Transcript Generators

Generators
EMF from Motion
 A moving conductor causes
electrons to move.
• Magnetic force applied to
electrons
 The electrons drift to one
end.
• Create an electric field
• Matching EMF
 At equilibrium forces
balance.
FM  qvBsin 
FE  qE
  EL 
FE L
q
  vBL sin 
Falling Bar
 A bar 1.0 m long is dropped in
an east-west orientation. The
earth’s field is 2.0 x 10-5 T.
 Find the induced EMF after the
bar falls for 4.0 s.
 The problem gives the length
and the field, but the velocity is
needed.
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Use basic kinematics
v = v0 + gt
v = (9.81 m/s2)(4.0 s)
v = 39 m/s.
 Now use the equation for
induced EMF.
• E = vBL
• E = 0.78 mV
Cutting Lines
 As the sliding bar moves the
circuit increases in size.
 The area increases by a rate
equal to DA/Dt = vL.
 The number of field lines cut
per second is the area change
times the field.
• vLB in a uniform field
 The EMF equals the number of
field lines cut per second.
Power Bar
 The sliding bar creates an
EMF
• Potential difference in volts
 The potential can create a
current in a resistor.
 There is power output from the
moving bar.
BDA
DV  vBL 
Dt
V
1  BDA 
P  IDV 
 

R R  Dt 
2
2
Alternating Generator
 A turning loop of wire in a magnetic field generates EMF.
• Loop area changes with respect to field lines
AC Generation
 The moving loop alternates
creating current one way
then the other.
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Sinusoidal pattern
Loop turns at rate w=/t
EMF based on loop area
Extra turns increase the
EMF
 This is called alternating
current or AC.
v  wr
  vBL sin 
  wrBL sin wt
  NABw sin wt
DC Generator
 A DC generator can be made
by using a commutator and
brushes.
• Like a motor
• Voltage pulses but doesn’t
reverse
 Better DC generators use
multiple commutators.
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