20'3 Electrical Generator

1
20.3 Electrical Generator

• Electrical Generators
• Magnetic induction

2
Applications of Faradays Laws Current
generation by changing magnetic flux
Electrical generator Magnetic tape
recorder Ground fault interrupter Magnetic
induction
3
Electrical generator
AC generator -Alternating current
voltage alternates between positive and
negative- sinusoidal dependence
R
I
V
V
0
t
4
A wire loop of area A rotating in a magnetic
field with constant angular velocity ?.
Normal to the plane
?
B
e
The flux through the loop
angular velocity (radians/s)
5
The emf is proportional to the slope
B
BA
t
-BA
maximum slope
BA?
from calculus
-BA?
6
?
BA
2?
The maximum slope is two times higher when the
angular frequency is increased by 2.
t
-BA
7
The emf generated by a loop of N turns rotating
at constant angular velocity ? is
NBA?
0
t
-NBA?
8
35. In a model ac generator, a 500 turn
rectangular coil 8.0 cmx 20 cm rotates at 120
rev/min in a uniform magnetic field of 0.60 T. a)
What is the maximum emf induced in the coil?
The maximum value of e
9
AC generator
e
t
10
DC generator
generator
11
A generator is a motor acting in reverse
Back emf When a motor is running it
generates an emf that opposes the driving voltage.
V
R
motor
back emf
V
generator
12
Generation of voltage by a tape recorder Changing
magnetic flux?emf
As magnetic tape moves under the gap it induces a
changing flux in the coil. The changing flux
produces a voltage that is amplified to give the
signal.
Iron core electromagnet
Gap
Magnetic tape
V
13
Ground fault interrupter
sensing coil
Net current through ring is zero No voltage in
sensing coil
e0
I
electrical appliance
circuit breaker
I
iron ring
14
Ground fault interrupter
sensing coil
Induced voltage in sensing coil triggers circuit
breaker to shut off current
e
I
circuit breaker
I
iron ring
short to ground
15
Magnetic Induction
Force due to eddy currents
Eddy currents in conducting ring produce a
magnetic dipole that opposes the change in the
field in the core.
F
conducting ring
coil
iron core
16
Magnetic Induction
Energy transfer due to eddy currents
conducting ring
The induced currents can do work. Energy transfer
though space
coil
iron core