Peoples Physics Book Version-2

15.2. Sources of Magnetic Fields http://www.ck12.org

15.2 Sources of Magnetic Fields

In the electricity chapter, we learned that static electric fields have, as their source, some arrangement of charges.
On the other hand, there are no sources of magnetic charge: every magnet, no matter how small, has a ’north’
and ’south’ pole. Nonetheless, there exist ’magnetic materials’ that create fields and experience forces from other
magnetic materials. In this chapter, we study magnetic fields produced by two different phenomena.

Permanent Magnets

Permanent magnets (like refrigerator magnets) consist of atoms, such as iron, for which the magnetic moments
(roughly electron spin) of the electrons are “lined up” all across the atom. This means that their magnetic fields add
up, rather than canceling each other out. The net effect is noticeable because so many atoms have lined up. The
magnetic field of such a magnet always points from the north pole to the south. The magnetic field of a bar magnet,
for example, is illustrated below:

If we were to cut the magnet above in half, it would still have north and south poles; the resulting magnetic field
would be qualitatively the same as the one above (but weaker).

Charged Particles in Motion (Wires)

Charged particles in motion also generate magnetic fields. The most frequently used example is a current carrying
wire, since current is literally moving charged particles. The magnitude of a field generated by a wire depends on
distance to the wire and strength of the current(I):

Bwire=

μ 0 I

2 πr

[1] Magnetic field at a distancerfrom a current-carrying wire

Whereμ 0 = 4 π× 10 −^7 Tm/A [2] Permeability of Vacuum (approximately same for air also)

Meanwhile, its direction can be found using the so calledfirst right hand rule: point your thumb in the direction of
the current. Then, curl your fingers around the wire. The direction your fingers will point in the same direction as
the field. Be sure to use your right hand!