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Theoretical questions: Basic electric concepts

  1. May 14, 2009 #1
    A wire and a battery (a AA Duracell battery just to keep things clear) are connected in series.

    The way I understand it, the wire (say, 1 meter in length) already has its fixed number of charges, both positive and negative. When those charges move, meaning when there is a potential difference, the charges move ALONG the wire.

    In other words, if we were to look at the wire (microscopically), we should see something like this:

    http://www.hasdeu.bz.edu.ro/softuri/fizica/mariana/Electricitatea/Practic/electricity%20book/images/animated_wire_1.gif" [Broken]

    Where, if we were to paint one electron BRIGHT GREEN, we would be able to follow it all along the wire, correct?
    That brings up another question..does the battery not supply or absorb any electrons? That would change the amount of charge, thus changing the current, would it not?
     
    Last edited by a moderator: May 4, 2017
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  3. May 14, 2009 #2

    berkeman

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    The negative charges (electrons) are what move, and the lattice of nuclei stays fixed. Yes, your bright green electron would be seen to move, from the - terminal of the battery, out around the wire, and back to the + terminal of the battery. And then inside the battery from the + terminal to the - terminal, and back out onto the wire.

    Think of the battery as a "pump". It does not add or subtract charges, it just pumps electrons from a low potential energy state (as they arrive at the + terminal of the battery) to a higher potential energy as they leave the - terminal and head out into the wire.
     
    Last edited by a moderator: May 4, 2017
  4. May 14, 2009 #3
    Thank you for noting that the lattice of the nuclei stays fixed; it was understood, but it is made more clear when it is actually said.

    Now, in practically every textbook (that I have used), the current we consider is the movement of positive charges, with the direction opposing the "electron current". First of all, are these "holes" that are moving (relative to the electrons), or is this opposing current caused by the movement of protons?
     
  5. May 14, 2009 #4

    berkeman

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    No, not + holes. That concept is useful in semiconductor physics, but not for electrical current flowing on a wire. I believe the + current convention was just a historical thing, before the nature of the movement of electrons in conductors was understood. You might be able to find the historical context with Google.
     
  6. May 14, 2009 #5
    My real question leads to alternating current.

    With the same model above, if we were to follow the one bright green electron we would see it moving back and forth, correct? Ideally, it should move an equal distance "right and left", so to speak (thinking of a sinusoidal input) always sliding past the same "mid point".

    Is this why AC is more efficient than DC for long distance transmission? A heavy duty wire, leading from the power plant all the way to a wooden post near homes, already has charges all along its length. So, ideally, the only charges that are being fed to the transformer are the ones at the tip of the wire closest to the homes, and they're being constantly reused. Is all this correct?

    That brings up another question...may be obvious, but why is AC bad for electrical components like a laptop, for example? Does this have to do with the frequency response of each component?
     
  7. May 14, 2009 #6

    chroot

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    Current can be considered as either the flow of negative charges in one direction, or the flow of positive charges in the other. There is absolutely no difference in the two views.

    In reality, only type of carrier actually moves in a specific kind of material, but that's often irrelevant.

    - Warren
     
  8. May 14, 2009 #7
    By 'flow of positive charges', you mean protons?

    I understand this; this makes sense.
     
  9. May 14, 2009 #8

    chroot

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    No problems there.

    Neither is inherently more efficient than the other in transmitting energy through a wire. The loss of efficiency in DC systems is due to the much more complex electronics that must be used to step-up and step-down voltages at each end of the line. AC can be stepped up and down with passive devices, transformers, which can be made quite efficient for much less cost than an equivalent DC system.

    Yep.

    Microprocessors are based on transistors, which act like tiny switches. It's imperative that each switch maintain its state until it supposed to change. If you tried to use AC to control the switches, they'd be constantly changing states in a way that would be difficult to control.

    - Warren
     
  10. May 14, 2009 #9

    chroot

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    In the case of a battery, the positive charge carries are usually protons. In the case of a current through a p-type semiconductor, the positive charges are "holes," places where an electron is missing. Interestingly, the "hole" can be thought of as moving through the sea of neutral atoms in a very similar way that an electron would move through a sea of lattice ions.

    - Warren
     
  11. May 14, 2009 #10
    ions and protons in some electrochemical setup, or a particle accelerator.

    but for everyday electronics, negative electrons flow and positive charges on which they cling remain stationary,
     
  12. May 14, 2009 #11
    it's a little more complicated. Consider this set up: you have an AC generator, a transmission line, and a termination load. If the impedances are not properly matched, two waves will form. One wave (a bunches of electrons moving back and forth) will be absorbed by the load. And a little bit of it will be reflected back to the generator in different phase. So now you have two bunches of electrons zipping back-forth but in different phase.

    In AC design it is important to have as little reflection as possible, because it is a waste of power

    That depends on the frequency of the AC source. From the frequency you can calculate the wavelength which is length of the wave in a cycle. On an atomic level, the electrons will be pushed up to half a wavelength before turning back to the source. When they reach half a wavelength, they will excite another bunch of electrons that will travel for another half a wavelength, and so forth.
     
    Last edited: May 14, 2009
  13. May 14, 2009 #12

    vk6kro

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    That brings up another question...may be obvious, but why is AC bad for electrical components like a laptop, for example? Does this have to do with the frequency response of each component?

    THere are a couple of problems with feeding AC into DC equipment.

    One is that AC comprises Positive and Negative voltages and the Negative voltages could be destructive for some electronic components that are not meant to have negaitive polarity on them. Most semiconductors and all electrolytic capacitors would not function properly with reversed polarity and may be destroyed by it.

    Another problem is that AC is a varying voltage. This may be obvious, but it can result in humming noises in sound or dark bands on a monitor screen at least. So, even if the laptop had a diode to protect against reverse polarity, it could still get strange effects from the varying DC that results.

    I used to believe that individual electrons travelled very quickly through conductors. This is apparently not true. Individual electrons travel at a rate of millimeters per second, but the wave of such movement travels quite quickly, approaching the speed of light.

    http://en.wikipedia.org/wiki/Speed_of_electricity

    Free electrons in a conductor vibrate randomly, but without the presence of an electric field there is no net velocity. When a DC voltage is applied the electrons will increase in speed proportional to the strength of the electric field. These speeds are on the order of millimeters per second. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field.[1]
     
  14. May 14, 2009 #13
    I was able to picture this exactly, thank you for this.


    Is that the case every time, when we walk about two components working in different phases? Is there any time when we do want this type of effect? I can immediately recall some words from a textbook about capacitors being -90 degrees out of phase, and inductors +90 degrees, but that's as far as my understanding goes on this topic...just words. I've made it a mission this semester in Uni to change (fix) that. /rant

    Great link, I'm always looking for more 'in depth' information like this. A lot of the time it seems like questions of this type even annoy some professors.
     
    Last edited: May 14, 2009
  15. May 14, 2009 #14

    vk6kro

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    I like this quote from Wikipedia:

    http://en.wikipedia.org/wiki/Drift_velocity

    As a numerical example,for a copper wire of 1 square mm area, carrying a current of 3 amperes, the drift velocity of electrons would be about 0.00028 metres per second (or just about an hour to travel one metre).

    So, forget about individual "green" electrons zooming along wires at amazing speeds. It just doesn't happen.
     
  16. May 15, 2009 #15

    chroot

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    Yep, once you get your head around those numbers, you begin to really understand just how powerful the electromagnetic force is!

    Another astonishing figure is that a kilogram of copper contains about one and a half million coulombs of free electrons. If you could pull out just one coulomb and hold it in your left hand, and another in your right hand, the two groups of charges would repel each other with two billion pounds of force!

    Kinda puts gravity to shame...

    - Warren
     
  17. May 17, 2009 #16
    Figured since there's plenty of good information on this thread, I'd post this here:

    Rather poor and clumsy writing, but he goes into great detail on the fundamentals of electric current and explores the "following individual electrons" notion.

    http://amasci.com/amateur/transis.html
     
  18. Jun 19, 2009 #17

    DC is a more inefficient mode of transmission, but due to the increased costs changing voltages; DC is only used in transmission over very long distances. Wiki search HVDC.
     
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