Transcript AP Physics III.E

AP Physics III.E
Electromagnetism
22.1 Induced EMF and Induced
Current
Astounding demo.
Induced EMF (electromagnetic
induction) from
• Changing magnetic field
• Changing the area of the coil
22.2 Motional EMF
EMF induced in a moving
conductor
The separated charges on the
ends of the moving conductor
create an induced EMF or
motional EMF. Note, the EMF is
induced only when the conductor
is moving.
“And the formula is . . . (drum
roll please)”
Ex. The rod in the illustration has a velocity of 5.0 m/s
perpendicular to a magnetic field with a strength of 0.80 T. The
length of the rod is 1.6 m and the bulb has a resistance of 96
Ohms. Find a) the EMF b) the induced current c) the electric
power dissipated by the bulb and d) the energy used by the bulb
in 60.0 s.
Motional EMF and Electrical
Energy (another magnetic force
to worry about)
Ex. What is the force (magnitude and direction) on the rod in the
previous direction?
The force on the conductor is in
the opposite direction of the
velocity. So where does the force
come from the light the bulb for
60 s?
Ex. An external agent provides 0.086 N of force to keep the rod
moving at 5.0 m/s for 60.0 s. Find the work done by the force.
The direction of current and
conservation of energy.
Conceptual Example 3.
p. 700: 2, 4-7, 9
2. a) yours
b) 3.0 m
4. 7800 V
6. 3.2 A (recall formula for R in terms of
resistivity)
22.3 Magnetic Flux
Any induced EMF can be described
in terms of magnetic flux. Magnetic
flux – the product of the strength of
the magnetic field and its crosssectional area. φ = BA (measured in
Webers)
A derivation
General expression for magnetic
flux.
Ex. A conducting coil is in a magnetic field of 0.50 T. The area
of the coil is 2.0 square meters. Find the flux for angles of 0.0º,
60.0º and 90.0º.
22.4 Faraday’s Law of
Electromagnetic Induction
Ol’ Michael (as well as Joseph
Henry) found that whenever there
is a change in flux through a loop
of wire, an EMF is induced inside
the loop. Faraday’s law unites
flux and a time interval.
Faraday’s Law of electromagnetic
induction
EMF is generated if magnetic
flux changes for any reason. So
change in flux depends on the
change in the magnetic field, area
or angle.
Ex. A coil of wire with 20 turns has an area of 1.5 EE –3 square
meters. A magnetic field is perpendicular to the surface of each
loop at all times. At the initial time, the initial magnetic field is
0.050 T. At 10.0 s the magnetic field is 0.060 T. Find a) the
average induced emf during this time and b) the average induced
emf if the magnetic field decreases from 0.060 T to 0.050 T in 0.10
s.
Ex. A coil of wire has an area of 0.020 square meters with 50
turns. At initial time the coil is oriented so that the normal of
the surface of the coils is parallel to a magnetic field of 0.18 T.
At 0.10 s, the angle is 30.0º to the normal. Find a) the induced
emf. b) What is the induced emf if the coil is returned to its
initial orientation in 0.10 s?
p. 701: 10-11, 17-21; Rev. 07B1, 2
10. a) 0.0056 Wb
b) 0 Wb
18. 2.8 EE -3 V
20. 1.5 m2/s
B1
a) 8.8 m/sb) drawing c) 63 N d) 0.27 e) yrz
B2
a) ? b) ? c) 1.7 EE 5 m/s d) 6500 V
22.5 Lenz’s Law
The polarity of a magnetic field
in a coil results from
• The original magnetic field that produces
the changing flux that leads to the emf.
• The induced current that creates its own
magnetic field
Lenz’s Law – the induced emf
resulting from changing magnetic
flux leads to an induced current
whose direction is such that the
induced magnetic field opposes
the direction of the original flux
change.
Strategy
• Determine if the flux that penetrates the coil
is increasing or decreasing
• Find the direction of the induced magnetic
field. It must be in a direction that opposes
the change in flux.
• Once the direction of the induced magnetic
field is determined, use RHR-2 to determine
the direction of the induced current.
A couple of examples.
78B4, 07B3, 07B4
78B4
a) 1.5 m/s to the ?
b) 8 N
c) 12 W
d) 4.5 J
07B3
a) Yours
b) Yours again
c) 533 Ohms
d) 0.15 A
e) 6.0 EE -6 F
07B4
a) 6 EE -6 m3/s
b) Q = Av is also vol. flow
rate (2.4 m/s)
a) Bernoulli’s Eq. (0.29
m)
b) Yours