# Strength of magnetic field at a point

1. Mar 26, 2009

### erinec

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

http://img291.imageshack.us/img291/9572/picture2ehv.png [Broken]

What is the strength of the magnetic field at point P?

The answer is 7.9×10^−5 T.

However, I used the equation B = u*I/(2*pi*d) and keep getting 5*10^-5 T.

2. Relevant equations
B = u*I / (2*pi*d)

3. The attempt at a solution

Nearer wire:
B = uI/(2*pi*d) = (1.257*10^-6)(5.0)/(2*pi*0.02) = 0.00005 T
Further wire:
B = uI/(2*pi*d) = (1.257*10^-6)(5.0)/(2*pi*0.01) = 0.0001 T
Total B:
B = 0.0001T - 0.00005T = 0.00005 T

What am I doing wrong?

Last edited by a moderator: May 4, 2017
2. Mar 26, 2009

### alphysicist

Hi erinec,

This formula gives the magnitude of the B-field of a long straight wire (at a distance d away from the wire).

But in this problem you don't have straight wires creating the magnetic field at P; the wires are semicircles. Look through your textbook again and see if you can find a formula that better fits this situation.

Last edited by a moderator: May 4, 2017
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