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The direction of an electrical current

  1. Jun 8, 2018 #1
    OK, so my last post seemed meaningless.
    Can anyone help with this, I understand some of you say current moves from the positive terminal of a battery to the negative terminal and some say the opposite.
    Lets say it is positive to negative just for this question.
    If I have a battery that the positive terminal is on the right and negative on the left. I wind 2 coils, 1 clockwise and the other counterclockwise, other than that the same starting point and same ending point and I place both coils in the between the battery terminals, positive right and negative left. The 2 coils will have opposite magnetic fields, so if the current is moving from positive to negative only and creating different magnetic fields based on the coil winding clockwise or counterclockwise, how can it be said that the magnetic field is the only determining factor of what direction the current is moving?
    In both coils the current is starting and ending at the same points, just rotating in different directions.
     
  2. jcsd
  3. Jun 8, 2018 #2

    berkeman

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    Can you post a schematic and drawing of your question? It's a little hard to follow for me. Thanks.
     
  4. Jun 8, 2018 #3

    phinds

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    Electrons move from the negative terminal to the positive terminal. Current flows from the positive terminal to the negative terminal. It's a convention that was started by mistake and has just hung on, as many accepted convention do, just BECAUSE it was a convention and everyone was used to it.

    I agree w/ Berkeman. The rest of your post is confusing. You should always post a circuit diagram when asking about a circuit.
     
  5. Jun 8, 2018 #4

    tech99

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    Even though you have wound a non-inductive coil, the circuit still has a magnetic field because the circuit itself is a single turn, or loop. So it you look at the magnetic field direction of the single turn it will tell you the current direction.
     
  6. Jun 8, 2018 #5

    Dale

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    In normal operation current moves from positive to negative terminal. Electrons move from negative to positive, and since electrons are negatively charged the current is in the opposite direction of their motion. There is no controversy about this at all. There is only a bit of confusion from students who mistakenly think that the current should be in the same direction of the charge carriers, regardless of the sign of the charge on the charge carriers.

    As @phinds mentioned, assigning a negative charge to the electrons is a convention and it could have been done the other way, but the convention is well established and universally adopted.

    I have never heard anyone say that previous to your post. You may have misunderstood whatever source you think said that. It might help to post a link to the source you are quoting.
     
  7. Jun 8, 2018 #6
    How about this untitled.JPG
     
  8. Jun 8, 2018 #7

    Dale

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    Current goes counter clockwise through this diagram. It goes from right to left through the inductors and from left to right through the battery itself.

    Btw, this circuit, as drawn, is a short circuit and will have infinite current. To make it a little more realistic you should include a resistor to model the resistance in the coil and the battery.
     
  9. Jun 8, 2018 #8
    That part I understand, I think you missed my point.
    Read my post again, its about the magnetic fields
     
  10. Jun 8, 2018 #9

    Dale

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    What about the magnetic fields? As drawn you simply have two independent inductors. There is nothing wrong with that. Inductances add in series, so you would simply have an overall inductance equal to the sum of the inductances.
     
  11. Jun 8, 2018 #10

    berkeman

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    Or if they are tightly coupled and have opposite directions, the inductance will be zero. Still nothing mysterious, IMO.
     
  12. Jun 8, 2018 #11

    anorlunda

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    @Wattif , let me repeat what @Dale and @berkeman told you in different words.

    If the two coils wound in different directions, are so close to each other, then the magnetic fields cancel each other and the inductance is zero. If they are so far apart that their fields don't interact with each other, then the direction of windings don't matter and the inductances add. Of course you can imagine a between case if we gradually brought the coils closer and closer.

    But there is also a flaw in your approach. There are two ways to analyze this problem, one with circuit analysis, CA, and the other with Maxwell's Equations. In circuit analysis, we assume that there are no magnetic fields outside the wires or components. Therefore inductances in series always add. In Maxwell's equation, you must consider the actual 3D coordinates of each tiny segment of wire in the coils and the connecting wires and perform the spatial integrals to solve for the 3D fields.

    You are confusing yourself by trying to imagine a case mixing CA and Maxwells. You drew a CA type schematic in 2 dimensions and without specifying the lengths of the wires, then asked a question about the magnetic fields. That is a recipe for failure.
     
  13. Jun 8, 2018 #12

    Dale

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    This between case can be analyzed using mutual inductance. You can vary the strength of the coupling to represent either extreme too.

    @Wattif Here is a page describing mutual inductance including an example for the case of interest of two inductors in series with opposite winding.
    https://www.electrical4u.com/mutual-inductance/

    The total inductance would be ##L_1+L_2-2\kappa\sqrt{L_1 L_2}## where ##0\le\kappa\le 1##
     
    Last edited: Jun 8, 2018
  14. Jun 8, 2018 #13

    Tom.G

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    Let's back up a little bit. The posts here so far are all valid but I don't believe they address your underlying question.

    Back when electricity was first being investigated (in 1800 an italian by the name of Allessandro Volta invented the first electric Battery), it was realized that there must be an excess of "something" and a shortage of "something" for this mysterious "stuff" to flow. (The "stuff" was initially thought to be a fluid of some sort.) Not knowing what was going on, an arbitrary decision was made to label the two different terminals as "+" and "-". This resulted in the convention of the "something" flowing from "+" to "-". Seems completely logical. This is now called "conventional current flow."

    In 1897 J.J.Thomson discovered the Electron, and in 1906 he received the Noble Prize for doing so. It turned out that the Electron was the "something" that was flowing in an electrical circuit. However the Electron moves from the terminal called "-" to the terminal called "+".

    To this day there is often confusion between "conventional current flow" ("+" to "-"), and "Electron flow" ("-" to "+").

    Btw, the electrical unit called the Volt is named honoring Allessandro Volta.

    Hope this helps.

    Cheers,
    Tom

    p.s. Watch out for the labels Anode and Cathode, that too gets confusing.
    An Anode is defined as the terminal where Electrons leave a device,
    and the Cathode is the terminal where Electrons enter a device.
     
  15. Jun 9, 2018 #14

    bhobba

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  16. Jun 9, 2018 #15

    CWatters

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    The direction the current is rotating is what determines the direction of the magnetic field.

    See... "Magnetic field caused by current in a wire" and the right hand rule..

    https://www.khanacademy.org/test-pr...es/magnetism-mcat/a/using-the-right-hand-rule

    It's just the way the universe works.

    There isn't really anything "special" about the direction of the magnetic field or the choice of hand. The choice of which hand to use is just convention just as by convention current flows from positive to negative.

    In addition to the current rotating in the coils it's also rotating as it goes around the circuit which in this case is a one turn loop. You can also apply the right hand rule to that loop.
     
  17. Jun 9, 2018 #16

    CWatters

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    PS your coils are marked clockwise and anti clockwise but the direction depends on which end you look at.
     
  18. Jun 9, 2018 #17
    Excuse me if my terminology is not correct.
    Again, You missed my point about current flow direction and the magnetic field it creates.
    current direction is usually based on the direction of the magnetic field lines, or poles, correct?
    My point is a DC current used to create opposite magnetic fields just by rotating the coils in a different direction.
    How can the magnetic field be the only determining factor for current flow direction if 2 different fields can be created
    using the known current, like DC?

    A link showing what I'm getting at
     
  19. Jun 9, 2018 #18

    Dale

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    No. I told you that you probably misunderstood whatever source you used to come to this idea.

    Please provide a link to the source that gave you this incorrect idea.
     
  20. Jun 9, 2018 #19

    phinds

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    No, current direction is dependent on the polarity of the power source. PERIOD. The magnetic field line direction follows from that.
     
  21. Jun 9, 2018 #20
    You are mixing up the schematics with the actual geometry.

    Example: there are several different routes you can take if you want to travel from Moscow to Washington. Whichever route you chose won't effect the fact that you will go from Moscow to Washington. But your wallet will know the difference, right?

    When you ask 'direction of electrical current' that's the departure and arrival. When you ask about magnetics, that's about geometry/route.
     
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