General electric flux definition

1. If the surface is made up of a mosaic of N little surfaces General electric flux definition • The field is not uniform • The surface is not perpendicular to the field

2. Magnetic Flux Whenever possible: Units: [Weber] = [Wb]=[T-m2]

3. Gauss’s Law Gauss asserts that the calculation for the flux through a closed surface from a point charge is true for any charge distribution!!! This is true so long as Q is the charge enclosed by the surface of integration.

4. Gauss’s Law for magnetism This is true because we cannot isolate a magnetic pole and because magnetic field lines are continuous. The net number of field lines passing through any surface is always zero!

5. Faraday’s Law When the magnet moves, a current is induced as if there was a source of emf (like a battery) in the circuit!

6. Active Figure 31.1 (SLIDESHOW MODE ONLY)

7. Faraday’s Law

8. Active Figure 31.2 (SLIDESHOW MODE ONLY)

9. Faraday’s Law of Induction • An induced emf is produced by a changing magnetic field. • Lenz’s Law – An induced emf is always in a direction that opposes the original change in the flux that caused the emf. Units: [Volts]

10. How can we change the flux? • Change flux by: • Change area • Change angle • Change field

13. 30. In Figure P31.30, the bar magnet is moved toward the loop. Is Va– Vbpositive, negative, or zero? Explain. Figure P31.30

14. Faraday’s Law – continued • Remember FB is the magnetic flux through the circuit and is found by • If the circuit consists of N loops, all of the same area, and if FB is the flux through one loop, an emf is induced in every loop and Faraday’s law becomes

15. Decaying uniform magnetic field P31.4 Assume we can change field: • Change flux by: • Change field • Change area • Change angle A R

16. Applications of Faraday’s Law – Pickup Coil • The pickup coil of an electric guitar uses Faraday’s law • The coil is placed near the vibrating string and causes a portion of the string to become magnetized • When the string vibrates at the same frequency, the magnetized segment produces a changing flux through the coil • The induced emf is fed to an amplifier

17. 1. A 50-turn rectangular coil of dimensions 5.00 cm × 10.0 cm is allowed to fall from a position where B = 0 to a new position where B = 0.500 T and is the magnetic field directed perpendicular to the plane of the coil. Calculate the magnitude of the average emf that is induced in the coil if the displacement occurs in 0.250 s. 3. A 25-turn circular coil of wire has diameter 1.00 m. It is placed with its axis along the direction of the Earth’s magnetic field of 50.0 μT, and then in 0.200 s it is flipped 180°. An average emf of what magnitude is generated in the coil? 6. A magnetic field of 0.200 T exists within a solenoid of 500 turns and a diameter of 10.0 cm. How rapidly (that is, within what period of time) must the field be reduced to zero, if the average induced emf within the coil during this time interval is to be 10.0 kV?

18. Linear Generator Charges stop moving when:

19. Linear Generator with Faraday’s Law By Lenz’s Law, what is the direction of current?

20. Active Figure 31.10 (SLIDESHOW MODE ONLY)

21. Power moving the bar Same result!

22. Breaking effect if power not added

23. 12. A 30-turn circular coil of radius 4.00 cm and resistance 1.00 Ω is placed in a magnetic field directed perpendicular to the plane of the coil. The magnitude of the magnetic field varies in time according to the expression B = 0.010 0t + 0.040 0t2, where t is in seconds and B is in tesla. Calculate the induced emf in the coil at t = 5.00 s. 19. An automobile has a vertical radio antenna 1.20 m long. The automobile travels at 65.0 km/h on a horizontal road where the Earth’s magnetic field is 50.0 μT directed toward the north and downward at an angle of 65.0° below the horizontal. (a) Specify the direction that the automobile should move in order to generate the maximum motional emf in the antenna, with the top of the antenna positive relative to the bottom. (b) Calculate the magnitude of this induced emf. 22. A conducting rod of length ℓ moves on two horizontal, frictionless rails, as shown in Figure P31.20. If a constant force of 1.00 N moves the bar at 2.00 m/s through a magnetic field B that is directed into the page, (a) what is the current through the 8.00-Ω resistor R? (b) What is the rate at which energy is delivered to the resistor? (c) What is the mechanical power delivered by the force Fapp?

24. Rotating Generators and Faraday’s Law 0 For N loops of wire

25. Induced emf in a Rotating Loop • The induced emf in the loop is • This is sinusoidal, with emax = NABw

26. Active Figure 31.21 (SLIDESHOW MODE ONLY)

27. Rotating Generators

28. DC Generators • The DC (direct current) generator has essentially the same components as the AC generator • The main difference is that the contacts to the rotating loop are made using a split ring called a commutator

29. Active Figure 31.23 (SLIDESHOW MODE ONLY)

30. 32. For the situation shown in Figure P31.32, the magnetic field changes with time according to the expression B = (2.00t3 – 4.00t2 + 0.800)T, and r2 = 2R = 5.00 cm. (a) Calculate the magnitude and direction of the force exerted on an electron located at point P2 when t = 2.00 s. (b) At what time is this force equal to zero? 45. A proton moves through a uniform electric field E = 50.0 j V/m and a uniform magnetic field B = (0.200i + 0.300j + 0.400k)T. Determine the acceleration of the proton when it has a velocity v = 200 i m/s. 59. A circular loop of wire of radius r is in a uniform magnetic field, with the plane of the loop perpendicular to the direction of the field (Fig. P31.59). The magnetic field varies with time according to B(t) = a + bt, where a and b are constants. (a) Calculate the magnetic flux through the loop at t = 0. (b) Calculate the emf induced in the loop. (c) If the resistance of the loop is R, what is the induced current? (d) At what rate is energy being delivered to the resistance of the loop?

31. Eddy Currents

32. Self-Inductance • When the switch is closed, the current does not immediately reach its maximum value • Faraday’s law can be used to describe the effect

33. Self-induced emf • A current in the coil produces a magnetic field directed toward the left (a) • If the current increases, the increasing flux creates an induced emf of the polarity shown (b) • The polarity of the induced emf reverses if the current decreases (c)

34. Self Inductance Define: Self Inductance

35. Inductance of a Solenoid • The magnetic flux through each turn is • Therefore, the inductance is • This shows that L depends on the geometry of the object

36. Inductance Units

37. 3. A 2.00-H inductor carries a steady current of 0.500 A. When the switch in the circuit is opened, the current is effectively zero after 10.0 ms. What is the average induced emf in the inductor during this time? 5. A 10.0-mH inductor carries a current I = Imax sin ωt, with Imax = 5.00 A and ω/2π = 60.0 Hz. What is the back emf as a function of time? 7. An inductor in the form of a solenoid contains 420 turns, is 16.0 cm in length, and has a cross-sectional area of 3.00 cm2. What uniform rate of decrease of current through the inductor induces an emf of 175 μV?

38. LR Circuits Charging Kirchhoff Loop Equation: Solution:

39. LR Circuits Discharging Kirchhoff Loop Equation: Solution:

40. Active Figure 32.3 (SLIDESHOW MODE ONLY)

41. 14. Calculate the resistance in an RL circuit in which L = 2.50 H and the current increases to 90.0% of its final value in 3.00 s. 20. A 12.0-V battery is connected in series with a resistor and an inductor. The circuit has a time constant of 500 μs, and the maximum current is 200 mA. What is the value of the inductance? 24. A series RL circuit with L = 3.00 H and a series RC circuit with C = 3.00 μF have equal time constants. If the two circuits contain the same resistance R, (a) what is the value of R and (b) what is the time constant?

42. Energy in a coil PE in an Inductor PE in an Capacitor

43. Energy Density in a coil PE in an Inductor

44. 31. An air-core solenoid with 68 turns is 8.00 cm long and has a diameter of 1.20 cm. How much energy is stored in its magnetic field when it carries a current of 0.770 A? 33. On a clear day at a certain location, a 100-V/m vertical electric field exists near the Earth’s surface. At the same place, the Earth’s magnetic field has a magnitude of 0.500 × 10–4 T. Compute the energy densities of the two fields. 36. A 10.0-V battery, a 5.00-Ω resistor, and a 10.0-H inductor are connected in series. After the current in the circuit has reached its maximum value, calculate (a) the power being supplied by the battery, (b) the power being delivered to the resistor, (c) the power being delivered to the inductor, and (d) the energy stored in the magnetic field of the inductor.

45. Example 32-5: The Coaxial Cable • Calculate L for the cable • The total flux is • Therefore, L is • The total energy is

46. Mutual Inductance

47. Mutual Inductance example

48. Induced emf and Electric Fields • An electric field is created in the conductor as a result of the changing magnetic flux • Even in the absence of a conducting loop, a changing magnetic field will generate an electric field in empty space • This induced electric field is nonconservative • Unlike the electric field produced by stationary charges • The emf for any closed path can be expressed as the line integral of E.ds over the path

49. General form of Faraday’s Law So the electromotive force around a closed path is: And Faraday’s Law becomes: A changing magnetic flux produces an electric field. This electric field is necessarily non-conservative.

50. E produced by changing B How about outside ro ?