W05D2 Dielectrics and Conductors as Shields

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W05D2Dielectrics and Conductors as Shields
Todays Reading Assignment Course Notes
Sections 5.4, 5.6, 5.8-5.9
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Announcements
Math Review Week 06 Tuesday 9-11 pm in
26-152 PS 5 due W05 Tuesday at 9 pm in boxes
outside 32-082 or 26-152 Add Date Week 05 Friday
W05D3 Reading Assignment Friday Problem
Solving Capacitance and Dielectrics Course Notes
Sections 5.6, 5.8-5.9
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Outline

• Dielectrics
• Electric Fields in Matter
• Conductors as Shields

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Capacitors and Dielectrics
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Dielectrics
A dielectric is a non-conductor or
insulator Examples rubber, glass, waxed
paper When placed in a charged capacitor, the
dielectric reduces the potential difference
between the two plates HOW???
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Molecular View of Dielectrics
Polar Dielectrics Dielectrics with permanent
electric dipole moments Example Water
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Molecular View of Dielectrics
Non-Polar Dielectrics Dielectrics with induced
electric dipole moments Example CH4
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Dielectric in Capacitor
Potential difference decreases because dielectric
polarization decreases Electric Field!
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Dielectric Constant
Dielectric weakens original field by a factor
Dielectric Constant
Dielectric constants Vacuum 1.0 Paper
3.7 Pyrex Glass 5.6 Water
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Group Problem Induced Surface Charge Density
A dielectric material with constant
completely fills the space between two conducting
plates that have a surface charge densities
as shown in the figure. Induced surface charge
densities appear on the surfaces of
the dielectric. Find an expression for in
terms of and .
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DemonstrationParallel Plate Capacitorwith
Dielectric E5
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numE205show0
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Dielectric in a Capacitor
Q0 constant after battery is disconnected
Upon inserting a dielectric free charge on plates
does not change, potential decreases, capacitance
increases
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Dielectric in a Capacitor
V0 constant when battery remains connected
Upon inserting a dielectric free charge on plates
increase
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Gausss Law with Dielectrics
In both cases
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Concept QuestionsDielectric in a Capacitor
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Concept Question Dielectric
A parallel plate capacitor is charged to a total
charge Q and the battery removed. A slab of
material with dielectric constant is
inserted between the plates. The charge stored
in the capacitor

1. Increases
2. Decreases
3. Stays the Same

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Concept Question Answer Dielectric
Answer 3. Charge stays the same

• Since the capacitor is disconnected from a
  battery there is no way for the amount of charge
  on it to change.

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Concept Question Dielectric
A parallel plate capacitor is charged to a total
charge Q and the battery removed. A slab of
material with dielectric constant in
inserted between the plates. The energy stored
in the capacitor

1. Increases
2. Decreases
3. Stays the Same

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Concept Question Answer Dielectric
Answer 2. Energy stored decreases

• The dielectric reduces the electric field and
  hence reduces the amount of energy stored in the
  field.
• The easiest way to think about this is that the
  capacitance is increased while the charge remains
  the same so
• Also from energy density

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Concept Question Dielectric
A parallel plate capacitor is charged to a total
charge Q and the battery removed. A slab of
material with dielectric constant in
inserted between the plates. The force on the
dielectric

1. pulls in the dielectric
2. pushes out the dielectric
3. is zero

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Concept Question Answer Dielectric
Answer 1. The dielectric is pulled in

• We just saw that the energy is reduced by the
  introduction of a dielectric. Since systems want
  to reduce their energy, the dielectric will be
  sucked into the capacitor.
• Alternatively, since opposing charges are induced
  on the dielectric surfaces close to the plates,
  the attraction between these will lead to the
  attractive force.

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Conductors as Shields
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Concept QuestionPoint Charge Inside Conductor
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Concept Question Point Charge in Conductor
A point charge Q is placed inside a neutral,
hollow, spherical conductor. As the charge is
moved around inside, the surface charge density
on the outside

1. is initially uniform and does not change when the
   charge is moved.
2. is initially uniform but does become non-uniform
   when the charge is moved.
3. is initially non-uniform but does not change when
   the charge is moved.
4. is initially non-uniform but does change when the
   charge is moved.

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Concept Question Answer Q in Conductor
Answer 1 is initially uniform and does not
change when the charge is moved.

• E 0 in conductor ? -Q on inner surface
• Charge conserved ? Q on outer surface
• E 0 in conductor ? No communication
• between Q Q ? Q uniformly distributed

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Concept Question Point Charge in Conductor
A point charge Q is placed inside a neutral,
hollow, spherical conductor. As the charge is
moved around inside, the electric field outside

1. is zero and does not change
2. is non-zero but does not change
3. is zero when centered but changes
4. is non-zero and changes

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Concept Question Answer Q in Conductor
Answer 2. is non-zero but does not change.

• E 0 in conductor ? -Q on inner surface
• Charge conserved ? Q on outer surface
• E 0 in conductor ? No communication
• between Q Q ? Q remains uniformly
  distributed so E stays unchanged

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Shielding By Conducting Shell Applet
Charge placed INSIDE induces balancing charge ON
INSIDE. Electric field outside is field of point
charge.
http//web.mit.edu/viz/EM/visualizations/electrost
atics/ChargingByInduction/shielding/shielding.htm
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Shielding by Conducting Shell Applet
Charge placed OUTSIDE induces charge separation
ON OUTSIDE. Electric field is zero inside.
http//web.mit.edu/viz/EM/visualizations/electrost
atics/ChargingByInduction/shielding/shielding.htm
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DemonstrationFaraday Cage D33
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numD2033show0
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Experiment 1Faraday Ice Cage