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DC generated current

  1. May 30, 2013 #1
    in DC there is a movement of electrons

    IE in a battery there is an excess of electrons chemically stored in one end that desire to travel to the other side of the battery where there are a shortage of electrons

    where do the electrons come from in a DC generated current ?

    if electrons do not travel down the line to the resistance in generated DC is this not AC ?
     
  2. jcsd
  3. May 30, 2013 #2
    What do you mean by DC generated current? a battery current?

    AC is the movement of electrons through a resistance in an alternating manner, unlike in DC where electrons move in one direction.

    I think you need to have an understanding of AC and DC.
     
  4. May 30, 2013 #3

    CWatters

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    A DC generator doesn't need to create a bucket full of "new" electrons. Metal atoms readily give up their outer shell electrons. The generator just needs to push these so called "free" electrons around.

    Consider a central heating system. That has a pump which circulates water around the system from the boiler (called a kettle in some countries) to the radiators. The pump doesn't need to create "new" water it just pumps water already in the pipes around.
     
  5. May 30, 2013 #4
    understood

    what i do not understand is the following

    there was a great debate among Edison and others as to what electricity Niagara falls should produce (AC or DC)

    AC won out because it loses less power in the transmission of it down the wires (energy loss comes mostly through heat loss which is why they cannot bury transmission wires as they would catch on fire)


    If they had decided to produce DC where were the electrons going to come from that moved down the current ? Would there have to have been a complete circuit bringing the electrons back to the generating plant at Niagara ?

    thx
     
  6. May 30, 2013 #5

    CWatters

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    DC vs AC it makes no difference.

    The electrons would come from the metal wire. See my previous reply.

    Yes but again that has nothing to do with AC vs DC. A return path is needed for BOTH.

    The planet earth can be used for the return path.
     
  7. May 30, 2013 #6

    nsaspook

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    They would come from the same place they do in a AC circuit, the wiring.

    http://www.physicsclassroom.com/Class/circuits/u9l2e.cfm
     
  8. May 30, 2013 #7
    Mr watters i am lost

    In a DC battery there are excess electrons at one end and a negative supply at the other

    when the circuit is complete the electrons flow from excess supply to negative supply

    when they have established equilibrium the battery is "dead"

    how would this "flow" of electrons happen if Niagara Falls produced DC?


    the way i understand it the electrons in AC come from the metal in the wire (as you pointed out) but they do not flow through the wire they agitate back and forth as the generator changes north pole to south pole agitating the electrons


    in DC the electrons flow from one atom to the next (in AC they stay in their original orbit) so there has to be a starting point for the electron

    once having left its starting point it needs to be replaced so we need an excess of electrons or a circuit that returns the electrons



    where in my thinking am i wrong?


    thx
     
  9. May 30, 2013 #8

    CWatters

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    Go back to the central heating model I posted earlier..

    DC - the pump runs in one direction all the time.
    AC - the pump changes direction at regular intervals.

    In neither case does there need to be new water added to the pipework, nor an "excess of water". Nor does there need to be a "starting point".

    It's also wrong to think of electrons as flowing all the way around a circuit. Even for DC it's best to imagine it as a queue of electrons. When you push the first one in the queue a wave travels down the line until the one at the end moves. It appears as if one electron has made the trip but that's not the case. Only the wave has made the trip. The waves moves very fast but electrons actually move quite slowly down the wire - it varies but it's on the order of mm per second.
     
  10. May 30, 2013 #9

    CWatters

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    There isn't a huge pile/excess of electrons sitting in the battery ready to go. Think of it as a chemical reaction that's capable of generating electrons one at a time. There is a limit to how fast the chemical reaction can go and that limits how much current the battery can deliver.
     
  11. May 30, 2013 #10
    thx

    i am starting to understand

    i just read where actual electron movement down the line is called "drift flow" and is very, very slow

    it is the excitment of electrons that travels quickly because of the potential difference between the ends of the circuit

    however, there is some movement down the line of electrons in DC where

    these electrons move into the orbit of another atom (unlike AC where the stay in the same orbit) but unlike AC there is a "drift flow" down the line so do they not have to be replaced?
     
  12. May 30, 2013 #11
    ok

    there is a potential difference between the ends of the battery ie the ability to excite the electrons into movement with an electronic signal

    when the circuit is turned on an electric signal travels at the speed of light charging the electrons(the charge itself does not travel)
     
    Last edited: May 30, 2013
  13. May 30, 2013 #12

    Drakkith

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    Not true. Individual atomic orbitals have little to nothing to do with this. In a solid metal the moving electrons all occupy the conduction band (which is shared by the entire metal) and easily flow everywhere throughout the metal. This happens in both AC and DC circuits. The only difference is that AC circuits alternate the polarity of the voltage and thus the direction of current.

    To understand how batteries work you would need to understand the chemical reaction taking place inside it. I leave that to someone more experienced that myself.
     
  14. May 30, 2013 #13
    then please explain this

    "You never need to replace electrons because they always come back.
    This is why you need two prongs on a plug for it to work. Electrons go in one and out the other. Passing through the device is what allows energy to be transferred."

    Dr. Ken Mellendorf
    Physics Instructor
    Illinois Central College


    here Dr Mellendorf states electrons flowing down the line (from high to low potential difference I presume) is what causes electricity to do its work does he not?



    clearly i am not well anointed in physics and greatly admire those who are
     
  15. May 30, 2013 #14

    nsaspook

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  16. May 30, 2013 #15
    i am trying real hard


    "The electrons do indeed flow. When the circuit is closed, an electric field is set up in the entire wire from the generating plant to the user and back again on another wire -- a closed circuit is required! A wire is a conductor precisely because some of the electrons are free to move when pushed by an electric field.

    The closed circuit is required so the electrons can flow in a closed circuit; no electrons are lost! "

    Dick Plano, Professor of Physics emeritus, Rutgers University





    so you do need a complete circuit

    grounding will not suffice (is Plano refering to DC or AC or both?)


    is the circuit needed to capture drift ( electron flow down the line )?

    is it the charge of each electron that transfers energy to the resistance?

    When an electron transfers energy does it lose its charge? then recapture it from the potential difference?


    i am getting contradictory answers but i suspect this is because each answer by itself is an incomplete picture


    cheers
     
  17. May 30, 2013 #16

    nsaspook

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  18. May 31, 2013 #17

    tiny-tim

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    does the earth act as a wire that is part of the circuit, or as a capacitor inserted into the circuit?
     
  19. May 31, 2013 #18

    thx a bunch


    my preconceptions are wrong (thank the public schools )


    have i got this right


    in your link it is stated

    "A battery can send electric energy to a light bulb, and the bulb changes electrical energy into light. "

    so what is energy?

    Energy is Charge * Voltage. correct?

    You need both the current flow and the voltage differential for the bulb to light up.

    Also

    the magnetic fields contain the energy, however they will not light something up until a potential change (voltage) in the field takes place.

    In short, energy causes the filament to light up, however one of the components of electrical energy is the existence of charge flow (current).

    so what kills you the energy (charge flow ie current * voltage ie potential diffeence) in the electro magnetic field or is it the current (charge flow) alone and separate from the energy ?

    in the link he says "When lightning strikes , the spreading electric currents in the ground can kill anyone standing nearby"

    he does not say the current times voltage (energy) kills you


    cheers
     
    Last edited: May 31, 2013
  20. May 31, 2013 #19
    In some commercial power systems, the return [negative] path is earth...in others it is ground wires..in either case the electric potential moves electrons along a CLOSED circuit..a continuous path.

    ok.....
    But CHARGE is not current flow...current flow is ' i'...or 'I'....

    ...POWER is current times voltage: P = IE, and charge, Q, is IT

    exactly.....

    Close, but you again have energy and power mixed up. POWER causes the filament to light.
    WATTS, for example, a common measure of bulb strength, is a measure of POWER. Power is energy per unit time.
     
  21. May 31, 2013 #20
    tremendous NATY

    getting the concepts is tricky for a Canadian from the country

    one more

    in commercial power stations the power is AC

    in AC i thought electrons move back and forth while staying within their own orbit (unlike DC where you do have drift) negating the need for a return wire whose job it is to return the electrons that have drifted

    if the power station produced DC (which can easily be done) then you would need a return wire(by ground wire do you mean return wire that goes back to the power plant to complete the circuit?)


    cheers from Canada
     
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