Electricity and Magnetism

Chapter 29

M-M-More Magnetic Sources

Recall the example from the previous chapter, in which a thin straight wire of length L

carries current I. At a field point, P, located distance a away from the wire, along its

perpendicular bisector, the magnitude of the magnetic field is

B(P) =

µ

0

I

2 π a

"

1 +



2 a

L



2

#

−

1

2

.

The direction of the field is perpendicular to the plane containing the field point and the

wire. The magnetic field wraps around the wire in a manner respecting the cylindrical

symmetry of the space outside the wire.

ASIDE: Each differential contribution to the field is p er pendicular to its current element source

[and hence to the straight wire], as well ...