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Basic question - current

Discussion in 'General Electronics Discussion' started by 8bit, May 13, 2015.

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  1. 8bit

    8bit

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    Oct 30, 2013
    If a current is applied across a wire will the current stop once all the electrons have moved the postive terminal of the battery? Can it run out of electrons?

    This is a real fundimental question,sorry, but I think it will help my overall understanding of this subject. :)
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi 8bit

    firstly current isn't applied across a wire (other circuit) ... a voltage potential difference is ( more simply a voltage)
    this potential difference results in an EMF electromotive force that causes the electrons to move from the negative terminal of the battery (or other power supply source ) towards the positive terminal


    The number of electrons in a given circuit is constant. For every electron that leaves the negative terminal, one enters the positive terminal

    Dave
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    basics of a battery ( you can google for a deeper understanding)

    A chemical reaction occurs within the electrolyte of the battery, this separates some of the electrons from their atoms
    The electrons move towards the negative terminal whilst the positively charged ions move to the positive terminal.
    This creates a voltage, a potential difference, between the two terminals

    An Ion is an atom that is short of a few electrons
    and thus it is now positively charged rather than being neutrally (zero) charged

    When a battery goes flat ... it isn't because it has run out of electrons, rather it is because the chemical reaction can no longer
    separate electrons from atoms

    Dave
     
  4. 8bit

    8bit

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    Oct 30, 2013
    Thanks for all the replies.

    So is it the battery that supplies elctrons or does it just force them along the circuit?
     
  5. BGB

    BGB

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    Nov 30, 2014
    (trying to explain as I understand it).

    it is basically like a chemical electron pump.

    in a battery, there is a chemical reaction which wants to occur (such as sulfuric acid turning lead metal into lead sulfate in a lead-acid battery), but it can't because electrons aren't where they need them to be.

    so, what parts of the reaction have occurred, creates an excess buildup of electrons in one place (the lead plate), and a relative lack of electrons in another place (the lead-dioxide plate), and the reaction mostly stops. this is seen by a voltage forming between the battery terminals.

    when a load is connected, then the electrons can start flowing along (with the amount of flow being the amperage).

    since there is no longer as much of an electron imbalance, the chemical reaction can take place (say, creating lead sulfate and water, and pumping more electrons along from one plate to another in the process).

    at some point, the reaction can no longer continue, as much of the exposed lead and sulfuric acid has since been turned into a sulfate buildup on the plates and leaving mostly water in the battery. as a result of this reaction starting to fizzle out, the electron pump slows down, and the voltage across the battery drops off as a result.

    when the battery is charged back up, this reaction is forced to go backwards, converting much of the lead sulfate back into lead, lead dioxide, and sulfuric acid.


    the basic idea is similar with other types of batteries, just with different chemical reactions.
     
    Last edited: May 14, 2015
  6. poor mystic

    poor mystic

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    Apr 8, 2011
    There is a sea of electrons associated with the wire. When a current flows along the wire, for every electron that enters the wire, an electron leaves it. The electrons are supplied by the metallic atoms which make up the wire.
     
  7. 8bit

    8bit

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    Oct 30, 2013
    Thanks for the reply Mystic.

    When you say 'enters the wire' do you mean the free eletrons that are already present?
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009

    yes, all conductive metals(materials) have electrons that are not bound to particular atoms and are free to move between atoms
     
  9. Ratch

    Ratch

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    Mar 10, 2013
    Whatever supplies the voltage, be it a battery, generator, or electronic power supply is irrelevant. The voltage source will send out charge carriers from one terminal and receive the same number of charge carriers at the opposite terminal.

    I sense that you do not understand what voltage is. Voltage is the energy density of the charge. Its unit is joules/coulomb. Charges do not like to get together because like charges repel each other. When charges are gathered together in a space, it requires energy. More charges in a specified space requires more energy. The same number of charges in a smaller space requires more energy. Charges at a higher energy density (voltage) will try to move to a lower energy density (lower voltage) provided there is a conduction path. A voltage source provides a higher energy density at one terminal relative to the other terminal. The wire provides the conduction path. When charges move, that is called a current.

    Because voltage is a density, it is semantically wrong to say that voltage is stored, because you cannot store a density. You can store energy and charge, but not voltage. You can accumulate voltage, however.

    Ratch
     
    garublador and Arouse1973 like this.
  10. 8bit

    8bit

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    Oct 30, 2013
    Thanks.

    I have always understood voltage to be the driving force of a current, between an area of high charge and an area of low charge

    What I am having trouble understanding is fact you have a collection of electrons in a wire that are forced along it to the other end and yet the conductor is never depleted of them.
     
  11. Anon_LG

    Anon_LG

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    Jun 24, 2014
    That is probably why it is called a circuit. The electrons move in a basic circuit from the negative terminal of the cell, through the wire, through the load, through the wire, through the positive terminal of the cell back to the negative terminal of the cell. That is the most basic explanation I can give. An analogy as to why the wire never runs out of electrons is, imagine a circle of people standing on a looped path (a circuit), if one moves forward, so do the others, the person behind takes the space of that person.

    I hope this helps,
     
  12. Ratch

    Ratch

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    Mar 10, 2013
    It is a higher energy density of charge carriers (voltage) and a conduction path that will move the charge carriers. The term "high charge" is meaningless. In a metallic conductor like a wire, there is an ocean of charge carriers (high charge amount of electrons) available. When ever an electron enters the wire at one end, another electron leaves at the opposite end. The electron that enters is not the same electron that leaves. It is analogous to a hose filled with marbles. Forcing one marble into the hose at one end will cause a different marble to pop out at the opposite end. The marbles will travel through the hose very slowly, but the time between the marble entry and marble leaving will be very fast. In a wire, this movement of the electrons is called the drift velocity. In a copper wire, charges move at the speed of cold molasses, although the charge carrier movement responds to the voltage very quickly. High currents can be achieved in a wire, because although the drift velocity is very small, there is huge amount of charge carriers that will move.

    Ratch
     
  13. 8bit

    8bit

    97
    3
    Oct 30, 2013
    I think I can see where some of my misunderstanding is coming from. I assumed the battery simply provided the 'push' that drove the electrons in the wire along. I was not aware the battery actually provided them as well. :)

    Thanks again for the information.
     
  14. davenn

    davenn Moderator

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    Sep 5, 2009
    Yes, that's correct
    have a look again at my comments in posts #2 and 3

    So for every electron that leaves the negative terminal of the battery one comes into the positive terminal and combines with an ion
    The overall charge of the battery doesn't change. The net charge in the battery is zero .... that is ... there are as many negatively charged electrons as there are positively charged ions


    Dave
     
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