Review Sheet for HourTest II  PHYS 212, Spring 2007
NOTE:
This review sheet is
NOT
a substitute for doing the assigned problems, handin problems, reading quizzes and
drills.
You
CAN'T
expect to do well on the test unless you successfully complete all of the problems.
Also, don't forget to
review ConcepTests
Also, we make no guarantees that everything on the test is covered in this review sheet.
We don’t even guarantee that
everything is correct on this sheet (the lecture notes take precedence).
Magnetic Flux
:
Be able to determine the flux for a uniform magnetic field and a flat surface:
φ
m
=
N
r
r
B
A
•
= NBAcos
θ
,
where
θ
is the angle between the area and magnetic field vectors and N is the number of turns in the coil.
Know that
r
A
points perpendicularly to the surface.
Faraday's Law and Lenz' Law:
EMF 
ε
=
m
dt
d
φ
=
)
(
A
B
N
dt
d
r
r
•
= N
)
cos
(
θ
BA
dt
d
.
Use this to calculate the emf
and/or current I (=
ε
/R) in a wire loop in a changing magnetic field.
If the mag. field is perpendicular to the loop (i.e.,
parallel to the area vector), then the problem boils down to either:
(1)
ε
= NAdB/dt or
ε
= NA(
Δ
B/
Δ
t) if the area is
constant; or (2)
ε
= NBdA/dt or
ε
= NB(
Δ
A/
Δ
t) if the magnetic field is constant (e.g., if there is a loop falling through a
region of constant magnetic field).
Make sure you know how to take derivatives:
"dB/dt" does not
mean magnetic field
divided by time or even the derivative divided by time > it is just the derivative, period.
For direction of emf and/or current, use Lenz's Law
, which states that the current will be induced in a direction that will
try to keep the flux from changing.
After you have determined how the flux is changing, decide what magnetic field
could be added to keep the flux the
same
, then use the right hand rule to determine what direction the induced current will
be to produce this added magnetic field.
Practically speaking, if the flux is increasing, then the induced current will
produce a magnetic field in opposite direction as original magnetic field.
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 Spring '08
 Ladd
 Diffraction, Frequency, Magnetic Field, Wavelength, Lenz

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