Is voltage just moving the electrons in a conductor or introducing new electrons from the source?

Discussion in 'General Electronics Chat' started by elara, Jan 12, 2016.

  1. elara

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    It maybe an obvious answer, but as I understand the potential energy from the voltage source in a circuit is responsible for introducing electron flow(assuming a closed circuit), but does the voltage source itself introduce new electrons in the conductor or just move the existing electrons that are present in the conductor?

    Thanks,
    Elara
     
  2. dl324

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  3. hp1729

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    You obviously have read a good bit of stuff already and did not find an answer ... Voltage is just the pressure to move the electrons that already exist. A magnetic field crossing a wire causes the electrons to move. A battery is a chemical source of electrons. When it runs out of electrons the battery is "dead".
     
  4. MikeML

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    I would say it differently. A battery creates a voltage. Electrons drift in the presence of a voltage gradient which is created by connecting the ends of a wire between two different voltages (+ and - plates of a battery).

    Kirchoff's Current Law says that the current must be the same everywhere in a series circuit, hence the current out of the positive plate of the battery equals the current in the wire equals the current into the negative plate of the battery. A battery is dead when it no longer has a voltage difference between its plates.
     
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  5. MrSoftware

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    Here is how I think of it, someone can correct me if my analogy is wrong. Think of a wire like a drinking straw that is full of water (electrons). When you put one drop of water in the bottom of the straw, another drop pops out the top. If you keep adding drops in the bottom, eventually your first drop will pop out the top. A wire is similar; it's already full of electrons, and as you add electrons in one side other electrons pop out the other side. Look up the math for electron propagation in a wire, it's slower than you would think.
     
  6. #12

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  7. elara

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    Thanks everyone. I am not sure if I have a definite answer yet. All I think know is that the result of some sort of "static electricity" stored in a "reservoir" aka voltage source - when this source is connected to a circuit - the electrons from this source starts to flow (electric field) creating voltage which means that electrons are transferred to the conductor and flow along with the existing electrons in it in a continuous loop ( in a closed circuit).

    MikeML, could you tell me in a series circuit, why does the current remain the same when the voltage is dropped across the resistor? I assumed the voltage affects the rate of flow of current. In other words, the greater the force, the greater the electron velocity. So if voltage has dropped, why doesn't the electrons flow at a relative rate? Apologies if this is a silly question.
     
    Last edited: Jan 12, 2016
  8. #12

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    That is a point of mistake. Amperes is more about velocity than voltage is.
    It's all about energy. The energy in a battery makes electrons flow. Electrons carry a charge. Moving a charge requires energy. That energy is dissipated as heat in the resistance of the circuit. Every electron is accounted for. For instance, you can calculate how many grams of metal will be moved in an electroplating process by counting the electrons.
     
  9. marcf

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    It is probably worth pointing out that the velocity of the signal in a conductor (due to electromotive force applied to charge carriers ) is much faster than the velocity (drift) of the individual charge carriers (electrons and/or holes).

    A typical velocity for the individual electrons in a circuit is in the range of a hundredth of a cm/sec, while the velocity of the signal is 0.5 to 0.3 the speed of light.
     
  10. #12

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    I thought of an exception. In a cathode ray tube, oscilloscope, TV picture tube, the velocity of the electrons is caused by a voltage difference. While the electrons are traveling in a vacuum, they are endowed with more energy (in the form of velocity) which is inflicted on them by an external source. They hit the front and, splat. The energy is dissipated by converting it to light and heat.

    In a solid conductor, the current is a flow of charge (energy) carried by electrons. Every electron that goes in one end comes out the other end. They all travel at the same speed. The only change is where the energy went. It became heat in a resistor.

    In the CRT, every electron that went in, came out. The only thing that changed was the amount of energy each electron carried. The energy changed, but the electron did not.
     
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  11. WBahn

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    You might find this useful:

    http://forum.allaboutcircuits.com/blog/a-battery-isnt-a-capacitor.588/
     
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  12. hp1729

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    That works.
     
  13. elara

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    Thanks again.

    Kirchoffs law of Current:
    I think I understand why the rate of current is the same even when voltage is dropped in the presence of a resistor/resistors i.e. in a series DC circuit:

    Previously, I was comparing electron flow to flow of water from a pipe which is clenched at one point and also imagining air into the equation. Since there is no "air" in the conductor and there is only one path, the "overall" current is the same throughout the single path. This however does mean that since the voltage has dropped, the "current" has dropped but remains equal along the path. No?

    Thanks again for any guidance.
     
  14. #12

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    Because of the resistance, the current, OVERALL has dropped from theoretically infinity to some reasonable number. That number of electrons is flowing in the path in equal numbers at all times and all places in that path.
     
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  15. MikeML

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    look at this
     
  16. WBahn

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    The water analogy can be very useful (though all analogies have limits) but you need to consider a closed system with no air. For example, a hydraulic system. If you introduce a restriction in a line then the overall flow is reduced but the flow at all points in the line must be the same. Kirchhoff's laws apply for pressure and fluid flow in the same way they do for an electric circuit's voltage and current. Many of the same ideal approximations and non-ideal models also apply.
     
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  17. hp1729

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    "Creating voltage", no. What is voltage? Normally an atom has the same number of electrons (negative charges) as it does protons (positive charges). When an electron is pulled away from an atom the atom has a positive charge. A charge that attracts an electron from a near by atom. This potential to move an electron is the source of this thing we call voltage as well as bonding between atoms. Electrons are always in motion between atoms. When we have an organized flow of these electrons we have an electrical current, but the voltage across a conductor comes first.
    My question ... if all atoms are the same why do some elements conduct better and more yet why do some form magnetic domains and a magnetic field?
     
  18. WBahn

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    Is this a question you are asking the TS or a question you are seeking an answer to?
     
  19. hp1729

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    To the group, in general.
    Maybe I should start a new topic on this and not hijack this one.
     
  20. #12

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    I agree.
     
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