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Currents in two same direction, they attract ?currents are going in opposite repel?

  1. May 24, 2009 #1
    I've read that when currents in two parallel wires are going in the same direction, they attract each other. The same text said that when the currents are going in opposite directions, the two wires repel each other. now i cant porve this for myself as i keep making the folowing mistake

    can someone have a look at what i doing worng?

    please forgive my spelling

    http://img29.imageshack.us/img29/7831/currentinwiresattarctor.th.jpg [Broken]
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 24, 2009 #2
    Re: currents in two same direction, they attract ?currents are going in opposite repe

    For proof see Smythe "Static and Dynamic Electricity" 2nd Ed, Sec 7.19 and 8.01. Take derivative of stored energy. Opposite currents repel.
     
  4. May 24, 2009 #3

    Doc Al

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    Re: currents in two same direction, they attract ?currents are going in opposite repe

    Don't think in terms of "polarity". The magnetic field from one wire exerts a force on the second wire. Use the right hand rule to find the direction of that magnetic field, then use it again to find the direction of the force that it exerts on the current in the second wire.
     
  5. May 24, 2009 #4

    jtbell

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    Re: currents in two same direction, they attract ?currents are going in opposite repe

    From Hyperphysics: Magnetic Force Between Wires has a diagram which may be helpful.
     
  6. May 24, 2009 #5
    Re: currents in two same direction, they attract ?currents are going in opposite repe

    Here is a demo:
     
    Last edited by a moderator: Sep 25, 2014
  7. May 25, 2009 #6
    Re: currents in two same direction, they attract ?currents are going in opposite repe

    are you speaking of right hand grip rule? do you mean flemings left hand rule if not how would i use grip rule it to find the force? even so if both the wires are carrying current in same direction right next to each othe rmy thumb points in same direction as do my fingers curl in same direction i.e magnetic feild lies in same direction :S

    sorry if i just sound ignorant/stupid ,i just cant cant get my head around it

    ps thanks for all the quick replies
     
  8. May 25, 2009 #7
    Re: currents in two same direction, they attract ?currents are going in opposite repe

    Yes, use the right hand grip rule or the corkscrew rule or whatever you prefer to find the field direction around one of the wires.The field due to that wire is at 90 degrees to the current in the second wire.Now use the left hand rule to find the force direction on the second wire.
     
  9. May 25, 2009 #8

    jtbell

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    Re: currents in two same direction, they attract ?currents are going in opposite repe

    Here's a diagram of the right hand rule for finding direction of magnetic force.
     
  10. May 25, 2009 #9

    Doc Al

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    Re: currents in two same direction, they attract ?currents are going in opposite repe

    Sorry if I wasn't clear. (And thanks to jtbell for providing helpful hyperphysics links.) I always use a "right hand" rule for everything. Makes it simpler (at least to me). To find the direction of the magnetic field from a straight current-carrying wire, use the right hand "grip" rule. Here's an illustration: http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magcur.html#c1"

    Once you find the magnetic field from wire 1 that acts on wire 2, then use a different version of the right hand rule to find the force. (See the links provided by jtbell.) This is equivalent to Fleming's left hand rule, of course.
     
    Last edited by a moderator: Apr 24, 2017
  11. May 25, 2009 #10

    jtbell

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    Re: currents in two same direction, they attract ?currents are going in opposite repe

    I didn't even know there was a left-hand rule until seeing this thread. I've always used my right hand for everything. And all the textbooks I've used so far have mentioned only right-hand rules.
     
  12. May 25, 2009 #11
    Re: currents in two same direction, they attract ?currents are going in opposite repe

    I don't like using those hand rules.They make me look like a total muppet.
     
    Last edited: May 25, 2009
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