Physics 212 Lecture 18: Displacement Current and Faradays Law

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Physics 212 Lecture 18 Displacement Current
and Faradays Law
Displacement Current
Magnetic fields are present inside capacitors
while charging/discharging. This is not possible
by Amperes law since current is zero inside the
capacitor.   What generates the magnetic
field? Changing electric field is present in
capacitor during charging/discharging. Therefore,
changing electric fields produce magnetic
fields.   Derivation of displacement current
(defined by Maxwell)
and
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Ampere-Maxwell Law

• - combines regular current with displacement
  current

Faradays Law Changing magnetic fields inside a
loop of circuit produce a voltage in the loop
for N loops from definition of magnetic flux
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Example Ch 30 380.2A flowing onto circular
capacitor with plate radii 10cm and 4mm apart.
Find (a) time rate of increase of E between
plates (b) B between plates but 5cm from center.
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Example Ch 31 5Electromagnet produces uniform
B1.6T over 0.2m2 cross-sectional area. Place
another coil with N200 and R20 Ohms. Then,
turn off current to inner magnet over a period of
20ms. Find I induced in outer coil.
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Questions for Groups

• 30.5 Is Amperes Law valid for all paths around
  a conductor? Why is it not useful to calculate B
  for all such paths?
• 30.7 Is the magnetic field inside a toroid
  uniform?
• 30.9 A hollow copper tube carries current along
  its length. What is B inside? Outside?
• 30.10 Describe the similarities between Gausss
  Law in electrostatics and Amperes Law in
  magnetism.
• 30.11 A conducting loop is in a B field along the
  x-axis. How should you orient it for minimum
  flux? Maximum flux?
• 30.12 What new concept did Maxwells generalized
  form of Amperes Law include?