Lenz s Law (Section 22.5)

Transcription

1 Lenz s Law (Section 22.5) : Thursday, 25 of February 7:00 9:00 pm Rooms: Last Name Room (Armes) Seats A - F G - R S - Z Phys 1030 General Physics II (Gericke) 1

2 1) Charging by contact and induction: Lecture 1 and Phys 1030 General Physics II (Gericke) 2

3 2) Coulombs Law: Lecture 2 F k q 1 r q 2 2 F 34 F net q 1 q 2 q 3 q 4 F 14 F 24 F F F F net F 34 F net F 24 F Phys 1030 General Physics II (Gericke) 3

4 3) The Electric Field: Lecture 3,4 E k q F net 2 r q 0 For a positive isolated point charge a positive test charge will move radially away from the charge Q. For a negative isolated point charge a positive test charge will move radially toward the charge Q. E Q k r r ˆ 2 Q Q E Q k r r ˆ Phys 1030 General Physics II (Gericke) 4

5 3) The Electric Field: Lecture 3,4 E k q F net 2 r q 0 For a positive isolated point charge a negative test charge will move radially toward the charge Q. For a negative isolated point charge a negative test charge will move radially away from the charge Q. E Q k r r ˆ 2 Q Q E Q k r r ˆ Phys 1030 General Physics II (Gericke) 5

6 4) The Electric Potential and Potential Energy: Lecture 6 V r Q k r E V d W electric q 0 V V V A B E mv mgh kx EPE Phys 1030 General Physics II (Gericke) 6

7 5) Electromotive Force and Current : Lecture 7 6) Ohm s Law: Lecture 7 V IR 7) Resistance and Resistivity: Lecture 7 R L A Phys 1030 General Physics II (Gericke) 7

8 8) Power and Power Dissipation: Lecture 7,8 9) Series and Parallel Resistor Circuits: Lecture 8,9 2 2 V P IV I IR I R R 10) Kirchoff s Rules: Lecture 9, Phys 1030 General Physics II (Gericke) 8

9 11) Magnetic Force on a Moving Charge: Lecture 11,12 F qvb sin B 12) Force on a Current in a Magnetic Field: Lecture 13,14 F ILB sin Phys 1030 General Physics II (Gericke) 9

10 13) Torque on a current carrying coil Lecture 13,14 IAB sin 14) Magnetic Fields Produced by Currents Lecture 14, 15 I 0 B 2 r T m A Phys 1030 General Physics II (Gericke) 10

11 15) Induced Current and Induced emf: Lecture 15, 16 E vbl 16) Magnetic Flux: Lecture Phys 1030 General Physics II (Gericke) 11

12 17) Faraday s Law: Lecture 16,17 E N t 18) Lenz s Law: Lecture 17, 18 (finish up today) Phys 1030 General Physics II (Gericke) 12

13 Faraday s Law (Section 22.4) Faraday s Law of Electromagnetic Induction The average emf induced in a coil of N loops is E N t Note that t is the average rate of change of through 1 loop! Any change in B, A or cos( ) will create an induced EMF Phys 1030 General Physics II (Gericke) 13

14 Lenz s Law (Section 22.5) Direction of induced current produces a magnetic field that opposes the change in flux. Area of loop increases: t 0 Induced I creates magnetic field out of page, opposite direction. Total magnetic field is reduced, a change which opposes the increase in flux Phys 1030 General Physics II (Gericke) 14

15 Lenz s Law (Section 22.5) For t 0 the induced I creates magnetic field into page (same direction to oppose the decrease)! Phys 1030 General Physics II (Gericke) 15

16 Lenz s Law (Section 22.5) LENZ S LAW: polarity of EMF The induced emf resulting from a changing Φ has a polarity that leads to an induced I whose direction is such that the induced B field opposes the original Φ change. Induced EMF Induced current Induced magnetic field opposing change in flux Phys 1030 General Physics II (Gericke) 16

17 Lenz s Law (Section 22.5) Using Lenz s Law: Reasoning Strategy 1. Determine whether the Φ that penetrates the coil is increasing or decreasing. 2. Find what the direction of the induced B field must be so that it can oppose the change in Φ by adding or subtracting from the original field. 3. Use RHR-2 to determine the direction of the induced I Phys 1030 General Physics II (Gericke) 17

18 Lenz s Law (Section 22.5) The EMF produced by a moving magnet A bar magnet is approaching a loop of wire. The external circuit consists of a resistance. Find the direction of the induced current and the polarity of the induced EMF. Reverse the magnet s direction of motion. What happens then? Reverse the magnet s orientation. What happens then? Phys 1030 General Physics II (Gericke) 18

19 Lenz s Law (Section 22.5) C&J A long straight wire lies on a table and carries a current I. As the drawing shows, a small circular loop of wire is pushed across the table from position 1 to position 2. Determine the direction of the induced current, clockwise or counterclockwise, as the loop moves past (a) position 1 and (b) position Phys 1030 General Physics II (Gericke) 19

20 i>clicker Lenz s Law (Section 22.5) Find the direction of I in each loop. Use CW, CCW or 0 to indicate the direction of the current for loops 1 to 5 A. CW, CCW, CCW, CCW, CW B. 0, CW, CW, CCW, 0 C. 0, CW, 0, CCW, 0 D. 0, CCW, 0, CW, 0 E. 0, CCW, CW, CCW, Phys 1030 General Physics II (Gericke) 20

21 i>clicker Lenz s Law (Section 22.5) Two circular coils of wire lie on a flat surface. The centers of the coils coincide. In the larger coil there is a switch and a battery. The smaller coil contains no switch and no battery. Describe the induced current that appears in the smaller coil when the switch in the larger coil is closed. A. It flows CCW, but only for a short period just after the switch is closed. B. It flows CW forever after the switch is closed. C. It flows CW, but only for a short period just after the switch is closed. D. It flows CCW forever after the switch is closed Phys 1030 General Physics II (Gericke) 21