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Electricity doesn't move at the speed of light?

  1. Sep 2, 2003 #1
    Electricity doesn't move at the speed of light?!?!

    I recently have read (in several aticles) that have presented evidence that "electrons" do not actually flow at the speed of light (and electron movement is thought to be "electricity"). If you argue that "Yes electricity DOES flow at the speed of light", then you must specify which of the 5 definitions of electricity you picked to represent "electricity". The electrons move actually very slow in the conductor, 0.2 cm p/s I was told. But the effect of the movement across the whole of the conductor is felt at once through the kinetic energy of the movement.

    There are also many logical things (That we don't ususaly think about (***Almost an Oxymoron***)) about how it is not possible for the electrons to move at the speed of light.

    Number one: Electrons have mass. Nothing that has mass can travel at the speed of light.

    Second: Even light slows down in a volume, it only travels at c in a vaccum. Copper or other conductors are even more dense than air(which slows down light), and thus electrons definately could not move at the speed of light in the volume of the conductor.

    Third: If they (electrons) were even moving at speeds half that of c the conductor would get so hot it would melt, the resistors would overload + burn, stuff would probably catch on fire, and (of course joking) it could even go plasma ;-)!

    All my points/comments (except for the one about plasma, Duh) hold a hard to debate validity in them.

    I would like to see other peoples comments on this.
     
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  3. Sep 2, 2003 #2

    chroot

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    I have no idea what you intend to discuss with this thread.

    Conduction electrons move very quickly inside the wire, with thermal energies on the order of 3/2 kT.

    In the absence of an electric field, the electrons do not have any net movement in any direction. When you apply a voltage and hence create an electric field, the electrons now preferentially move in the direction of highest potential. This net movement is slow, on the order of centimeters per second, and is called the drift velocity.

    Changes in electric and magnetic fields (of which light is an example) propogate at or below the speed of light. In a vacuum, changes in the field propagate at c. In a wire, the field propagates a little slower than c.

    Thus, when you throw a lightswitch, the electric field makes its presence known throughout the wire at nearly the speed of light. The electrons initially present in the wire, however, might take hours to actually make their exits, since they move towards the greatest potential on average only at their drift velocity.

    It turns out that the presence of matter around the wire is really integral in determining how fast the field propagates. A wire carrying a current builds a concentric magnetic field around it. This field must propagate through the material surrounding the wire. Thus, a bare wire in a vacuum will carry electrical signals faster than will a trace buried in a printed circuit board.

    - Warren
     
  4. Sep 2, 2003 #3
    To explain the "speed-of-light" part of electricity I will use some analogies. First of all you must understand that electrical "enery" is like sound waves that move throught "air". Where the waves are the energy (that move fast) and Air (which transmits the wave). Same with electrons and electrical energy.

    Ok now, say you have a straw full of marbles and it is (how ever many miles it is that light can travel in a second)<--miles long. You shove a nother marble in and almost immediately the marble in the other end flies out while the rest of the marbles (electons) barely moved. It transmitted a wave (using a domino effect w/ kinetic energy) through the straw to the other end.

    That is how it really works, or so I beleive and has been detailed in research.

    Comments?
     
  5. Sep 2, 2003 #4
    I have no idea what you intend to discuss with this thread.

    That's exactly what I wanted to hear. Your opinion on my post and where you stand in light of it. For it against it, or somewhere else.

    I just want to know if someone else besides be beleives this.

    My dad has been an electonics technition for 20 years and he doesn't fully accept the idea of electrons moving slow after having been told (like every body else) that that move at the speed of light.
     
    Last edited: Sep 2, 2003
  6. Sep 2, 2003 #5

    chroot

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    Tell your dad to pick up any first-year physics textbook. That electrons do not move at the speed of light is certainly not news. Anyone who says "electrons move at the speed of light" has absolutely no idea what they're talking about.

    There are, in fact, many simple experiments that can be done to demonstrate the drift velocity of electrons. The Hall effect, for example, provides one mechanism to measure it.

    - Warren
     
  7. Sep 2, 2003 #6
    There are, in fact, many simple experiments that can be done to demonstrate the drift velocity of electrons. The Hall effect, for example, provides one mechanism to measure it.

    My dad said that he would consider it if he had some visible proof in the form of a research paper or experiment. Do you know any sites that you can direct me to to find this information?

    Please help.
     
  8. Sep 2, 2003 #7

    marcus

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    I hope Warren wont object to my butting in for a moment. I just want to make sure, Cyberice, that you and your dad both understand that an electric signal can travel along a wire much faster than the electrons themselves.

    Say you have a mile long two-conductor wire leading to a lightbulb. You throw a switch at one end and the lightbulb will
    go on almost immediately down at the other end.

    The electrons between here and there might only be going a couple of centimeters per second, as you say----some drift velocity which can (as Warren says) be measured
     
    Last edited: Sep 2, 2003
  9. Sep 2, 2003 #8

    chroot

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    Start here: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html

    (Any first-year physics text will also include this information.)

    You can set up your very own apparatus and measure the Hall voltage with your own voltmeter. The Hall voltage is related very simply to the drift velocity of the charge carriers.

    Why don't you digest this information for a while and see if you can come up with your own apparatus that will be capable of showing you a clear, unambiguous Hall voltage?

    Let me know if you need some clarification.

    - Warren
     
  10. Sep 3, 2003 #9
    I've read that you can actually SEE electrons moving slowly on surface of metal wire. Iirc, electrons are what makes bare wire look "metallic", and under microscope you'd see bubble-like thingies moving.
    I wonder if thats true that you can see them.
     
    Last edited: Sep 4, 2003
  11. Sep 3, 2003 #10

    chroot

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    Uh... *rolls on the floor, laughing*

    - Warren
     
  12. Sep 3, 2003 #11
    I wonder if thats true that you can see them.

    I'm sorry, but who ever told you that lied. We got lucky just getting a picture of one *big* atom. (we have seen uranium - and I've seen the video clip of it) but seeing how uranium is 237 times bigger than hydrogen, and hydrogen has only one proton (no neutrons), and a proton is 2000 times bigger than an electron, I could safely say that we may not see an electron anytime soon (if ever).

    If we could see electrons clearly heinsburg would have a hayday.
     
    Last edited: Sep 3, 2003
  13. Sep 3, 2003 #12

    russ_watters

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    Re: Electricity doesn't move at the speed of light?!?!

    Two clarifications: the word "volume" is meaningless there. The word you are looking for is "medium."

    Second, light travels at C. Period. Unforturnately the glossed over high school physics approach to explaining light leads people to the misunderstanding that light itself slows down.

    It does not. Any normal medium is composed almost entirely of empty space. The volume of the elementary particles (electrons, protons, neutrons) is statistically insignificant in the total volume of an atom. And then the atoms are not necessarily touching. So its actually redundant to say "light travels at C through a vacuum." When light is light it is ALWAYS transiting a vacuum.

    In a medium, light will occasionally hit something and when it does several things can happen, but to use refraction as the example, the light will be absorbed and re-emitted. The time delay for the absorption and re-emission causes the light to APPEAR to have slown down while traveling through the medium. What has actually changed is only the AVERAGE speed.
     
  14. Sep 3, 2003 #13
    (Any first-year physics text will also include this information.)

    You can set up your very own apparatus and measure the Hall voltage with your own voltmeter. The Hall voltage is related very simply to the drift velocity of the charge carriers.

    Why don't you digest this information for a while and see if you can come up with your own apparatus that will be capable of showing you a clear, unambiguous Hall voltage?

    Let me know if you need some clarification.

    - Warren



    Well first of all what is the formula for it and how does it work? What elements determine the values used in the formula and what law (or laws) can be used to back up what it built upon?

    I am asking strictly for proof. Other wise no convincing will come over the people I tell it to.

    The way I found out that *electrons* don't move at the speed of light is that I read an article called "electricity misconceptions". It explainned that it was the electrical energy that traveled at the speed of light, not electrons.

    Well, WHAT IS THAT ENERY? Certainly not Heat or light. And don't say "electrical energy", because that is using a word to to describe it self. For instance don't say in reply to the question "what is fire?": and answer "fire."

    I want to know what electrical energy is. Is it kinetic motion of particles? (It is described as a kinetic wave, but what kind)? If so then how could it possibly heat up a light bulb?

    Please explain to me (with as much scientific EVIDENCE as possible - assume that I am stubborn and don't beleive what I hear :smile: ) what the energy is, how fast the ELECTRONS flow, what formulas and calculations does it use, what laws does it follow or build on. Please be as factual as possible. Thnx.
     
    Last edited: Sep 3, 2003
  15. Sep 3, 2003 #14

    chroot

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    Did you not read the page on the Hall effect? Did you not look in a first-year physics textbook as I told you to do? I'm sorry, but I don't have the time or inclination to type out here what is readily available elsewhere. Read the page to learn how the Hall effect works, and how you can measure drift velocity with it.
    What is energy? Energy is the ability to do work.

    What is work? Work is the effort it takes to roll a bowling ball up a hill, or the effort expended by a mishandled bottle of soda when it expells its contents onto your favorite shirt.

    Electrons are like the bowling balls, and the electric field is like the hill. An electric field exerts a force on charged particles within it. The force they feel is expressed very simply:

    F = qE

    where both E and F are vector quantities, and q is the charge. q is in units of coulombs, F is in units of newtons, and E is in units of newtons per coulomb or (equivalently) volts per meter.

    Where did "volts" sneak in? Well, voltage is a measure of potential energy per unit charge, and is thus equivalent to joules per coulomb. When a coulomb of charge moves through an electrical potential of one volt, one joule of work is done. When this work is positive, energy is released, usually as heat or sound or light.

    When you roll a bowling ball down a hill, the same concepts apply. The force felt by the bowling ball is

    F = mg

    where m is the mass, in units of kilograms, and g is the gravitational acceleration, in units of meters per square seconds. You can identify q and m as being "analogous quantities," and E and g also.

    If you want to think about the potential experienced by the bowling ball, you know it will have to be in units of joules per kilogram. You should know that the gravitational potential energy of a bowling ball at height h above the ground is

    U = -mgh

    Don't worry about the minus sign, it's only there to adhere to physicists' conventions.

    Look at the units:

    U (joules) = -m (kilograms) g (meters/sec^2) h (meters)

    You can rearrange this as:

    U/m (joules/kilogram) = gh (meters^2/second^2)

    Lo and behold, the quantity (gh) is equivalent to both meters^2/seconds^2 AND joules/kilograms. gh is therefore a measure of the potential energy per unit mass of a gravitational field.

    Remember now that voltage is a measure of the potential energy per unit charge in an electric field, and you should see at once how similar the concepts are. The only difference is that we routinely lump the electrical analogue of (gh) into one quantity, and call it "voltage." There is no special name for the quantity (gh), however.

    These two sentences are analogous:

    When a coulomb of charge moves through an electrical potential of one volt, one joule of work is done.

    When a kilogram of mass moves through a gravitational potential of one meter^2/second^2, one joule of work is done.

    In the case of a 1 kilogram bowling ball moving through a vertical distance of 1 meter on earth's surface, where g is 9.8 meters/second^2, 9.8 joules of work are done.

    Now, you plug a light bulb into the socket and a generator many miles away applies a potential to it through the connecting wires. The potential is 120 volts. The resistance of the filament in the light bulb is 120 ohms, so one ampere of current flows. An ampere is one coulomb of charge per second. Apply the relationship figured out above:

    When a coulomb of charge moves through a potential of 120 volts, 120 joules of work are done. Since one coulomb of charge is moving through the light bulb every second, 120 joules of work are being done every second. The watt is the unit of power: energy per unit time. 120 joules per second is equal to 120 watts of power. Your light bulb is dissipating 120 watts of power.

    When you drop a bowling ball, the energy goes into breaking the floor, making a loud noise, and scaring the neighbors.

    When you turn on a lightbulb, the energy goes into jostling the atoms in the filament, causing their temperature to increase, with the ensuing radiation of visible light.
    Now, exactly how big is a coulomb? Well, it takes 6.25*10^18 electrons to equal one coulomb of negative charge. A cubic meter of copper, by contrast, contains about 8.46 * 10^28 electrons, or about 13 billion coulombs.

    Let's take a piece of copper wire 5 mm in diameter. The area of the cross section is 1.9*10^-5 square meters. A meter of this wire contains, 1.9*10^-5 cubic meters of copper, or about 265,000 coulombs of charge.

    If you wire up your 120W lightbulb, one columb of charge per second is moving through it -- but every foot of wire contains 265,000 coloumbs of charge! You can see immediately that the charge doesn't have to move very fast. If all the charge in that meter of wire were to move through the wire in one second, all the electrons moving at just one meter per second, your lightbulb would dissipate 31 million watts of power, as much power as consumed by a large university campus. Clearly, electrons flow much more slowly than that!

    Does this make sense?

    - Warren
     
    Last edited: Sep 3, 2003
  16. Sep 4, 2003 #15
    Cyberice is correct.

    Let me try to explain it how I learned it.

    Assume for a moment that a pipe has been filled with ping pong balls, if a ball is forced into the end of the pipe, the ball at the other end will be forced out. Each time a ball enters one end of the pipe, another ball is forced out the other end. This principle is also true for electrons in a wire. There are billions of electrons in a wire. If an electron enters one end of a wire, another electron is forced out the other end.

    Again assume for the moment that a wire is long enough to be wound around the earth 10 times. If a power source and switch were connected at one end of the wire and a light at the other end, the light would turn on the moment the switch was closed. But it would take light approximately 1.3 seconds to travel around the earth 10 times.

    What we call this phenomenon is the "impulse of electricity"
     
  17. Sep 4, 2003 #16
    1) Get a bunch of marbles in a tube, filling it from one end to the other.

    2) Stick your finger in one end, and push at any rather slow speed you like.

    Despite the slow motion of each marble, you will note that (if the tube was full, the marbles packed up against each other), the marble at the far end moved pretty much at the exact moment you pushed the marble at the near end.

    The individual marbles move slowly. The effect moves pretty damn quick.
     
  18. Sep 4, 2003 #17

    chroot

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    This is an analogy. Real electrons do not behave this way.
    This is patently, egregiously incorrect. Changes in the electric field propagate at or below the speed of light, depending upon the index of refraction of the medium. The lamp would absolutely not turn on instantaneously. Such behavior would actually violate a very large number of physical laws.
    I have never heard anyone call anything, much less this non-existent effect, the "impulse of electricity." Sorry. In the future, please don't post when you don't know what you're talking about.

    - Warren
     
  19. Sep 4, 2003 #18
    Excuse me?
    Come again?

    The Impulse of electricity is a very real theory--see ELECTRON THEORY. Since you do not know what I am talking about then maybe you should shut up and listen and don't assume I am wrong without acknowledging you're own lack of knowledge on the subject--That is immature and rude!



    Because you have never heard of it does that mean I am wrong? Who are you to say such a thing? If you want to debate this please go ahead but don't ever talk like that to anyone and except them to take your word for it.

    ps Please follow your own advice--last sentence in your post
     
  20. Sep 4, 2003 #19

    chroot

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    Hey Integral,

    Little help?

    - Warren
     
  21. Sep 4, 2003 #20
    Changes in the electric field propagate at or below the speed of light

    Changes? Changes? What changes are you talking about? Elaborate on changes. Because I want to know what DOES move at the speed of light. I beleive it is the a impulse as stated earlier. If you say "changes in polarity" then that would describe voltage. What effect is felt at the speed of light? A wave? Heat? Kinetic energy? (certainly not-->) Electrons? How did we even measure it? What is IT?

    I have to go with the elctrical impulse idea here, in the form of a kinetic energy wave of the whole, that is what moves at the speed of light.
     
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