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Motion in the field (electric, magnetic) questions

  1. Apr 22, 2009 #1
    1. The problem statement, all variables and given/known data
    Why cant i upload the image file here? anyway i'll try to explain it in words

    1.
    One block of conductor is positioned in the magnetic field (into page) and the block is moving up with certain velocity. What will be the separation of the induced charges?


    2. Two current flowing wires are positioned parallel to each other (1 m apart). They are carrying the current of I in the same direction. If there is point P which is midway between two wires and the magnetic field strength experience by at point P due to one wire is B, then what is the magnitude of magnetic field strength on Point P due to both wires?

    2. Relevant equations



    3. The attempt at a solution
    1. I thought that since when i use the flemings right hand rule my middle finger points left side so left side of conducting block is negatively charged and right hand side is positively charged but the answer is just opposite. Isnt the direction where the middle finger is pointing is the direction of the current? and the direction of current is from positive to negative?

    2. As the current is flowing in the same direction, the N pole and S pole of each wire would face the different pole and that's why they are not canceling each other out so i tought that the magnitude of magnetic field strength would be doubled, but the answer was Zero

    Thankyou for reading and sorry for my poor English.
     
  2. jcsd
  3. Apr 22, 2009 #2
    Okay, I'll talk about problem number 2.

    So, we have two parallel wires 1 m apart. The currents are in the same direction. We can use the right hand rule for currents in a wire to determine the direction of the magnetic field from each wire on the point in-between.

    Do you know what I'm talking about?

    The right-hand rule for currents in a long wire says that if you put your thumb along the wire pointing in the direction of the current flow, then your fingers will curl around the wire in the direction of the magnetic field. Therefore, you can determine from this that the magnetic field due to one of the wires is pointing one way, and the magnetic field due to the other wire is pointing the opposite way. Therefore, they cancel each other out because they have the same magnitude (they are the same distance away from the point in question) and opposite directions.

    So, the magnitude of the magnetic field at that point is zero when we take into consideration both wires.
     
  4. Apr 22, 2009 #3
    --l-----l--
    --l-----l--
    N-l-S-N-l-S
    --l-----l--
    --l-----l--

    (Please ignore the dotted horizontal line it had to be there to draw it properly)
    Doenst it be something like this when the current on both wire is flowing at the same direction? I can understand that if i use the right hand grip rule the magnetic field act opposite but i think what i have drawn here is also right...and as magnetic field is not canceling out between the two wires if my drawing is right....From where i'm wrong?
     
  5. Apr 22, 2009 #4
    inbetween the two wires, one is south and one is north... that's two vectors pointing in opposite directions.. Therefore, they DO cancel each other out :)
     
  6. Apr 22, 2009 #5
    Oh i think i got it. Thanks alot
     
  7. Apr 22, 2009 #6
    You're welcome.
     
  8. Apr 22, 2009 #7
    anyone question 1?
     
  9. Apr 22, 2009 #8
    The diagram for question 1 is like this
    l <- shows the direction of motion of conducting block
    x <- shows that the direction of magnetic field is into the page

    .................... l
    .................... l
    ................... l
    xxxxxxxxxxxxx l xxxxxxxxxxxxxxxxx
    xxxxxxx conducting xxxxxxxxxxxxx
    xxxxxxx block xxxxxxxxxxxxx
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

    if the conducting block is moving up like that (it doesnt look like but it is straight line) under the magnetic filed which is go into the page, what will be the separation of induced charges? will it be

    +..........-
    +block -
    +..........-
    +..........-

    or

    -.......+
    -block+
    -.......+
    -.......+

    ?
     
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