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Today’s agenda:
Induced Electric Fields.
You must understand how a changing magnetic flux induces an electric field, and be able
to calculate induced electric fields.
Eddy Currents.
You must understand how induced electric fields give rise to circulating currents called
“eddy currents.”
Displacement Current and Maxwell’s Equations.
Displacement currents explain how current can flow “through” a capacitor, and how a timevarying electric field can induce a magnetic field.
Back emf.
A current in a coil of wire produces an emf that opposes the original current.
Eddy Currents
You have seen how a changing magnetic field can induce a
“swirling” current in a conductor (the beginning of this lecture).
If a conductor and a magnetic field are in relative motion, the
magnetic force on charged particles in the conductor causes
circulating currents.
These currents are called “eddy currents.”
Eddy currents give rise to magnetic fields that oppose any
external change in the magnetic field.
Eddy Currents
Eddy currents are useful in generators, microphones, metal
detectors, coin recognition systems, electricity meters, and
roller coaster brakes (among other things).
However, the I2R heating from eddy currents causes energy
loss, so if you don’t want energy loss, you probably think eddy
currents are “bad.”
Eddy Current Demos
cylinders falling through a tube
magnetic “guillotine”
hopping coil
coil launcher
magnetic flasher
Conceptual Example: Induction Stove
An ac current in a coil in the stove
top produces a changing
magnetic field at the bottom of a
metal pan.
The changing magnetic field gives
rise to a current in the bottom of
the pan.
Because the pan has resistance, the current heats the pan. If
the coil in the stove has low resistance it doesn’t get hot but
the pan does.
An insulator won’t heat up on an induction stove.
Remember the controversy about cancer from power lines a few years back? Careful studies
showed no harmful effect. Nevertheless, some believe induction stoves are hazardous.