The Biot-Savart Law

Consider a small piece of wire of length ds carrying a current I. This defines a vector ds that points in the direction of the current. The magnetic field dB set up by this piece of current-carrying wire at a point a distance r away is:

• proportional to 1/r2

• proportional to I, the current, and ds, the length of the wire

• in a direction perpendicular to both ds and r, the vector from the wire to the point

• proportional to sin(q), where q is the angle between ds and r

All of these observations can be satisfied by the equation:

dB = (mo / 4p ) I ds ´ /r2

where the constant mo is known as the permeability of free space and has a value of

mo = 4p x 10-7 T m /A

Compare the result for dB to the electric field dE we get from a point charge dq:

dE = ( 1 / 4p eo ) dq /r2

To find the total field at a point from an entire wire, simply integrate all the dB's:

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

The process is very similar to what we did to find electric field from charge distributions.