# Magnetic field problem

1. Mar 22, 2005

### formulajoe

A square conducting loop of side 2a lies in the z=0 plane and carries a current in the counterclockwise direction. Show that at the center of the loop
H = sqrrt2*I/pi*a in the z direction.

I am stuck in this problem. Heres what Ive got. I placed the center of the loop at the origin. Im using Biot-Savarts Law. I found an R vector to be sqrt x^2 + y2 in the rho direction and cos x/sqrt x^2 + y2 in the phi direction. Heres where Im stuck. When I go to do the integral, my dl depends on the side im integrating. So if im my dl is dx, i dont know how to cross that why R because R has the unit vectors rho and phi.

2. Mar 22, 2005

### Gamma

I didn't quiet understand your approach. Find the magnetic field due to one wire of length 2a at a distance a from it's center. Contribution from each wire is same. So B = 4 * B(A).

|
| a
|-----A
|
|

From the center of the wire at a distance l choose current element dl.
Distance between dl and A is r.

Using biot law,

$$B(A) = \frac{\mu_0}{4\pi}\int_{-a}^{a} \frac{I dl X n}{r^2}$$

$$B(A) = \frac{\mu_0 I}{4\pi}\int_{-a}^{a} \frac{dl sin \theta}{a^2+l^2}$$

$$sin \theta = a / \sqrt (a^2 + l^2)$$

Do the integration.

$$B(A) = \frac{\mu_0 I}{4\pi a}* \sqrt 2$$

$$H = 4* H(A) = \frac{I}{\pi a} * \sqrt 2$$

3. Mar 22, 2005

### formulajoe

where does the sin theta come from in that second step

4. Mar 22, 2005

### Andrew Mason

You only have to do one side because they all add together. Call them sides 1, 2, 3, 4.

$$B_1 = \mu_0I \int \frac{d\vec L \times \vec r}{4\pi r^2} = \frac{\mu_0I}{4\pi} \int \frac{rdlsin\theta}{r^2}$$

In polar coordinates:

$dL = cos\theta d\theta$
$r = a/cos\theta$

So:
$$B_1 = \frac{\mu_0I}{4\pi} \int_{-\pi/4}^{\pi/4} \frac{(a/cos\theta)cos\theta d\theta sin\theta}{(a/cos\theta)^2}$$

$$B_1 = \frac{\mu_0I}{4\pi} \int_{-\pi/4}^{\pi/4} \frac{asin\theta cos^2\theta}{a^2}d\theta$$

Since $B = 4B_1$:

$$B = \frac{\mu_0I}{\pi a} \int_{-\pi/4}^{\pi/4} sin\theta cos^2\theta d\theta$$

You have to use some trigonometric identities to integrate that. Better check my math too.

AM

5. Mar 22, 2005

### Gamma

In my first integral, n is the unit vector along the direction of vector r. $$sin \theta$$ comes in when you take the cross product of dl X n. $$sin \theta$$ is the angle between r and dl.

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