#### The Magnetic Field along the Axis of a Circular Loop

We derived an expression for the magnetic field at the center of a circular current loop of radius R. What is the field at some general point a distance x from the center along the axis?

Should we apply the Biot-Savart Law or Ampere's Law to solve this problem?

1. Biot-Savart Law
2. Ampere's Law

Biot-Savart is appropriate here. The points on the axis are unique, as the magnetic field changes magnitude as we move along the axis, so there's no easy way to apply Ampere's Law.

If the current in this loop is out of the page at the top of the loop and into the page at the bottom, in which direction is the net magnetic field on the axis of the loop at some point to the right?

1. right
2. left
3. up
4. down
5. a combination of two of the above

The net field on the axis points along the axis. In our case it points away from the loop, to the right.

When we do the integral all we need to worry about is the components to the right.

The net magnetic field is B = ( mo I / 4p ) ò ds ´ /r2

Adding up only the x-components, ds ´ = ds sin(q)

r2 = x2 + R2

sin(q) = R/r

All these are constants, so we get:

B = ( mo I / 4p ) R/(x2 + R2)3/2 * 2pR

B = mo IR2/2(x2 + R2)3/2

When x = 0 we get the correct result for the field at the center of a current loop.

When x is much larger than R the magnetic field falls off as 1/x3.