Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Just to know ?

  1. Jul 26, 2009 #1
    Just to know...??

    This is no homework,, this is just my effort trying to solve this out...

    If we ever succeed to transfer electrical power without the use of wires (conduction) would that mean that the speed of light is reached? And If your answer is no, then how come the electric power that lights a bulb earns the speed of light when it's changed into light?
     
  2. jcsd
  3. Jul 26, 2009 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Just to know...??

    What do YOU mean by "transfer electrical power"? Every radio, cell phone, or television broadcast transfers electrical power- at the speed of light, of course!

    If you mean transfering enough electrical power to actual 'power' something- run a motor for example, rather than just send a signal- that has also been done. The difficulty, the reason it is not a viable way of sending power, has nothing to with the speed of light but with the fact the 'signal' spreads out spherically. In order to transmit a large amount of electrical power to a specific spot, you would have to waste an enormous amount of energy that was not picked up. In addition, there are concerns about the health or environmental problems with broadcasting large amounts of energy.
     
  4. Jul 26, 2009 #3

    negitron

    User Avatar
    Science Advisor

    Re: Just to know...??

    Electricity does travel at the speed of light. It travels at the speed of light through the insulating medium surrounding the wire. For a wire in free space, this is the same as c; in air it's about 98% c. For wires with typical plastic insulation, this can be anywhere from about 50% to 80% c, depending on the dielectric constant of the insulator.
     
  5. Jul 26, 2009 #4
    Re: Just to know...??

    Hmmm. Are you sure about that?
    An electron has mass, thus can not travel at c.
    I think that particle accelerators have brought them close to c, but this is certainly not something experienced in electrons moving in wiring.

    Anyway, those are just my thoughts.
     
  6. Jul 26, 2009 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Re: Just to know...??

    The electron's drift velocity is not the speed of electricity. Electrical impulses travel much faster.
     
  7. Jul 26, 2009 #6

    negitron

    User Avatar
    Science Advisor

    Re: Just to know...??

    Right. The speed of electricity is really the speed of the EM wave which propagates around the conductor, not the speed of the electrons moving in it.
     
  8. Jul 26, 2009 #7
    Re: Just to know...??

    OK, that makes it clearer... to me anyway.
     
  9. Jul 26, 2009 #8
    Re: Just to know...??

    I'm a bit confused about the role of the electron and EM wave when electricity flows through wires. Electricity is the flow of electrons that results from a difference in potential energy...Waves carry energy and so do electrons. So what is the role of a EM wave if the electron is transferring the energy?
     
  10. Jul 26, 2009 #9

    negitron

    User Avatar
    Science Advisor

    Re: Just to know...??

    Electrons don't transfer energy, they carry charge. Accelerating charges releases electromagnetic energy. The carrier particle of electromagnetic energy is the photon, which travels at the speed of light.
     
  11. Jul 26, 2009 #10
    Re: Just to know...??

    okay. hwbout when electrons move through a lightbulb filament: does the resistance of the bulb cause electrons to decelerate thus releasing electromagnetic energy which lights up the bulb? Do electrons keep accelerating through a conductor or is it at a constant speed?
     
  12. Jul 26, 2009 #11
    Re: Just to know...??

    I always understood electrical energy as the potential energy of a charge sitting in an electrical field, and not energy radiated by an accelerating charge. So the only way to transfer electrical energy through a wire is to move the charges (electrons) themselves through the wire. Capacitors aren't charged by absorbing radiation, they are charged by moving some electrons from one plate to the other, creating an electrical potential, and thus giving the charges a certain potential energy. Similarly, electrical motors depend up the moving charge carriers themselves, and aren't at all powered by radiation.

    The lightbulb is a good example to mention. The lightbulb works by sending a current, ie moving electrons, through a resistor. The resistor converts some of the electrical energy of the electrons into thermal energy. The resistor heats up. Any object that has a temperature above absolute zero emits electromagnetic radiation, because the electrons in the material are jiggling, or accelerating. Once the temperature of the resistor is high enough, the radiation emitted is energetic enough to fall within the visible range of the EM spectrum. So I guess lightbulbs utilize both electrical energy and radiative energy to operate, hehe.
     
  13. Jul 26, 2009 #12

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Re: Just to know...??

    That's partly true but not really the point.
    Electricity does move through a wire by moving electrons but the electrons don't have to move the full lenght of the wire. An electron moves (a small distance) which creates an electric field that moves at near light speed to the next electron which it causes to move. This moving electron creates a field that moves the next and so on.....
    It's a bit like a train starting off by bumping wagons, or a mexican wave in a stadium - the signal travels along faster than the individual wagon or football fan can move.
     
  14. Jul 26, 2009 #13
    Re: Just to know...??

    The light bulb analogy makes alot of sense. And mgb_phys cleared up the role of the EM wave. this is interesting stuff :) thx
     
  15. Jul 27, 2009 #14
    Re: Just to know...??

    Yeah, I've learned a few things here myself!
    Gotta love PF.
     
  16. Jul 27, 2009 #15
    Re: Just to know...??

    Classically speaking, current theory is that any information traveling through a medium of massive objects will travel below light speed, and any information traveling through a massless medium will travel at light speed, no matter what the form of that information.
     
  17. Jul 27, 2009 #16
    Re: Just to know...??

    You can measure this with a waveform generator, oscilloscope, and a length of coaxial cable, all which can be found in an introductory university lab.
     
  18. Aug 1, 2009 #17
    Re: Just to know...??

    Thanks for the correction! What you say makes sense now that I think about it more. In a DC circuit with a battery, a capacitor, and a switch and no resistor, the time constant would be roughly zero meaning the capacitor would charge almost immediately.
     
  19. Aug 2, 2009 #18
    Re: Just to know...??

    In wires, you don't just have one electron. Basically, it's the old pool ball system. An electron ( or a lot of them) bang into one end of a wire. This shifts the electrons all through the wire, and some come out the other end to give you a great shock or whatever.

    The same thing happens in the filament of a light bulb. However, I would assume that due to its structure, many of the electrons get banged about within the filament. All of this internal movement causes heat and light.

    The actual explanation involves a lot more detail, but that's the basic concept.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook