# Two Parallel Wires( in 2 hrs!)

1. Oct 17, 2007

### Winzer

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 Files:

• ###### TwoParallelWires.jpg
File size:
10.1 KB
Views:
99
2. Oct 17, 2007

### PiratePhysicist

Why do you think that the Magnetic field is not zero?

3. Oct 17, 2007

### Winzer

Thats what I thought originally , but it is incorrect.

4. Oct 17, 2007

### 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.

5. Oct 17, 2007

### TVP45

Anti-parallel.

6. Oct 17, 2007

### Winzer

Thats what I used to think too, but when I entered 0 T it is incorrect.
there is something else I am missing.

7. Oct 17, 2007

### Winzer

got an idea?

8. Oct 17, 2007

### TVP45

Go back and ask your instructor what he means by anti-parallel. It has more than one meaning. It shouldn't but it does.

9. Oct 17, 2007

### Winzer

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.

10. Oct 17, 2007

### PiratePhysicist

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.

11. Oct 17, 2007

perhaps his answer is wrong by mistake? talk to other people in the class

12. Oct 17, 2007

### Winzer

he is not, there is an answer I people got. There is some trick though, but my math tells me otherwise,

13. Oct 17, 2007

### nrqed

??? 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.

Last edited: Oct 17, 2007
14. Oct 17, 2007

### Staff: Mentor

Good.
True.
Show how you got that answer--it's not dimensionally correct, for one.

15. Oct 18, 2007

### TVP45

OK, now that I can see your diagram, I see the instructor used anti-parallel in the correct sense.