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Electricity & Energy

  1. Sep 20, 2006 #1

    I am trying to understand how energy propogates in an electric circuit and found this website :


    Assuming the site is correct, and it kinda makes sense to me, I wanted
    to clarify some things :

    Does most of the energy flow closest to the wires of the circuit as
    opposed to in the diagrams were it appears that there is as much
    flowing directly between the battery and load as there is flowing
    next to the wires ?

    The em field shown does not vary so how is there energy propogation
    at all ?

  2. jcsd
  3. Sep 20, 2006 #2
    I didn't look through the page very carefully, but qualitatively, it seems correct. (I'm a EE major who knows a bit about microwave circuits, so bear with me.) Basically, every wire in a circuit can actually be treated as a special case of a more general transmission line, which is in itself a special case of the more general waveguide. At low frequencies, these effects are irrelevant: the wire behaves "classically," and the voltage across the wire changes very little as a function of position. In essence, what happens is that the wire can be treated electrostatically, and the metal will have the same voltage all the way across it. At high frequencies (where the order of the wavelength of the signal is approaching the length of the wire), the wire's internal capacitances and inductances become important, and you can actually measure the effects of the standing EM wave across the wire.

    In fact, this is why you need to use something like coaxial cable if you want to transmit high frequencies. The coaxial cable, in essence, "confines" the electric field to between the two conductors of the cable. If you tried to transmit a signal across one wire, your line's characteristic impedance would depend largely on environmental factors: the humidity in the air, the line's distance from the ground, whether it is raining or not, etc. This is a bad thing if you want your devices to behave the same way every time.
  4. Sep 20, 2006 #3
    What you say is interesting but raises more questions for me.

    I'm trying to get to grips with the fundamentals and that is why
    a simple DC circuit is of interest to me. To my mind there is no frequency
    involved at all, so how does the energy propogate ? Assuming the
    energy does propogate as shown on the website is it mainly concentrated
    around the wires ?

  5. Sep 20, 2006 #4

    Andrew Mason

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    The source of energy is the applied power source which produces and maintains an electric field. When subjected to that electric field, electrical charges gain kinetic energy (ie. they move, or flow) due to the applied potential difference. It is the energy of those moving charges that allows work to be done within the circuit.

    In the case of a resistance, the energy of the flowing charges is transferred to the molecules of the resistive conductor and increases their kinetic energy (temperature) which can result in heat or light (incandescent). In the case of an inductor, charge flow results in the creation of a magnetic field which can can do mechanical work (eg. as the result of the interaction of that field with another as in an induction motor). In the case of a capacitor, it can be used to build up and store electrical energy.

  6. Sep 20, 2006 #5
    I can understand the electrons having kinetic energy but is there also
    energy stored in the em field caused by the movement of the electrons ?
    It seems to me that there must be, yet it is not intuitive as the field
    is static - unlike em radiation.

  7. Sep 20, 2006 #6

    Andrew Mason

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    Charges have potential energy (relative to ground potential) so long as they are at a position in the circuit where the potential is higher than ground. It is the conversion of that potential energy into kinetic energy that produces electrical energy.

    The motion of the charges also creates a magnetic field around the conductor. When charge starts to flow until it stablizes, there is some energy stored in that magnetic field. If the flows is continually changing, an em wave will result and that radiates energy away from the circuit (eg. radio waves).

  8. Sep 21, 2006 #7
    Only electron kinetic energy produces heat and light energy ? With electrons being so small and moving so slowly (although there are lots of them) is there sufficient energy ?

    The energy imparted to an electric motor comes from the em field outside the wire ? You say the magnetic field only stores energy when the current
    starts but an electric motor will run constantly under a constant current ?
    That is why the static nature of the em field confuses me.

  9. Sep 21, 2006 #8
    I think what you are wondering about is the EMF, electro-motive force, which is distinct from the actual, very slow, movement of electrons, called "electron drift". EMF is near instantaneous while electron drift is actually quite slow. EMF is the same as voltage, and is the pressure in the circuit. As Andrew Mason said, pressure is a relative proposition, not absolute, and we can only speak of pressure between two points. In a DC circuit the pressure which is of most interest is the pressure between the positive and negative imputs, whether these be from a battery or a rectified AC source.

    The source of this pressure is the fact that electrons repell each other. The more of them you pack together, the greater the pressure. In a common battery there are more electrons packed into the negative side of the cell than the positive. (This is the result of the chemical rections inside.) Therefore, given a free path, the electrons, and perhaps more importantly, the pressure they exert, will conduct along the path to the positive side. Electrons are attracted to positive charges: protons, and will move toward any atoms that have vacant space for more electrons. However, they will also move to any place where the electron density is lower, seeking to spread themselves out evenly from each other. In this way, any place where the electrons are less densely packed will act as "ground": a desirable destination given a free path. Conducting materials like metal wire, form the path of least resistance for electron movement. Insulators, like air, offer high resistance to it. The energy of the EMF is actually in the wires where the electrons are pushing against each other, so to speak, trying to get away from each other.

    I find the site you linked to to be confusing for not mentioning EMF and seeming to say that the electric and electromagnetic fields in the space around the wires are the only important thing to be aware of.

    DC motors are designed so that the current to the coils is continually cut off and on by the action of the commutator despite the fact that the imput current is steady. Each coil is continuously being turned on and off, on and off as the commutator rotates on the shaft first aligning with the imput contacts, called "brushes", and then moving away from them. The magnetic fields around the coils are therefore constantly expanding then collapsing, expanding, then collapsing.

    In AC motors the imput current, itself, is constantly surging, first in one direction then the other causing the magnetic fields around the coils to expand, collapse, then expand with the opposite polarity, then collapse. No commutator necessary.
  10. Sep 21, 2006 #9
    Thanks so far guys. Sorry if I'm going off at a bit of a tangent here.

    If you connect the two terminals of a battery with cooper wire the
    battery will drain pretty quickly and I take it all the chemical energy
    will be converted into heat in the circuit ?

    What happens if you use a superconducting material where there should
    be little or no losses ? The battery will still drain but what will have
    happened to the energy ?

    In the case of the dc motor, it is just wire, so is it the case that the
    interaction of the circuits em field with the magnet's magnetic field
    causes an increase in resistance in the cicuit ?

  11. Sep 21, 2006 #10
    In both these cases all that happens is that the pressure within the circuit is relieved and becomes zero. You will have wasted the available energy by not making it do any work in the process. In the first, room temperature, battery, only a small amount of the available energy will have been converted to heat because the resistence of a plain piece of copper wire is so low. In the superconducting material even less will have been converted to heat. Electric current only converts to heat when it forces its way through materials that offer alot of resistance. So, what happens to the energy is that it is used up pointlessly causing chemical reactions at the positive ends of the battery.
    Yes! When you make the magnetic field around the coil do work by exerting force on a mounted permanent magnet it increases the resistence of that part of the circuit. (It is also true that if you apply any mechanical force opposed to the rotating shaft, which is what is refered to as a "load" the resistance of the circuit increases. A freely rotating motor doing no work requires much less power than a motor that is being made to operate a machine.)
  12. Sep 22, 2006 #11
    Okay, I think maybe it is energy I don't understand.

    Am I right in saying that the chemical energy in the battery will be converted to either heat, light or mechanical energy, or will be converted
    back to chemical energy?

    In a circuit with little or no resistance most of the energy gets converted
    back to chemical energy when the electrons arrive at the positive
    terminal. But they will still arrive back in a circuit that generates heat,
    light etc., they will just arrive back more slowly.

    My hurt brains.

  13. Sep 22, 2006 #12
    Energy is the ability to do work. It has many common forms. The current of a battery is one of them.

    No. See the next:
    No, it isn't converted back to chemical energy, just back into chemicals. It participates in a chemical reaction which renders it now unusable in the circuit. No more ability to do work.

    Think of a stone falling. There is energy there: you could use the falling stone to do work. You could use it to crack a nut open. You could put a nut on the ground where the stone is going to hit. Or, you could do nothing. If you do nothing, the stone hits the ground and the energy of the stone gets converted to heat and sound which radiates away through the earth and air. The energy in the drop of that stone is not available to be reused. It's been used up doing something useless to you. You could reuse the stone itself, just like you could reuse the battery, but you'd have to get energy from somewhere else to lift the stone or to separate the charges in the battery.
    Also no. If the energy is used to create heat, light, etc, it does not make it back to the battery. Once it becomes heat or light or sound or mechanical force or whatever, it has been removed from the circuit.

    Think of money. If you spend some money you get something for it. Let's say you get a shirt. Now that money is gone. You don't have that money anymore to spend. Instead, you have a shirt. So, if you "spend" some electric current, you get heat or light or sound etc and that's what you have. You don't still have the electric current.

    The electricity is trying to get to the other side of the battery, just like the stone is trying to get to the ground. If we put something in its way, though, it doesn't make it back to the battery or to the ground.
    Last edited: Sep 22, 2006
  14. Sep 22, 2006 #13

    Andrew Mason

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    Yes. The drift velocity is the average speed at which individual electrons migrate through the conductor. But they are continually gaining kinetic energy and then slamming into atoms and bouncing around in all directions within the conductor. Have a look at the Wikipedia article on drift velocity
    Energy is stored in the magnetic field of a conductor. Energy from the current is transferred into the magnetic field only while the current is building up. Once the current stabilises there is no transfer of energy to the magnetic field. When the current decreases, there is a transfer of energy from the magnetic field to the wire.

    Resistance is a property of the conductor. The constant change in direction of current within the copper wired armature produces inductive reactance, which reduces current. Inductive reactance is not resistance, but it limits current. The current is limited due to the 'back emf' or voltage induced by the changing magnetic field.

  15. Sep 22, 2006 #14
    Okay thanks guys, that clears up my original question. I'm off to think
    a bit more about energy, conservation of, and potential, then maybe
    start a new topic.


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