# Homework Help: Determine magnetic field from figure

1. Jun 20, 2008

### scholio

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

find the magnetic field at point P in the figure below: (see attachment)

2. Relevant equations

magnetic filed B = integral[dB] = mu_0/4pi (integral[I(dL)/r^2]) where mu_0 is constant = 4pi*10^-7, I is current, dL is change in length, r is radius

3. The attempt at a solution

based on looking at the diagram, i assume i as the current, to be constant, and how do i factor in the L, specifically the change in L, dL from the diagram, which length am i supposed to use/measure, is it just L/2? or is the radius r = L/2?

#### Attached Files:

• ###### magfield.JPG
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Last edited: Jun 20, 2008
2. Jun 21, 2008

### kreil

After examining the diagram, it appears the following are true:

- The left and right hand short wires that go up have different current directions, so the fields cancel

- The beginning and ending straight lines cannot contribute to the field, since magnetic fields are normal to the current

From these, it seems as though the only part of the wire that contributes to the B-field is the straight line segment of length L at the top of the rectangle.

3. Jun 21, 2008

### scholio

so using that information and the equation in the original post, dL = r = L correct?

what are my integration limits?