How Do You Calculate the Magnetic Field Near a Current-Carrying Ribbon?

[Amy Marie]

Homework Statement

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?

Homework Equations

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

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?

 

[mfb]

I don't understand your calculations.

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

This is the field 2.16cm outside, okay. The units are missing.

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

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.

 

[Amy Marie]

I don't understand your calculations.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.

I figured the problem out, but thank you.

 

[SHOORY]

I figured the problem out, but thank you.

how did you solve it
using what equation

 

[mfb]

This thread is from 2014 and OP didn't post anything in this forum in the last three years, it is unlikely that you will get an answer.