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Today4/23
Magnetic Induction Review
HW Questions?
Exam 4/24
Force and motion of charged particles
B-fields due to wires
Force and Torque on current loops
Forces from currents (like 4/16 Wires 2)
Induced current (like 4/17)
Magnetic Induction
The current only flows while:
 the B-field is changing
the ring is moving
the magnet is moving
It’s all about a “changing” B field!!!
Look at whether the # of field lines are
increasing or decreasing
Force on Current Loops
Ch 21: Force/Torque arises from a
battery operated loop in a static B-field.
Ch 22: Current in loops “induced” by a
“changing” external field. The loop
then reacts as in Ch 21.
In fact, battery operated loops resist the
changing field caused by themselves!
Called “Self Induction” see 22.8
Faraday’s & Lenz’z Law
Loop “Voltage”
E = - 
t
Lenz’s Law
 is the # of field
lines inside loop
 can change in many ways:
changing B-field
changing loop area
changing loop angle
changing B-field angle
This is the “equation” you will see on the exam
Using
E = - 
t
Identify initial field direction
Identify final field direction
Determine direction of the “change”
Use Lenz’s law to determine B-field
due to the current induced around the
loop
Use RHR #2 to find direction of
induced current
Energy Considerations
It takes energy to push current around
a loop
Therefore, forces arising from induced
currents must always serve to retard
(or slow down) the object