Loop Model for Electron Orbits
Eq. 32-28  can be obtained with the nonquantum
derivation.
Assuming  an electron moves along a circular
path with a radius that is  much larger  than an
atomic radius
Imagine  an electron moving at constant speed v in a
circular path of radius r  counterclockwise as shown in
Fig. 32-11.
FIG. 32-11 An electron moving at constant speed v in a
circular path of radius r that encloses an area A. The
electron has an orbital angular momentum Lorb.: and an
associated orbital magnetic dipole moment μorb. A clockwise
current i of  positive charge is equivalent to the
counterclockwise circulation of the negatively charged
electron.
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The magnitude  μorb of such a current loop  is
obtained with N = 1
A  the area enclosed by the loop.
The direction  of this magnetic dipole moment  from
the right-hand rule of Fig. 29-22 downward in Fig. 3211.
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Which is Eq. 32-28  obtained by "classical"  (non
quantum) analysis
The derivation  invalid for that situation  this line of
reasoning yields other results that are contradicted by
experiments.
Loop Model in a Nonuniform Field
FIG. 32-12 (a) A loop model for an electron orbiting in an
atom while in a non uniform magnetic field Bext.
(b) Charge -e moves counterclockwise; the associated
conventional current i is clockwise.
(c) The magnetic forces dF on the left and right sides of the
loop, as seen from the plane of the loop. The net force on the
loop is upward.
(d) Charge -e now moves clockwise.
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(e) The net force on the loop is now downward.
The current  along an element dL in a magnetic field Bext
 experiences a magnetic force  given by Eq. 28-28:
On the left side  of Fig. 32-12c  the force dF  directed
upward  rightward
On the right side  the force dF  large  directed
upward  leftward.
The two forces  have the same angles
Horizontal components  cancel and the  vertical
components add.
The same  is true at any other two symmetric points on
the loop.
The net force  on the current loop of Fig. 32-12b must be
upward 
The net force  on the loop in Fig.32-12d  downward
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32-8 Magnetic Materials
Each electron  in an atom has  orbital magnetic dipole
moment  spin magnetic dipole moment  combine
vectorially.
The resultant  of these two vector quantities combines
vectorially  with similar resultants for all other electrons
in the atom,
The resultant  for each atom combines with those for all
the other atoms in a sample of a material.
The combination  of all these magnetic dipole moments
produces a magnetic field  then the material is magnetic.
There are three  general types of magnetism
diamagnetism  paramagnetism  ferromagnetism.
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32-9 Diamagnetism
Classical Explanation the physical properties of the
diamagnetic material  provide with the loop model of
Figs. 32-11 and 32-12.
In the absence  Bext  the number of electrons orbiting
in one direction is the same as that orbiting in the opposite
direction  the result that the net upward magnetic dipole
moment of the atom equals the net downward magnetic
dipole moment.
In the present  Bext  of Fig. 32-12  the material
develops  downward magnetic dipole moment and
experiences  upward force.
When Bext  removed  both the dipole moment and the
force disappear.
In general
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Fig. 32-13  frog  diamagnetic  as is any other animal
 it was placed in the diverging magnetic field near the
top end of a vertical current carrying solenoid.
Every atom  in the frog  repelled upward away from
the region of stronger magnetic field at that end of the
solenoid.
The frog  moved upward into weaker and weaker 
magnetic field until the upward magnetic force balanced
the gravitational force on it  The frog  hung in midair
A person  can levitate in midair  due to the person's
diamagnetism  if a very large solenoid built.
Ans: a) away;
b) away;
c), less
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