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Physics 1161: Lecture 10, Slide 1
Faraday’s Law
• Today’s lecture will cover Textbook
Sections 23-1 -- 23-
Physics 1161: Lecture 10
Changing Magnetic Fields create Electric Fields
Exam
II
Physics 1161: Lecture 10, Slide 2
Faraday’s Law
• Key to EVERYTHING in E+M
– Generating electricity
– Microphones, Speakers and Tape Decks
– Amplifiers
– Computer disks and card readers
– Ground Fault Interrupters
Physics 1161: Lecture 10, Slide 3
Motional EMF, Preflight 10.
F = q v B sin(θ)
Potential Difference ≡ F d/q
EMF = q v B sin(θ) L/q
B
L v
Velocity
Moving + charge feels force downwards:
B Moving + Charge still feels force downwards:
Physics 1161: Lecture 10, Slide 4
Preflight 10.
• Which bar has the larger
motional emf? a b
v
v
E = v B L sin(θ)
θθθθ is angle between v and B
Case a: Case b:
Physics 1161: Lecture 10, Slide 5
Motional EMF circuit
• Direction of Current?
• Direction of force (F=ILB sin(θ)) on bar due to
magnetic field?
I = E /R
• Magnitude of current
Moving bar acts like battery E = vBL (^) B
V
= vBL/R
Physics 1161: Lecture 10, Slide 6
Preflight 10.4, 10.
Increase Stay the Same Decrease
Suppose the magnetic field is reversed so that it now points OUT of the page instead of IN as shown in the figure.
To keep the bar moving at the same speed, the force supplied by the hand will have to:
True False
To keep the bar moving to the right, the hand will have to supply a force in the opposite direction:
Physics 1161: Lecture 10, Slide 10
Lenz’s Law (EMF Direction)
Emf opposes change in flux
• If flux increases:
New EMF makes new field _________ original field
• If flux decreases:
New EMF makes new field _________ original field
Physics 1161: Lecture 10, Slide 11
Preflight 10.
The magnetic field strength through the loop is cut in half (decreasing the flux). If you wanted to create a second magnetic field to oppose the change in flux, what would be its direction?
a
n
B
a
n
B
Left Right
Physics 1161: Lecture 10, Slide 12
ACT: Change Area
v v
Which loop has the greatest induced EMF at the instant shown above?
L
W
v
Loop 1 Loop 2 Loop 3
Physics 1161: Lecture 10, Slide 13
Change Area II
V
t= ΦΦΦΦ 0 =BLW
t ΦΦΦΦt=BL(W+vt)
t
BLW vt BLW t t
t (^) = + − −
ε^0
L
W
V
W vt
EMF Direction: B is out of page and ΦΦΦΦ is increasing so EMF creates B field (inside loop) going ________.
I
EMF Magnitude:
Φ = B A cos(φ)
= =
Physics 1161: Lecture 10, Slide 14
ACT: Change B
As current is increasing in the solenoid, what direction will current be induced in ring?
- Same as solenoid
- Opposite of solenoid
- No current
N
S
Physics 1161: Lecture 10, Slide 15
ACT: Change B II
S
N
N S
Which way is the magnet moving if it is
inducing a current in the loop as shown?
1) Up
2) Down
If I reduce the resistance in the wire, the magnet will fall
Faster slower at the same speed
Physics 1161: Lecture 10, Slide 19
Motional EMF circuit
Moving bar acts like battery E = vBL B
V
x x x x x x x x x x x x x x x xx xx x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x xx xx x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
• Direction of Current x x
• Direction of force (F=ILB sin(θ)) on bar due to
magnetic field
• Magnitude of current
Physics 1161: Lecture 10, Slide 20
See you Friday!
• Read Sections 23-1 -- 23-
