|Themes > Science > Physics > Electromagnetism > Magnetostatics > Magnetic Field > Magnetic Fields of Currents > Magnetic Field of a Long Straight Conductor|
We can use the Biot law to calculate the magnetic field of a long straight conductor carrying a current of I. We will measure this field from an arbitrary point P outside the conductor as shown on the illustration at right (click on the illustration for a VRML model of the long straight conductor). The length of the conductor will be 2L, and the midpoint is at the origin.
The current element we will use to calculate the field is a distance r away from point P. The horizontal component of r is y; the vertical is z. So:
But since L is so much greater than z,it can be dropped out from inside the radical:
Notice that we replaced the z with an R. We did this so that you can think of the point as a distance of a radius around the wire.