# Homework Help: Magnetic fields due to currents help

1. Feb 23, 2010

### Nyasha

1. The problem statement, all variables and given/known data

In the figure 29-49, four long straight wires are perpendicular to the page, and their cross sections form a square of edge length a = 20 cm. The currents are out of the page in wires 1 and 4 and into the page in wires 2 and 3, and each wire carries 19 A. What is the magnitude of the net magnetic field at the square's center?

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c29/fig29_45.gif

Solution

I don't understand how they got the diagram with the directions of b1,b2,b3 and b4

Last edited by a moderator: Apr 24, 2017
2. Feb 23, 2010

### collinsmark

There are a couple of confusing things about the picture in the solution:

Firstly, Each vector seems to be going out on its own angle. This is not true, the drawing is not to scale. Treat both B2 and B4 having the same angle, and B1 and B3 having the same angle. I guess the artist is simply trying to make it clear that there are 4 separate vectors, even though two of them are identical, and another two of them are identical.

Secondly, Ignore the +1, -2, -3, and 4 in the corners of the square. I don't know what those are about. They don't match up with the original wires labeled 1, 2, 3, and 4 at all. So when you are trying to figure out which wire goes with which B, look at the figure in the problem statement to get the correct wire number.

If you adhere to the above, the directions fit with the right hand rule.