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Two Parallel Wires( in 2 hrs!)

by Winzer
Tags: parallel, wires
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Winzer
#1
Oct17-07, 02:30 PM
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
[tex] B= \frac{u_{0}I}{2\pi r} [/tex] 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:
[tex] B_{totalx}=\frac{u_{0} I R}{\pi \sqrt( (d/2)^2 +R^2)}[/tex]
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
TwoParallelWires.jpg  
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PiratePhysicist
#2
Oct17-07, 02:45 PM
P: 68
Why do you think that the Magnetic field is not zero?
Winzer
#3
Oct17-07, 02:49 PM
P: 605
Thats what I thought originally , but it is incorrect.

PiratePhysicist
#4
Oct17-07, 03:03 PM
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.
TVP45
#5
Oct17-07, 03:12 PM
P: 1,127
Anti-parallel.
Winzer
#6
Oct17-07, 03:15 PM
P: 605
Quote Quote by PiratePhysicist View Post
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.
Winzer
#7
Oct17-07, 03:50 PM
P: 605
Quote Quote by TVP45 View Post
Anti-parallel.
got an idea?
TVP45
#8
Oct17-07, 03:52 PM
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.
Winzer
#9
Oct17-07, 04:00 PM
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.
PiratePhysicist
#10
Oct17-07, 04:02 PM
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.
sadakaa
#11
Oct17-07, 04:04 PM
P: 19
perhaps his answer is wrong by mistake? talk to other people in the class
Winzer
#12
Oct17-07, 04:17 PM
P: 605
he is not, there is an answer I people got. There is some trick though, but my math tells me otherwise,
nrqed
#13
Oct17-07, 04:50 PM
Sci Advisor
HW Helper
P: 2,887
Quote Quote by PiratePhysicist View Post
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.
Doc Al
#14
Oct17-07, 05:23 PM
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P: 41,315
Quote Quote by Winzer View Post
[tex] B= \frac{u_{0}I}{2\pi r} [/tex] 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:
[tex] B_{totalx}=\frac{u_{0} I R}{\pi \sqrt( (d/2)^2 +R^2)}[/tex]
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.
TVP45
#15
Oct18-07, 06:32 AM
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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