1. From last time… Faraday: Lenz: induced currents oppose change in flux
Tue. Nov. 2, 2009
Physics 208, Lecture 18

2. Faraday’s law EMF no longer zero around closed loop EMF around loop
Magnetic flux through surface bounded by path
Make comparison to battery. Show that this acts just like battery. Maybe use shaking flashlight to show that this works.
EMF no longer zero around closed loop
Tue. Nov. 2, 2009
Physics 208, Lecture 18

3. Lenz’s law Induced current produces a magnetic field. Lenz’s law
Interacts with bar magnet just as another bar magnet
Lenz’s law
Induced current generates a magnetic field that tries to cancel the change in the flux.
Here flux through loop due to bar magnet is increasing. Induced current produces flux to left.
Force on bar magnet is to left.
Do demo with magnet and copper plate. Copper plate in nitrogen.
Tue. Nov. 2, 2009
Physics 208, Lecture 18

4. Question
Which way is the magnet moving if it is inducing a current as shown?
A. Up
B. Down S N
Current creates flux up. This must be opposing increase in flux down. So magnet must be falling down N S
Tue. Nov. 2, 2009
Physics 208, Lecture 18

5. Question Solenoid current SN
As current is increasing in the solenoid, what direction will current be induced in ring?
A. Same as solenoid
B. Opposite of solenoid
C. No current
Solenoid current
Tue. Nov. 2, 2009
Physics 208, Lecture 18

6. What is the direction of the force on the can?
Question
I(t) increases with time.
What is the direction of the current induced in the can?
Into page
Out of page CW
CCW
None of the above
Can
Coil
I(t)
Ican?
End view
What is the direction of the force on the can?
Tue. Nov. 2, 2009
Physics 208, Lecture 18

7. Question What is the direction of the force on the can? Up Down Inward
Coil
I(t) Up
Down
Inward
Outward
None of the above
End view
Tue. Nov. 2, 2009
Physics 208, Lecture 18

8. (sign is relative to coil current)
Can
Coil
I(t)
The current I(t) looks like this:
TIME (ms)
CURRENT ( A )
400
200 2 1
0.2
TIME (ms)
INDUCED CURRENT A
The current induced in the can looks like…
TIME (ms)
INDUCED CURRENT B
Do this one after the crushing demo. Then shoot the can from the coil of wire.
TIME (ms)
INDUCED CURRENT
(sign is relative to coil current) C
TIME (ms)
INDUCED CURRENT
Question D

9. Eddy current separation
side view
top view
Tue. Nov. 2, 2009
Physics 208, Lecture 18

10. A copper guillotine blade falls toward a victim
A copper guillotine blade falls toward a victim. It enters the field from strong magnets on way down. v
Model the blade as a square loop, and the field as a uniform with limited spatial extent.
What is the direction of the current induced in the loop (blade) as it enters field?
After questions, do demo. Then ask how induced current and force depends on blade velocity. Then argue that blade reaches terminal velocity. CW
CCW
Depends on field direction
Tue. Nov. 2, 2009
Physics 208, Lecture 18

11. Again model the blade as a loop, and the field as a uniform field of limited spatial extent.v
What is the direction of the magnetic force on the blade (loop) as it enters the field region? Up
Down
Left
Right
Depends on field direction.
After questions, do demo. Then ask how induced current and force depends on blade velocity. Then argue that blade reaches terminal velocity.
Tue. Nov. 2, 2009
Physics 208, Lecture 18

12. Eddy Currents In an extended conductor, currents are more complicated:
But same idea – force opposes motion
Tue. Nov. 2, 2009
Physics 208, Lecture 18

13. Eddy current braking Shinkansen Electro- magnets Steel Rail
ICE 3 near the Oberhaider Wald Tunnel on the Cologne-Frankfurt high-speed rail line
Shinkansen
Tue. Nov. 2, 2009
Physics 208, Lecture 18

14. Motional EMF Charges in metal feel magnetic force
Charges move, build up at ends of metal
Equilibrium: electric force cancels magnetic force
Tue. Nov. 2, 2009
Physics 208, Lecture 18

15. A square conductor moves through a uniform magnetic field
A square conductor moves through a uniform magnetic field. Which shows the correct charge distribution?
Magnetic field out of page
Tue. Nov. 2, 2009
Physics 208, Lecture 18

16. Motional EMF: circuits
Moving conductor in mag. field develops motional EMF
Can drive circuit currents like a battery
Tue. Nov. 2, 2009
Physics 208, Lecture 18

17. Using Faraday’s law The same result can be obtained with Faraday’s law
Change in flux in time t
Tue. Nov. 2, 2009
Physics 208, Lecture 18

18. Rotating loop An AC (alternating current) generator Tue. Nov. 2, 2009
Physics 208, Lecture 18

19. Question
Which position of the loop corresponds to zero EMF, and zero current? A. B. C.
Explain how this can be understood with either magnetic flux or with motional emf.
Tue. Nov. 2, 2009
Physics 208, Lecture 18