1. Some Weird Stuff
November 5, 2010
Induction

2. calendar Today – QUIZ Discussion about Lenz and Induction
Experiments on Inductors
Monday – Finish the Unit
Additional Discussion
Exam Date will be Announced
Labs will be collected again shortly … make sure you are ready.
calendar
Induction

3. Important Definition – Magnetic Flux
Magnetic Field
AREA
Important Definition – Magnetic Flux
Induction

4. Induction

5. The Essence of this Topic
Consider a conductor that is shaped in a loop but is continuous.
The conductor has a magnetic field through the loop that is not necessarily uniform.
There is a MAGNETIC FLUX through this loop.
If the FLUX CHANGES, an “emf” will be induced around the loop.
This emf can cause a current to flow around the loop.
Induction

6. How Can You Change the Magnetic Flux Going Through The Loop? Huh?
Divide the area of the loop into a very large number of small areas DA.
Find the Magnetic Field through each area as well as the angle that it makes with the normal to the area.
Compute the total flux through the loop.
Induction

7. The Magnetic Flux Going Through The Loop:
Add up all of these pieces
that are INSIDE the loop.
Induction

8. Changing F Change any or all of the And the Flux will change! Bi DAifi
Change the SHAPE of the loop
Change the ANGLE that the loop makes with the magnetic field (subset of above)
And the Flux will change!
Induction

9. WAIT A SECOND ……. You said that there is a conducting loop.
You said that there is therefore a VOLTAGE or emf around the loop if the flux through the loop changes.
But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop?
‘tis a puzzlement!
Induction

10. Induction

11. REMEMBER when I said E Fields start and end on CHARGES???
DID I LIE??
Induction

12. BUT WAIT, YOU STILL LIE! The truth
Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously.
Electric Fields created by changing magnetic fields can actually be shaped in loops.
BUT WAIT, YOU STILL LIE!
Induction

13. Why do you STILL think I am a liar?
Because you said that an emf is a voltage so if I put a voltmeter from one point on the loop around to the same point, I will get ZERO volts, won’t I
Induction

14. Yes … but this doesn’t make me a liar! Let Me Explain.
Induction

15. The POTENTIAL between two points
Is the WORK that an external agent has to do to move a unit charge from one point to another.
But we also have (neglecting the sign): Ds
Induction

16. So, consider the following:
x x x x x x x E
So, consider the following:
Conductor
Induction

17. THEREFORE WHAT WILL A VOLTMETER READ FROM A to A?
x x x x x x x E
A The emf
B Zero
C Can’t tell  A
Conductor
THEREFORE WHAT WILL A VOLTMETER READ FROM A to A?
Induction

18. How Big is the emf? Faraday’s Law
Michael Faraday ( )
MINUS????
Induction

19. Q: Which way does E point?
A: The way that you don’t want it to point! (Lenz’s Law).
Lenz’s Law Explains the (-) sign!
Q: Which way does E point?
Induction

20. Faraday’s Experiments?
Faraday’s Experiments
Induction

21. Insert Magnet into Coil
Induction

22. Remove Coil from Field Region
Induction

23. Push a magnet into a coil of two wires and a current is produced via an emf.
In this case, 2 coils, each have the SAME emf so we add..
Ohm’s Law still works, so
Induction

24. The Strange World of Dr. Lentz
Induction

25. Murphy’s Law for Magnets
Lenz’s Law
Induced Magnetic Fields always FIGHT to stop what you are trying to do!...
Murphy’s Law for Magnets
Induction

26. Example of Nasty Lenz The induced magnetic field opposes the
field that does the inducing!
Example of Nasty Lenz
Induction

27. Induction

28. Let’s do the
Lentz Warp
again !
Induction

29. An inductor resists CHANGES in the current going through it.
2/27/2019
Induction

30. An inductor resists CHANGES in the current going through it.
2/27/2019
Induction

31. An inductor resists CHANGES in the current going through it.
2/27/2019
Induction

32. Faraday says this is the emf!
If the FLUX changes a bit during a short time Dt, then the current will change by a small amount Di.
Faraday says this is the emf!
This is actually a
calculus equation
Inductance Defined
2/27/2019
Induction

33. So … E= The UNIT of “Inductance – L” of a coil is the henry. SYMBOL:
There should be
a (-) sign but we
use Lenz’s Law
instead!
The UNIT of “Inductance – L” of a coil is the henry.
SYMBOL:
So …
2/27/2019
Induction

34. Move on to the experiment in UNIT 14
Induction