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7.2 Faradays law

• Christopher Crawford
• PHY 417
• 2015-02-25

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Outline

• Motional EMF from Lorenz force law
• Faradays law THREE experiments, one result
• EMF, Lenz law, special relativity
• Examples AC generator, solenoid ring, SC
  levitation
• Differential form THREE Ampère-like laws, one
  form
• Magnetic vector potential potential momentum
• Mutual inductance transformers, inductors
• Example overlapping solenoids
• Inductor THREE electrical devices, one
  calculation
• Example LR circuit
• Magnetic field energy back EMF or electrical
  inertia
• Compare electrical field energy

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Review Electromotive Force (EMF)

• Better name electromotance
• The force that pushes electrons around a circuit
• Chemical (battery) isnt chemical just
  electric forces?
• Mechanical (piezoelectric, Van de Graaff)
• Light (photocell)
• Thermal (thermocouple)
• Magnetic analogy magnetomotance and reluctance
  HW5 3
• Hopkinsons law (Rowland)

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Motional EMF

• Precursor to Faradays law
• Write f v x B as a magnetic flux law
• Magnetic force does no work!

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Motional EMF

• General proof

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Faradays law THREE experiments
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Example AC generator
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Example solenoid and ring

• What is I(t) in the ring?
• Which direction?

• Levitation of ring
• Permanent for superconductors

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THREE Ampère-like laws

• Ampères law Vector potential Faradays law
• Relation between E and A will need to revisit
  electric potential!

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Mutual Inductance

• Overlapping solenoid and ring mutual inductance
  M
• Self-inductance L

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Inductance 1 of THREE components

• Capacitor Resistor Inductor
• Each the ratio of FLUX / FLOW
• Units
• Complex impedance

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Example LR-circuit
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Magnetic field energy

• Work against the back-EMF is stored in the
  magnetic field
• It acts as electrical inertia to keep current
  moving