MAGNETIC FIELDS CREATED BY CURRENT-
CARRYING WIRES
As we said at the beginning of this chapter, the source of magnetic fields are
electric charges that move; they may spin, circulate, move through space, or flow
down a wire. For example, consider a long, straight wire that carries a current I.
The current generates a magnetic field in the surrounding space that’s proportional
to I and inversely proportional to r, the distance from the wire.
Magnetic Fields vs.
Electric Fields
Electric fields start at
a point charge and end
at another point charge,
whereas magnetic fields
are unending loops.
Electric charges can be
positive or negative and
exist by themselves. There
is no such thing as a
monopole for magnets.B ∝
The magnetic field “lines” are actually circles whose centers are on the wire. The
direction of these circles is determined by a variation of the right-hand rule.
Imagine grabbing the wire in your right hand with your thumb pointing in the
direction of the current. Then the direction in which your fingers curl around the
wire gives the direction of the magnetic field lines.