Ch20-2s10

# Ch20-2s10 - Chapter 20(Lecture 2 Induced Voltages and...

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Chapter 20 (Lecture 2) Induced Voltages and Inductance

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Lenz’s Law revisited square6 The direction of the induced emf is given by Lenz’s Law, which predates Faraday’s Law and is contained in it. It is best to use Faraday’s Law to find only the magnitude of an induced emf, using Lenz’s Law to find its direction. square6 Lenz’s Law square6 The current caused by the induced emf travels in the direction that creates a magnetic field with flux opposing the change in the original flux through the circuit
Lenz’s Law revisited, cont. square6 Use Lenz’s Law in three steps. square6 Step 1: Find the direction of the magnetic field and whether the magnitude of the magnetic flux is increasing or decreasing with time. square6 Step 2: Guess the direction of the induced emf. (You have a 50% chance to be right.) square6 Step 3: Using Right Hand Rule #2, find the direction of the magnetic field due to the current driven by such an emf. square6 If that magnetic field opposes the change found in Step 1, then your guess is correct. If not, the direction of the induced emf must be opposite of your guess.

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Lenz’s Law revisited Example 1 (from book) square6 A bar magnet is moved to the right toward a stationary loop of wire (a) square6 Step 1: The magnetic field in the loop points to the right. As the magnet moves, the magnetic flux increases with time. square6 Step 2: Guess the induced current is as shown by the purple arrow. square6 Step 3: Such a current produces a magnetic field that points to the left inside the loop. square6 This magnetic field opposes the change noted in Step 1. So the current direction guessed in Step 2 is correct. square6 Reversing the direction of the movement will reverse the direction of the induced current. (Find it out in three steps.)
Lenz’ Law revisited – Example 2 (demo & quiz) square6 A bar magnet is dropped, with its north pole facing down, into a vertical copper pipe. Describe how the direction of the induced emf in the pipe changes with time at a fixed position P . square6 Step 1: The magnetic field of the bar magnet points down; the strength of the magnetic flux in a horizontal loop that goes through P first increases as the magnet approaches and, after the magnet has passed, decreases. square6 Step 2: Let’s guess that the direction of the induced emf is first clockwise, as viewed from above, and then counterclockwise. square6 Step 3: The direction of the magnetic field produced by a current driven by such emf is first downward and then upward. These directions enhance rather then oppose the changes found in Step 1. square6 The correct answer should be, therefore, first counterclockwise then clockwise.

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Lenz’s Law revisited – Example 3 square6 In the figure, the sliding bar is pulled to the right with a given velocity by an applied force (for instance, by you). Find the direction of the induced emf.
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