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.