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

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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]
 
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Answers and Replies

  • #2
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For proof see Smythe "Static and Dynamic Electricity" 2nd Ed, Sec 7.19 and 8.01. Take derivative of stored energy. Opposite currents repel.
 
  • #3
Doc Al
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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
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.
 
  • #4
jtbell
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  • #5
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Here is a demo:
 
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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.
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
 
  • #7
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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.
 
  • #8
jtbell
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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?
Here's a diagram of the right hand rule for finding direction of magnetic force.
 
  • #9
Doc Al
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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?
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.
 
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  • #10
jtbell
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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.
 
  • #11
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I don't like using those hand rules.They make me look like a total muppet.
 
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