Simple Nature - Light and Matter

c/The magnetic version of

figure a. A magnetically perme-

able material is placed at the

center of a solenoid.

d/Example 24: a cutaway

view of a solenoid.

relationshipD=Ewould actually be a good approximation. Be-

fore a material’s maximum polarization is reached, it may actually

spark or burn up.

self-check I

Suppose a parallel-plate capacitor is built so that a slab of dielectric

material can be slid in or out. (This is similar to the way the stud finder

in figure b works.) We insert the dielectric, hook the capacitor up to

a battery to charge it, and then use an ammeter and a voltmeter to

observe what happens when the dielectric is withdrawn. Predict the

changes observed on the meters, and correlate them with the expected

change in capacitance. Discuss the energy transformations involved,

and determine whether positive or negative work is done in removing

the dielectric. .Answer, p. 1061

11.7.3 Magnetic materials
Magnetic permeability
Atoms and molecules may have magnetic dipole moments as
well as electric dipole moments. Just as an electric dipole contains
bound charges, a magnetic dipole has bound currents, which come
from the motion of the electrons as they orbit the nucleus, c/1. Such
a substance, subjected to a magnetic field, tends to align itself, c/2,
so that a sheet of current circulates around the externally applied
field. Figure c/3 is closely analogous to figure a/3; in the central
gray area, the atomic currents cancel out, but the atoms at the
outer surface form a sheet of bound current. However, whereas like
charges repel and opposite charges attract, it works the other way
around for currents: currents in the same direction attract, and
currents in opposite directions repel. Therefore the bound currents
in a material inserted inside a solenoid tend toreinforce the free
currents, and the result is to strengthen the field. The total current
isI=Ifree+Ibound, and we define an altered version of the magnetic
field,
H=

B

μ

,

and rewrite Amp`ere’s law as

ΓH=Ithrough, free.

The constantμis the permeability, with a vacuum value ofμo =
4 πk/c^2. Here are the magnetic permeabilities of some substances:
substance μ/μo
vacuum 1
aluminum 1.00002
steel 700
transformer iron 4,000
mu-metal 20,000

Section 11.7 Electromagnetic properties of materials 737