# Magnetic Field homework

1. Oct 19, 2014

### Amy Marie

1. The problem statement, all variables and given/known data
Figure 29-61 shows a cross section of a long thin ribbon of width w = 4.91 cm that is carrying a uniformly distributed total current i = 4.61 * 10^-6 A into the page. What is the magnetic field at a point in the plane of the ribbon at a distance 2.16 cm from its edge?

2. Relevant equations
B = ((1.26 * 10^-6)i)/(2*pi*R)

3. The attempt at a solution
I tried using the Biot-Savart law:

0.0491 m + 0.0216 m = 0.0707 m

((1.26 * 10^-6)(4.61 * 10^-6))/(2*pi*0.0707) = 1.3 * 10^-11 T

((1.26* 10^-6)(4.61 * 10^-6))/(2*pi*0.0216) = 4.28 * 10^-11 T

(4.28 * 10^-11) - (1.3 * 10^-11) = 2.98 * 10^-11 T

When I checked, the answer was 22.3 pT instead of 29.8 pT.
Can anybody please let me know what I should be doing differently?

2. Oct 19, 2014

### Staff: Mentor

This is the field 2.16cm outside, okay. The units are missing.
What is this?
And where is the point in subtracting the two values?

Does the question really ask for a point outside? That would make the details about the ribbon pointless.

3. Oct 19, 2014

### Amy Marie

I figured the problem out, but thank you.