Induction and Inductance Chapter 30

1. Induction and Inductance Chapter 30

2. Magnetic Flux

3. Insert Magnet into Coil

4. Remove Coil from Field Region

5. From The Demo .. Second experiment First experiment A changing magnetic field INDUCES a current in a circuit loop.

6. Faraday’s Experiments ? ?

7. That’s Strange ….. These two coils are perpendicular to each other

8. Definition of TOTAL ELECTRIC FLUX through a surface:

9. Magnetic Flux:F THINK OF MAGNETIC FLUX as the “AMOUNT of Magnetism” passing through a surface.

10. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Consider a Loop • Magnetic field passing through the loop is CHANGING. • FLUX is changing. • There is an emf developed around the loop. • A current develops (as we saw in demo) • Work has to be done to move a charge completely around the loop.

11. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Faraday’s Law (Michael Faraday) • For a current to flow around the circuit, there must be an emf. • (An emf is a voltage) • The voltage is found to increase as the rate of change of flux increases.

12. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Faraday’s Law (Michael Faraday) We will get to the minus sign in a short time.

13. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Faraday’s Law (The Minus Sign) Using the right hand rule, we would expect the direction of the current to be in the direction of the arrow shown.

14. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Faraday’s Law (More on the Minus Sign) The minus sign means that the current goes the other way. This current will produce a magnetic field that would be coming OUT of the page. The Induced Current therefore creates a magnetic field that OPPOSES the attempt to INCREASE the magnetic field! This is referred to as Lenz’s Law.

15. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx How much work? emf Faraday's Law A magnetic field and an electric field are intimately connected.)

16. MAGNETIC FLUX • This is an integral over an OPEN Surface. • Magnetic Flux is a Scalar • The UNIT of FLUX is the weber • 1 weber = 1 T-m2

17. We finally stated FARADAY’s LAW

18. From the equation Lentz Lentz

19. Flux Can Change • If B changes • If the AREA of the loop changes • Changes cause emf s and currents and consequently there are connections between E and B fields • These are expressed in Maxwells Equations

20. Maxwell’s Equations(chapter 32 .. Just a Preview!) Gauss Faraday

21. The Flux into the page begins to increase. An emf is induced around a loop A current will flow That current will create a new magnetic field. THAT new field will change the magnetic flux. xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx Another View Of That hopeless minus sign again …..SUPPOSE that B begins to INCREASE its MAGNITUDE INTO THE PAGE

22. Lenz’s Law Induced Magnetic Fields always FIGHT to stop what you are trying to do!

23. Example of Lenz The induced magnetic field opposes the field that does the inducing!

25. Don’t Hurt Yourself! The current i induced in the loop has the direction such that the current’s magnetic field Bi opposes the change in the magnetic field B inducing the current.

26. Lenz’s Law An induced current has a direction such that the magnetic field due to the current opposes the change in the magnetic flux that induces the current. (The result of the negative sign!) …

27. #1 CHAPTER 30 • The field in the diagram • creates a flux given by • FB=6t2+7tin milliWebers • and t is in seconds. • What is the emf when • t=2 seconds? • (b) What is the direction • of the current in the • resistor R?

28. This is an easy one … Direction? B is out of the screen and increasing. Current will produce a field INTO the paper (LENZ). Therefore current goes clockwise and R to left in the resistor.

29. #21 Figure 30-50 shows two parallel loops of wire having a common axis. The smaller loop (radius r) is above the larger loop (radius R) by a distance x >>R. Consequently, the magnetic field due to the currenti in the larger loop is nearly constant throughout the smaller loop. Suppose that x is increasing at the constant rate of dx/dt = v. (a) Determine the magnetic flux through the area bounded by the smaller loop as a function of x. (Hint: See Eq. 29-27.) In the smaller loop, find (b) the induced emf and (c) the direction of the induced current. v

30. q B is assumed to be constant through the center of the small loop and caused by the large one.

31. q The calculation of Bz

32. dx/dt=v More Work In the small loop:

33. q Which Way is Current in small loop expected to flow?? B

34. What Happens Here? • Begin to move handle as shown. • Flux through the loop decreases. • Current is induced which opposed this decrease – current tries to re-establish the B field.

35. moving the bar

36. Moving the Bar takes work v

37. What about a SOLID loop?? Energy is LOST BRAKING SYSTEM METAL Pull Eddy Currents

38. Inductors Back to Circuits for a bit ….

39. Definition Current in loop produces a magnetic field in the coil and consequently a magnetic flux. If we attempt to change the current, an emf will be induced in the loops which will tend to oppose the change in current. This this acts like a “resistor” for changes in current!

40. Remember Faraday’s Law Lentz

41. Look at the following circuit: • Switch is open • NO current flows in the circuit. • All is at peace!

42. Close the circuit… • After the circuit has been close for a long time, the current settles down. • Since the current is constant, the flux through the coil is constant and there is no Emf. • Current is simply E/R (Ohm’s Law)

43. Close the circuit… • When switch is first closed, current begins to flow rapidly. • The flux through the inductor changes rapidly. • An emf is created in the coil that opposes the increase in current. • The net potential difference across the resistor is the battery emf opposed by the emf of the coil.

44. Close the circuit…

45. Moving right along …

46. Definition of Inductance L UNIT of Inductance = 1 Henry = 1 T- m2/A FB is the flux near the center of one of the coils making the inductor

47. Consider a Solenoid l n turns per unit length

48. So…. Depends only on geometry just like C and is independent of current.

49. Inductive Circuit • Switch to “a”. • Inductor seems like a short so current rises quickly. • Field increases in L and reverse emf is generated. • Eventually, i maxes out and back emf ceases. • Steady State Current after this. i

50. THE BIG INDUCTION • As we begin to increase the current in the coil • The current in the first coil produces a magnetic field in the second coil • Which tries to create a current which will reduce the field it is experiences • And so resists the increase in current.