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Physics 1161: PreLecture 16
Generators and Motors
• Textbook Section 23-6
Review: Two uses of RHR’s
• Force on moving charge in
Magnetic field
+ + + +v
F
I
– Thumb: v (or I)
– Fingers: B
– Palm: F on + charge
Palm: out of page.
• Magnetic field produced by
moving charges
– Thumb: I (or v for + charges)
– Fingers: where you want to know B
– Palm: B
Thumb: out
Fingers: up
Palm: left.
x
•
Review: Two uses of RHR’s
• Force on moving
charge in Magnetic
field
• Magnetic field produced by
moving charges
– Thumb: I (or v for + charges)
– Fingers: curl along B field
F
I
– Thumb: v (or I)
– Fingers: B
– Palm: F on + charge
+ + + +v
Palm: out of page.
I
Review: Induction
• Lenz’s Law
– If the magnetic flux (B) through a loop changes, an
EMF will be created in the loop to oppose the change
in flux
– EMF
current (V=IR)
additional B-field.
• Flux decreasing => B-field in same direction as original
• Flux increasing => B-field in opposite direction of original
• Faraday’s Law
– Magnitude of induced EMF given by:

 f  i
 

t
t f  ti
Review: Rotation Variables
v, , f, T
• Velocity (v):

– How fast a point moves.
– Units: usually m/s
r
• Angular Frequency ():
– How fast something rotates.
– Units: radians / sec
v
v
v= r
• Frequency ( f ):
– How fast something rotates.
– Units: rotations / sec = Hz
f =  / 2
• Period (T):
– How much time one full rotation takes.
– Units: usually seconds
T = 1 / f = 2
/
Generators and EMF
EMF is voltage!
side 1 = v B L sin(q)
v = r
side 1 = r B L sin(q)
side 2 = r B L sin(q)
loop = side 1 + side 2
 2r B L sin(q)
2rL = A
loop =  A B sin(q)
loop =  A B sin(t)
1
•

v
2
AB
x
q
v
r

t
AB
ACT: Generators and EMF
 =  A B sin(q)
•
θ
x
1
2
3
At which time does the loop have the greatest emf
(greatest / t)?
1) Has greatest flux, but q = 0 so  = 0.
2) (Preflight example) q  30 so   AB/2.
3) Flux is zero, but q = 90 so  = AB.
Comparison:
Flux vs. EMF
Flux is maximum
– Most lines thru loop
EMF is minimum
– Just before: lines enter from left
– Just after: lines enter from left
– No change!
Flux is minimum
– Zero lines thru loop
EMF is maximum
– Just before: lines enter from top.
– Just after: lines enter from bottom.
– Big change!
Generators and Torque
 =  A B sin(q)
Voltage!
Connect loop to resistance R use I=V/R:
I =  A B sin(q) / R
Recall:
t = A B I sin(q)
=  A2 B2 sin2(q)/R
•

v
x
r
Torque, due to current and B field, tries to slow
spinning loop down. Must supply external
torque to keep it spinning at constant 
q
v