The Strange World of Dr. Lentz
W14D1
GOOD FRIENDS??
Faraday and Lentz
Calendar Thing
Monday Wednesday Friday Saturday
14
Chapter 22Gauss
16 (Read rest of chapter)2 WebAssignsChapter 22Gauss
18QUIZ
Chapter 22Lentz & More
21
Chapter 23Faraday
23QUIZ
Chapter 23Faraday
HOLIDAY
28
Chapter 23Faraday
30
Chapter 24EM
2Chapter 24 EMOptional Exam
(22-24)
3
FINAL EXAM
Remember Flux?
What is the magnetic flux through the rectangle?A) 0.2 Tm2
B) 0.5 Tm2
C) 1.2 Tm2
D) 3.0 Tm2
E) 4.2 Tm2
Solenoid radius 2 cm. Ring radius 10 cm. B = .5 T inside
solenoid. B≈0 outside the solenoid.
What is the magnetic flux through the outer ring?A) 0 Tm2
B) 6.3e–4 Tm2
C) 6.3 Tm2
D) 1.6e–2 Tm2
E) 3.14e–2 Tm2
Ring around the Ring
From Faraday
d d demf dA
dt dt dt
BB •n A
This-dB/dt
is important
NC
So, continuing
The power supply is slowly turned up, increasing the current in the
solenoid.
While the current is increasing, what is the direction of inside the solenoid?
A) = 0B) To the rightC) To the leftD) Toward New York
/d dt B
Last Time - Maxwell
0
0 . 0
0
emf=-
( + )
inside
MNC
Einside path
qdA
dA
dd
dtd
d Idt
E n
B •n
E l
B l
Coulomb/Gauss
Gauss – Magnetism
Faraday’s Law
Extended Ampere’s Law
A Parallel Universe
0
0 . 0
0
emf=-
( + )
inside
MNC
Einside path
qdA
dA
dd
dtd
d Idt
E n
B •n
E l
B l
Lentz’s Law
Induced Magnetic Fields always FIGHT to stop what you are
trying to do!
i.e... Murphy’s Law for Magnets
Lenz’s Law
Example of Nasty Lenz
The induced magnetic field opposes thefield that does the inducing!
From Before .. as a reminder
d d demf dA
dt dt dt
BB •n A
This-dB/dt
is important
So, back to the question
The power supply is slowly turned up, increasing the current in the
solenoid.
While the current is increasing, what is the direction of inside the solenoid?
A) = 0B) To the rightC) To the leftD) Toward New York
/d dt B
The power supply is slowly turned up, increasing the current in the solenoid.
The initial B is show on the diagram.
The current increases so B increases as well by dB in time dt.
Effect is a field reduction caused by the induced current which must be opposing the original current (- sign).
The net emf is reduced as is the current. (Ohm’s Law). The induced current opposes I.
BdBdt
dBdt
( )d d d
emf dAdt dt dt
BB •n A
The total emf is changed (reduced ) by
this amount.
The power supply is slowly turned up, increasing the current in the solenoid.
B
( )d d d
emf dAdt dt dt
BB •n A
The total emf is changed (reduced ) by
this amount.
What does Lentz tell us???
Remember the Pipe Demo
Do you remember it?Can you explain it?
Mini Activity on the Pipe Whiteboard
Presentation
Aluminum Pipe
S
N
ACTIVITY #2
Let’s look at how a “transformer” might work.