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Determining the direction of current on a wire?

  1. Mar 13, 2013 #1
    In each of parts (a), (b), and (c) of the figure below, find the direction of the current in the wire that would produce a magnetic field directed as shown.

    19-p-044.gif

    part (a) right to left
    part (b) out of the page
    part (c) lower left to upper right

    Ughhhhh
    So, the right hand rule, predictably doesn't work for this...
    Part A for instance, the top half of the diagram....since the magnetic field is coming OUT of the page towards you (or the magnetic field/flux out of the page is INCREASING, a current will generate a magnetic field in such a way that it goes INTO the page...this cancels any changes in the magnetic flux - Lenz's law.
    So I do the right hand rule with my right hand and find out that for the TOP half of the diagram, the current is CLOCKWISE (goes from LEFT to RIGHT)

    http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/magnetic/imgmag/magcur.gif [Broken]

    THEN I tried doing the bottom half, which was the opposite...
    The current was COUNTERCLOCKWISE (goes from RIGHT to LEFT)

    -___-
    So how is the answer, overall, Right to left???
    In situations like Diagram A and Diagram C do I ONLY look at the half that has the dots (and not the X's)??
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Mar 13, 2013 #2

    Doc Al

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    No, the right hand rule works like a charm. :smile:
    The top half shows the magnetic field going into the page. It's even labeled Bin!
    This problem is just the magnetic field created by a current. It does not involve induced currents, changing flux, or Lenz's law.
    Just grab the wire with your right hand. Which way must your thumb point so that the fingers curl in the direction of the field?
     
  4. Mar 13, 2013 #3
    OHHHHH...
    Right to left!!
    I think my problem was that I confused the X's and the dots.
    Because I always though X's meant the arrow/magnetic field line is coming TO YOU OUT of the page and the dots meant that the arrow/magnetic field line is going INTO the page AWAY from you...
    So ESSENTIALLY, if the diagram is labeled with Bin if there are X's, then that tells me the magnetic field lines are going INTO the page, and if it says Bout, when there are dots, the magnetic field lines are coming OUT of the page...

    But what if they don't tell me this on a test?
    If there are just X's, do I THEN assume that it's Bout (and not Bin), or do the X's ALWAYS mean Bin?
     
  5. Mar 13, 2013 #4

    Doc Al

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    When in doubt, you can always ask.

    But the use of dots and Xs is pretty standard. Here's what it means:

    Imagine the field as a arrow. The dot represents the tip of the arrow, so where you see dots the field points out of the page. The X is supposed to represent the tail of the arrow (feathers, I guess), so where you see Xs the field points into the page.

    Make sense?
     
  6. Mar 13, 2013 #5

    PeterO

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    HINT: For the crosses and dots, think of an arrow - as in archery.

    If the arrow is going away from you, you see the "feather" end - the crosses.
    If an arrow is coming towards you, you see the point end - the dot.

    Doc Al just beat me!!
     
  7. Mar 13, 2013 #6
    Ohhh I get it!
    That's a neat analogy :eek:

    thanks!!
    And you too PeterO!
     
  8. Mar 13, 2013 #7
    Wait one more quick question though...

    So, if X's represent the arrow going IN, why is the direction of the force on a proton (a positively charged particle) moving through the magnetic fields in Diagram A to the left??

    2. Find the direction of the force on a proton (a positively charged particle) moving through the magnetic fields in the figure, as shown.

    19-p-002.gif

    This time youre supposed to use Fleming's Right hand rule

    http://www.ustudy.in/imagebrowser/view/image/3476/_original [Broken]

    Does the THUMB represent velocity V and the middle finger represent direction of force on a proton? Because like the diagram I posted here on top, the other diagrams on other websites say that the current and voltage is the middle finger and the thumb is the direction of force on a proton/electron. And that's what I followed to answer these questions... but got A wrong... Because what I did was, since they said voltage/current is the middle finger, magnetic field is the index finger, and direction of force on a proton is thumb, I got that the direction of force on the proton (for part A) was to the RIGHT and not the left :(
     
    Last edited by a moderator: May 6, 2017
  9. Mar 13, 2013 #8

    Doc Al

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    If you insist on using that version of the right hand rule, then the first finger represents current (or direction of motion of the positive charge), the second finger represents the field, and the thumb represents the the force on the moving charge.
     
    Last edited by a moderator: May 6, 2017
  10. Mar 13, 2013 #9
    What but, based on what the answer is ("to the left"),
    the index finger is magnetic field
    the thumb is velocity
    and the middle finger is direction of motion on a positive charge.
    :(:(:(

    So is that a different kind of Fleming's Right Hand rule?
    So I'm guessing THIS is for finding

    Direction of force on a moving positive/negative charge (quick question: for the negative charged particle, do I use the LEFT hand or do I still use the right hand?)

    Index finger: magnetic field
    Thumb: velocity
    Middle finger: direction of motion on a positive charge.

    And to find

    Direction of the deflection of charged particles

    Index finger: magnetic field
    Thumb: direction of the deflection force a moving charge
    Middle finger: velocity

    Example)

    19-p-004.gif
     
  11. Mar 13, 2013 #10

    PeterO

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    That "dynamo Rule" shows you the direction of the current in a dynamo or just a wire, when it is forced to move [perpendicularly to its length] in the direction of the thumb, in a field in the direction of the first finger. the current will be induced in the direction of the middle finger.
    The reason it seems backwards / counter-intuitive is that a dynamo in a circuit takes the place of a battery.
    Everyone learns that current flows from the positive terminal of a battery to the negative terminal. But that is out side the battery - in the circuit. Look at what happens inside the battery.
    That is the reason many people get the wrong answer when trying to predict which end of a wire moving in a magnetic field will be positive. Inside a battery current flows to the positive terminal.
     
    Last edited by a moderator: May 6, 2017
  12. Mar 13, 2013 #11

    Doc Al

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    Read what I wrote. What I wrote is for finding the direction of the force on a moving charge in a magnetic field: [itex]\vec{F} = q\vec{v}\times\vec{B}[/itex]

    If it was a negative charge, I would still use the right hand rule but know that the answer will be opposite to the thumb direction.
     
  13. Mar 13, 2013 #12

    PeterO

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    Those two descriptions both look backwards to me.

    If you want to hold your right hand like that:

    Index finger = direction of field
    Thumb = direction positive charge is moving
    Middle finger = direction of the deflecting force.

    If the moving positive charges are a stream of protons, that is the direction the stream will be defected.
    If the "moving positive charges" represents conventional current - then the middle finger gives the diection of the deflecting force on the wire. The physical set up will determine if the wire will actually move that way - it may not be physically possible for that to happen.

    As for negative charges: You work out which way positive charges would go, and the negative ones go in the opposite direction.
     
  14. Mar 13, 2013 #13

    Doc Al

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    You might find this discussion of right hand rules useful: Right Hand Rule

    As must be obvious by now, there are many ways to apply the right hand rule correctly, all of them equivalent. You just need to find one that you can remember!
     
  15. Mar 13, 2013 #14
    So ESSENTIALLY, the only one I can remember is this:


    Index finger: magnetic field
    Thumb: direction of velocity OR direction of moving charged particle
    Middle finger: direction of motion on a positive charge OR direction of deflection of positive charge
     
  16. Mar 13, 2013 #15
    I have ONE more really quick question...How would I go about determining the direction of the induced current for this one? I know I can use the right hand grip rule to make it seem as if I'm holding the wire but the diagram doesn't indicate that much to help me out...

    5. In the figure below what is the direction of the current induced in the resistor at the instant the switch is closed?

    20-p-014.gif

    a) The current out of the page.
    b) No current is present.
    c) The current is into the page.
     
  17. Mar 13, 2013 #16

    Doc Al

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    That's fine. I would say that the middle finger represents the direction of the force on a positive charge.
     
  18. Mar 13, 2013 #17

    Doc Al

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    Since this deals with induced current, we'll need Lenz's law.
    You can apply the same right hand rule, but it might be easier to use another for finding the direction of the field through a current loop: Curl your fingers in the direction of the current and your thumb will give the direction of the field inside the loop. See: Magnetic Field of Current Loop and Solenoid

    To solve the problem, ask yourself: When the switch is closed, what direction is the field through the resistor circuit? Is it increasing or decreasing? Then apply Lenz's law to figure out the direction of the field due to the induced current. (Once you have the direction of that field, use the same right hand rule to figure out the direction of the induced current.)
     
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