# Two Parallel Wires( in 2 hrs!)

by Winzer
Tags: parallel, wires
 P: 605 1. The problem statement, all variables and given/known data Two long parallel wires are a center-to-center distance of 1.50 cm apart and carry equal anti-parallel currents of 1.80 A. Find the magnetic field intensity at the point P which is equidistant from the wires. (R = 4.00 cm). 2. Relevant equations $$B= \frac{u_{0}I}{2\pi r}$$ infinite wire 3. The attempt at a solution Ok I have been workin this problem for a while. In terms of vectors, the y's cancel out. For the X direction I get: $$B_{totalx}=\frac{u_{0} I R}{\pi \sqrt( (d/2)^2 +R^2)}$$ I dont get why I am wrong, I took the sum of the b-fields in terms of vectors. And no the answer is not 0 T! Attached Thumbnails
 P: 68 Why do you think that the Magnetic field is not zero?
 P: 605 Thats what I thought originally , but it is incorrect.
 P: 68 Two Parallel Wires( in 2 hrs!) If they have equal, anti-parallel currents, and it's a point that's equidistance from the wires, then the answer has to be zero. Unless you decide to ignore the fact that magnetic fields are vectors.
 P: 1,127 Anti-parallel.
P: 605
 Quote by PiratePhysicist If they have equal, anti-parallel currents, and it's a point that's equidistance from the wires, then the answer has to be zero. Unless you decide to ignore the fact that magnetic fields are vectors.
Thats what I used to think too, but when I entered 0 T it is incorrect.
there is something else I am missing.
P: 605
 Quote by TVP45 Anti-parallel.
got an idea?
 P: 1,127 Go back and ask your instructor what he means by anti-parallel. It has more than one meaning. It shouldn't but it does.
 P: 605 I agree, word choice could be better. However I don't have contact with my prof. right now, this assignment is due in an hour.
 P: 68 If I was you I would give your best guess (ignore that you know it's "not right") and argue for points later. Chances are others having the same problem.
 P: 19 perhaps his answer is wrong by mistake? talk to other people in the class
 P: 605 he is not, there is an answer I people got. There is some trick though, but my math tells me otherwise,
HW Helper
P: 2,940
 Quote by PiratePhysicist If they have equal, anti-parallel currents, and it's a point that's equidistance from the wires, then the answer has to be zero. Unless you decide to ignore the fact that magnetic fields are vectors.
??? The two B fields add up, they don't cancel!!
It's when the currents are in the same direction that the total B field is zero at the point midway between them.
Mentor
P: 41,471
 Quote by Winzer $$B= \frac{u_{0}I}{2\pi r}$$ infinite wire
Good.
 3. The attempt at a solution Ok I have been workin this problem for a while. In terms of vectors, the y's cancel out.
True.
 For the X direction I get: $$B_{totalx}=\frac{u_{0} I R}{\pi \sqrt( (d/2)^2 +R^2)}$$ I dont get why I am wrong, I took the sum of the b-fields in terms of vectors.
Show how you got that answer--it's not dimensionally correct, for one.
 P: 1,127 OK, now that I can see your diagram, I see the instructor used anti-parallel in the correct sense.

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