# Homework Help: Properly Applying Ampere's Law

1. Oct 16, 2009

### Oijl

1. The problem statement, all variables and given/known data
Different question, same problem.
I edited this post from what I orignially posted it as (in which my issue was that I misread the problem).

In Figure 29-63, a long circular pipe with outside radius R = 2.4 cm carries a (uniformly distributed) current i = 3.40 mA into the page. A wire runs parallel to the pipe at a distance of 3.00R from center to center. Find the magnitude and direction of the current in the wire such that the net magnetic field at point P has the same magnitude as the net magnetic field at the center of the pipe but is in the opposite direction.

2. Relevant equations
Ampere's Law

3. The attempt at a solution

I want to find the net field at the center of the pipe.

By thinking about it and by looking at equations for B (which have R in the denominator), isn't the magnetic field at the center of the pipe due to the pipe zero?

If the field due to the pipe at the center of the pipe is zero, then the wire cannot produce a field at point P that is in the opposite direction than the net field at the center of the pipe.

So the net field at center of the pipe cannot be zero? Or where else could I be wrong?

Last edited: Oct 16, 2009
2. Oct 16, 2009

### kuruman

You didn't do what the problem asked you to do. You found how much current you need in the wire to have equal contributions from wire and pipe at point P.

You need to find first how much magnetic field you have at the center of the pipe then figure out how much current you need in the wire to get a field of the same magnitude (as the center) but in the opposite direction. What you did and what you are supposed to do are not the same

3. Oct 16, 2009

### Oijl

Well, do I ever feel sheepish.

4. Oct 16, 2009

### Oijl

I edited the first post, so that I'm asking a new problem now, because even reading the problem correctly I can't see it properly.

5. Oct 16, 2009

### kuruman

I don't know what you changed, but what you are asked to find remains the same. At the center of the pipe you only have the field from the wire. At point P you have the field from the wire and the pipe. My suggestion in posting #2 is unchanged.