1. Lenz and The Haunting of Physics
Lenz’s Lab
Dr. Lenz strikes again.

2. Last Time We looked at induced currents
We indirectly covered Lenz’s Law
Today we will explore induction further and learn about a new circuit element.
Friday we will have a quiz. (Remember my threat … it might happen! Or not.)

3. Last Time
We looked at currents and magnetic fields induced by changing magnetic fields.
We defined magnetic flux.
We can now state Lenz’s Law in the following way:
If you try to change the magnetic FLUX through a closed surface, the induced current will be in such a direction as to OPPOSE the change that you are trying to make!
Or – “The toast will always fall buttered side down!”

4. HEY BINDELL – WAIT A SECOND …….
You said that there is a conducting loop.
You said that there is therefore a current induced 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?
Or a current???
‘tis a puzzlement!
Induction
7/19/2018

5. Induction
7/19/2018

6. REMEMBER when I said E Fields start and end on CHARGES???
DID I LIE??
Induction
7/19/2018

7. The truth BUT WAIT, YOU STILL LIE!
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
7/19/2018

8. 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? How can there be a current??
Induction
7/19/2018

9. Yes … but this doesn’t make me a liar! Let Me Explain.
Induction
7/19/2018

10. 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
7/19/2018

11. So, consider the following:
x x x x x x x E
Conductor
Induction
7/19/2018

12. THEREFORE WHAT WILL A VOLTMETER READ FROM A to A?
x x x x x x x
A The emf
B Zero
C Can’t tell E  A
Conductor
Induction
7/19/2018

13. How Big is the emf? Faraday’s Law
Michael Faraday ( )
MINUS????
Induction
7/19/2018

14. 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!
Induction
7/19/2018

15. 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
7/19/2018

16. Check Your Knowledge

17. An inductor resists CHANGES in the current going through it.
Induction
7/19/2018

18. An inductor resists CHANGES in the current going through it.
Induction
7/19/2018

19. An inductor resists CHANGES in the current going through it.
Induction
7/19/2018

20. Faraday says this is the emf!
Inductance Defined
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
Induction
7/19/2018

21. 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:
Induction
7/19/2018

22. Series LR Circuit
Induction
7/19/2018

23. RL or LR Series Circuit
Switch is open .. no current flows for obvious reasons.
Switch closed for a long time:
Steady current, voltage across the inductor is zero. All voltage (E) is across the resistor.
i=E/R
Induction
7/19/2018

24. RL or LR Series Circuit
When the switch opens, current change is high and back emf from L is maximum. i
E/R t
As the current increases, more voltage is across R, the rate of change of I decreases
and as the current increases, it increases more slowly.
Induction
7/19/2018

25. RL Circuit When L=0, the current rises very rapidly (almost instantly)
As L increases, it takes longer for the current to get to its maximum.
Induction
7/19/2018

26. RL Circuit - Kirchoff Stuff
Induction
7/19/2018

27. The Graphic Result – Current Growth}
63% of
maximum
e= …
Induction
7/19/2018

28. Decay – Short out the battery
Magnetic field begins to collapse, sending its energy into driving the current.
The energy is dissipated in the resistor.
i begins at maximum (E/R) and decays.
Induction
7/19/2018

29. Solution
Induction
7/19/2018

30. Lab Time …
if enough time!