Agenda for Today. Elements of Physics II. Lenz Law. Emf opposes change in flux Faraday s Law Induced EMF in a conducting loop

Transcription

1 Lenz Law Physics 132: Lecture e 22 Elements of Physics II Agenda for Today Emf opposes change in flux Faraday s Law Induced EMF in a conducting loop Physics 201: Lecture 1, Pg 1

2 Lenz s Law Physics 201: Lecture 1, Pg 2

3 Lenz s Law Why? Does lenz's law apply to the moment of the change in flux or will the induced B field be the "permanent" field after the change. Physics 201: Lecture 1, Pg 3

4 Lenz s Law Reasoning Strategy 1. Find the direction of the magnetic flux that penetrates the coil. 2. Determine whether the magnetic flux that penetrates the coil is increasing or decreasing. Increasing: induced b-field opposes magnetic flux Decreasing: induced b-field aligned with magnetic flux 3. Use RHR-2 to determine the direction of the induced current. Physics 201: Lecture 1, Pg 4

5 Clicker Question 1: The magnetic field points out of the page and is increasing. (a) The induced current will flow counterclockwise. (b) The induced current will flow clockwise. (c) There will be no induced current. Physics 201: Lecture 1, Pg 5

6 Clicker Question 1: Correct: The loop creates an induced magnetic field opposite of the actual magnetic field to keep the magnetic flux constant. Since the induced field points into the page, the current will flow clockwise because of the right hand rule. The change in flux is increasing out of the page, so by the righthand rule a clock-wise current is needed to induce a magnetic field pointing into the page. When the magnetic field is increasing the induced field of the induced current is pointing in the opposite direction of the magnetic field. I used the curly right hand rule : thumb points in direction of induced magnetic field and fingers curl in direction of induced current. Science Incorrect: The external magnetic field points out, the induced field must always counteract this field and point in. Physics 201: Lecture 1, Pg 6

7 Clicker Question 2: The magnetic field points out of the page and is decreasing. (a) The induced current will flow counterclockwise. (b) The induced current will flow clockwise. (c) There will be no induced current. Physics 201: Lecture 1, Pg 7

8 Clicker Question 3: The two loops of wire in the figure are stacked one above the other. Does the upper loop have a clockwise current (from above), a counterclockwise current, or no current at the following times? Before the switch is closed. (a) The current will flow counterclockwise. (b) The current will flow clockwise. (c) There will be no current. Physics 201: Lecture 1, Pg 8

9 Clicker Question 4: The two loops of wire in the figure are stacked one above the other. Does the upper loop have a clockwise current (from above), a counterclockwise current, or no current at the following times? Immediately after the switch is closed. (a) The current will flow counterclockwise. (b) The current will flow clockwise. (c) There will be no current. Physics 201: Lecture 1, Pg 9

10 Clicker Question 5: The two loops of wire in the figure are stacked one above the other. Does the upper loop have a clockwise current (from above), a counterclockwise current, or no current at the following times? Long after the switch is closed. (a) The current will flow counterclockwise. (b) The current will flow clockwise. (c) There will be no current. Physics 201: Lecture 1, Pg 10

11 Clicker Question 6: The two loops of wire in the figure are stacked one above the other. Does the upper loop have a clockwise current (from above), a counterclockwise current, or no current at the following times? Immediately after the switch is reopened. (a) The current will flow counterclockwise. (b) The current will flow clockwise. (c) There will be no current. Physics 201: Lecture 1, Pg 11

12 Example: The bar magnet is pushed toward the center of a wire loop. Which is true? A. There is a clockwise induced current in the loop. B. There is a counterclockwise induced current in the loop. C. There is no induced current in the loop. Physics 201: Lecture 1, Pg 12

13 Clicker Question 7: A circular cu conducting loop is being moved upward (toward a current-carrying wire) at a constant speed. What will be the direction of the induced current? (a) No current will be induced (no flux change). (b) Current will be induced clockwise. (c) Current will be induced counter-clockwise. clockwise Physics 201: Lecture 1, Pg 13

14 Clicker Question 8: If a coil is rotated as shown, in a magnetic field pointing to the left, in what direction is the induced current? a) clockwise b) counterclockwise c) no induced current Physics 201: Lecture 1, Pg 14

15 Faraday s Law An emf is induced in a conducting loop if the magnetic flux through the loop changes. The magnitude of the emf is: The direction of the emf is such as to drive an induced current in the direction given by Lenz s law. Physics 201: Lecture 1, Pg 15

16 Using Faraday s Law If we slide a conducting wire along a U-shaped conducting rail, we can complete a circuit and drive an electric current. We can find the induced emf and current by using Faraday s law and Ohm s law: Physics 201: Lecture 1, Pg 16

17 Example: A 4 cm 3 cm rectangular a loop is made of a wire with resistance of 3.5 Ohm. The loop is placed in a region of uniform magnetic field, B = 5 T. The direction of B is perpendicular to the plane of the loop and points into the page as shown. The magnetic field starts to increase at a uniform rate of 0.1 T/sec. What is the magnitude of the induced current in the loop? (a) I = 0 A (b) I = A (c) I = A (d) I = 0.02 A (e) I =48A 4.8 Physics 201: Lecture 1, Pg 17

18 Example: A 4 cm 3 cm rectangular loop is made of a wire with resistance of f35oh 3.5 Ohm. The loop is placed din a region of uniform magnetic field, B = 5 T. The direction of B is perpendicular to the plane of the loop and points into the page as shown. The magnetic field starts t to increase at a uniform rate of 0.1 T/sec. What is the magnitude of the induced current in the loop? Physics 201: Lecture 1, Pg 18

19 Clicker Question 9: A 4 cm 3 cm rectangular a loop is made of a wire with resistance of 25 Ohm/m. The loop is placed in a region of uniform magnetic field, B = 5 T. The direction of B is perpendicular to the plane of the loop and points into the page as shown. The magnetic field starts to increase at a uniform rate of 0.1 T/sec. What is the direction of the induced current? (a) counterclockwise (b) clockwise (c) There is no induced current. Physics 201: Lecture 1, Pg 19

20 Example: A solenoid with cross sectional area A = m 2 is m long and has 2500 turns. The magnetic field at the center of the solenoid is T pointing into the page. A square wire loop 1.5 cm on each side is fixed inside the solenoid as shown. The current in the solenoid is uniformly decreased to zero over 10 seconds. What is the magnitude of the induced emf in the square loop? (a) V (b) V (c) V (d) V (e) V Direction? CW Physics 201: Lecture 1, Pg 20

21 Example: A solenoid with cross sectional area A = m 2 is m long and has 2500 turns. The magnetic field at the center of the solenoid is T pointing into the page. A square wire loop 1.5 cm on each side is fixed inside the solenoid as shown. The current tin the solenoid idis uniformly decreased dto zero over 10 seconds. What is the magnitude of the induced emf in the square loop? Physics 201: Lecture 1, Pg 21

22 Applications of Faraday s Law Electric Guitar A vibrating string induces an emf in a coil A permanent magnet inside the coil magnetizes a portion of the string nearest the coil As the string vibrates at some frequency, its magnetized segment produces a changing flux through the pickup coil The changing flux produces an induced emf that is fed to an amplifier Physics 201: Lecture 1, Pg 22

23 Applications of Faraday s Law Microphone Physics 201: Lecture 1, Pg 23

24 Applications of Faraday s Law Apnea Monitor The coil of wire attached to the chest carries an alternating current An induced emf produced by the varying field passes through a pick up coil When breathing stops, the pattern of induced voltages stabilizes and external monitors sound an alert Physics 201: Lecture 1, Pg 24