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F16 PH 333 4.1.1-4.1.4

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Richard Datwyler

on 30 October 2018

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Transcript of F16 PH 333 4.1.1-4.1.4

Polarization
4.1.1 Dielectrics
4.1.2 Induced Dipoles
4.1.3 Alignment of Polar Molecules
4.1.4 Polarization
Conductors vs. Dielectrics
All about the electrons
Conductors have 1-2 electrons per atom, able to move anywhere they are needed.
Dielectrics behave as insulators, where the electrons are attached to the atoms or molecules
yet they can move within the atom or molecules
This movement is what makes them dielectrics.
Induced Dipoles
When a neutral atom is placed in an E field, will pull the electron cloud one way and the nucleus another.
This induces a dipole, of strength
Where E is the external field, and alpha is the
Atomic Polarizabilty, measured by experiment, (a few shown in a table here)
E field a distance d from the center of a -q uniformly charged sphere
-q
+q
Equilibrium is reached when the nucleus is moved to the location the internal field equals the external field. Solving for this dipole moment
Molecules
Cute example of an atom, not perfect, but close. Molecules are much harder. Often require directional polarizabilty
note, you can choose an axis to simplify down to 3 terms.
also note, p will not point in the same direction as E
Polar molecules
Not the same as induced dipoles
Water, and others, have an intrinsic permanent dipole moment
Net force is zero. (in a constant field)

Net torque is (tries to line up dipole to the field)
nonuniform field, force becomes:
Polarization
Thus these two things will happen if a molecule is placed in an external E field
If neutral atom - induced dipole in direction of E
If polar molecule - torque moves dipole to E
Both produce same result: Polarization
Polarization = dipole moment per volume
O
H
H
"Can you explain the relation of ∆E and equation 1.35 a little more in depth, I'm not sure I fully understand it."
"What is the overall picture of dipoles? In other words why are dipole relevant?"
"what again is a tensor? so then what is a polarizability tensor? "
"i don't relay understand the effects of an E field in these situations "
" It then said that because of this, the second mechanism (the stretching) dominates the polarization. Why is that? It the polarization like the inertia in a sense? The harder the mechanism, the more sway it holds in the polarization?
"What do r and d represent in the different equations?"
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