.

    Except that it's not really a back and forth cycle like that, it's a continuous operation. Current flow starts out at some small level, and increases at whatever rate happens to create a voltage drop just large enough to prevent any more current than is supposed to from flowing. This continues indefinitely, an inductor is never fully charged like a capacitor, as no matter how much current is flowing through it, more can always flow. At least until the inductor bursts into flames, or more likely reaches other limits, like it's wire's resistance, or a circuit breaker or a nice fun cirucit that only connects it to voltage for short periods of time.

    The problem I've always had with books is that they always explain it as if it's the rate of current change that creates the voltage. It is, but that's not a useful way to look at it. It's more useful (to me at least) to look at it as that it's the voltage across the inductor that determines the rate of current change.

    Then, when you reduce that current, the inductor creates a voltage equal to the rate at which the current is decreasing. The quick, dirty, and simple way of looking at this is to say that the inductor creates whatever voltage it has to in order to preserve that current flow, but that for the larger the voltage you force it to create, the shorter period of time it creates that voltage.

    So if you just disconnect it completely, it will for a short instant create the thousands of volts necessary to force current through air so that it can maintain it's previous current flow. On the other hand, if you've got one end connected to +12v and the other end connected to ground through a 10 ohm resistance, 1.2A will flow through the inductor once it's magnetic field is fully charged. If you then instantaneously replace the 10 ohm resistance with 100 ohms, the inductor will (for a very breif instant) cause it's end connected to the resistor to rise to +120v, as that is what is required to maintain that 1.2A of current flow in the same direction through the new 100 ohm resistance. This is one way that voltage is increased, though usually the resistors are left out as there are much better ways to control the current flow.

    > Do 0Ω superconductors also generate electromagnetic fields in the presence of a current?

    Yes, it's the current that makes the field. Generating the field will consume some power, and of course to do that there must be a voltage drop across the superconductor, but it's a voltage drop due completely to it's inductance. Normal inductors will also have a small part of that voltage drop due to their DC resistance, and that energy is simply turned into heat, but since the superconductor is superconductive, it doesn't lose any energy to heat. Once the field is fully generated (as strong as it's going to get for that current) that voltage drop will be gone and then it will be truely superconductive. When you later reduce the current flowing through the superconductor, you'll get a sort of reverse voltage drop (I hope you can imagine what I mean, one that increases the voltage at the other end instead of decreasing it) that returns the energy taken by the first voltage drop. It'll basically work just like any other inductor, except that it has a zero DC resistance, and so all energy it consumes is returned, so long as it wasn't used to do some work, like turn a motor or something.

    Anyway, I hope I've been more helpful than unhelpful...
  3. n9xv

    Senior Member

    Jan 18, 2005
    For a given value of inductance, whether it be a simple strand of wire, a coil, or transformer etc. two variables can be manipulated to increase the "induced" voltage across the inductance.

    Induced voltage = ∆ i / ∆ t

    Its the change of current per given unit of time,


    change of time per given level of current that produces the induced voltage.

    You've probably heard of the idea of swiping the leads of an inductor across a 9-volt DC battery and producing a large voltage spike actross the coil. Thats because the current rises very fast (relatively speaking) in the coil causing it to produce a large induced voltage. Another way to specify it is that as the frequency increases, the induced voltage increases across the coil. This is true up to a point where the "Q" of the coil and other more complex factors come in to play, but thats the basic idea.

    I hope Santa gets peajay a chair for christmas. I got my chair about 6-years ago for christmas. Its survived a 7-year old, now being subjected to a (very mobile) 2-year old. Its been thrown down the basement steps once. Its been thrown away at least once. I retreved it out of desparation - I digress - but its still going strong!

    Never under-estimate the value of a descent chair!
  4. dpSkliroS

    New Member

    Dec 22, 2005
    Quantum Mechanics --->

    Electric and magnetic fields are essentially photons
    (massless particles of light) that come into and out of excistence in a random manner and this is why these [el. and mag.] interactions travel at the speed of light and not at the speed of e.g. an electron which is certainly not massless! The number of photons in a typical electric or magnetic field is not constant while being so large that it is more convenient (not to mention necessary due to the randomness of photon production and annihilation) to express it in terms of a continuum, i.e. a field, the electromagnetic field.

    Special Relativity--->

    In fact a thought experiment based on the theory of special relativity suggests that electric and magnetic fields are two different aspects of the same quantity. For example, suppose that the only field present is the electric field and envision the net motion of the electrons through a current loop which is lying on a table. Because they are in motion they will suffer a Lorentz contraction (, and hence the charge density of the electrons will be greater at any one point because charge is conserved. Hence, if a second current loop wire is placed concentrically and above the first, then if the current through this second wire is in the same direction as the first a volume element of charge will also suffer a Lorentz contraction (just as above) and hence the charge density will increase. Therefore, we have two wires which seem to be negatively charged relative to our reference frame and hence... they will repel! If we just used conventional or classical physics the only way we could get a repulsion (as required by exp.) out of this scheme is by assuming the presence of a "magnetic field" which produces a magnetic force f=vBsin θ. In fact putting the numbers in and actually doing the math in both cases gives the same result!

    But you may say: how can charge be conserved then if the loops seem charged relative to us? remember, it is space itself that contracts during Lorentz contraction so that the circumference occupied by the electrons will shrink whereas that occupied by the ions (stationary protons and electrons) will not as one would expect.

    Classical Physics--->

    According to Lenz's law circuits like to remain in a stationary state with stationary currents e-fields, b-fields, etc... In fact they adore that state so much that they will do what they can to resist and oppose any changes imposed upon them. Consider a solinoid: A current is flowing through it and according to Ampere's law, the "right hand rule" or Biot-Savart's law one can immediately determine the presence and direction of a magnetic field inside the solinoid (remember, a magnetic field does not look like something... its what makes a compass work...). Now let us vary the current in some random manner, let us reduce it. What can the circuit do so that it opposes the change of state the best it can? Remember Faraday's law: A changing e-field produces a b-field and vice versa.... So what it wants to do is to oppose the change in current, i.e. the change in e-field,... by... producing a b-field, the direction of which will be such that it produces an new e-field opposite to that of the old e-field! ---This is the basis of the inductor.

    I hope I have been as accurate as possible throughout and that the first two sections broadened your views of magnetic and electric fields.
  5. Dave

    Retired Moderator

    Nov 17, 2003
    Hi dpSkliroS, welcome to the forums.

    You post has some interesting and valid points, however the above highlighted sections doesn't read as many would expect - the idea of things "coming into and out of existence" could be as confusing as the whole concept of electromagnetic fields, perhaps this could be clarified for the benefit of the topic.