Announcements

1
Announcements

• Homework 6 is due at end of class on Friday,
  March 17
• Homework 7 is due at end of class on Friday,
  March 24
• Will cover induction and AC circuits (HRW
  chapters 30-31)
• Will post after class today
• Midterm 2 Plans
• Goal is to have it cover topics in magnetism
  (i.e., chapters 26 31)
• Several conflicts arose with proposal for having
  exam on Wed., April 5
• No date will be perfect, so lets schedule
  midterm 2 for Friday, April 7

2
Eddy Currents

• Faradays law applies generally, not just to
  loops / coils
• Consider a solid material like a sheet of
  aluminum with a uniform magnetic field B normal
  to the sheet
• The magnetic flux through a loop of radius r is
• If the magnetic field is changing, there will be
  an EMF around this loop, and thus an electric
  field

3
Eddy Currents

• Electric field in a conductor produces a current
• For a good conductor, these currents can be quite
  large
• Example dB/dt 1 T/s, Aluminum conductivity is
  3.6 x 107 /ohm meter gives a current density of
  185 A/cm2 at a radius of 10 cm
• These currents can produce large forces on the
  objects
• Lenzs law direction of force will be to try to
  keep flux in material from changing

4
Self-Inductance

• Suppose we have a coil of wire producing a
  magnetic field
• The magnetic field produced by this current
  yields a magnetic flux through the coil
• If we try to change the current in the coil, and
  thereby change the magnetic flux, an additional
  source of EMF will be produced in the coil due to
  its self-inductance
• Lenzs Law sign of the EMF is such that it acts
  to oppose the change in current

5
Self-Inductance of a Solenoid

• Consider a solenoid of length d with N windings
  and current I and radius r
• The magnetic flux is given by
• Flux is proportional to current - for solenoid

6
EMF from Self-Induction

• If the current changes, we produce an EMF
• We call L the inductance of our coil
• SI unit of inductance is the Henry
• Named after the physicist Joseph Henry
• A 1 Henry coil will produce an EMF of 1 V when
  the current changes by 1A/s