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Chapter 23

Chapter 23. Magnetic Flux and Faraday’s Law of Induction (Lecture II). Outline. Lenz’s law Several examples Motional EMF A qualitative discussion A quantitative discussion Mechanical work/electrical energy An example Real world applications Electric generators and motors. Lenz’s Law.

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Chapter 23

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  1. Chapter 23 Magnetic Flux and Faraday’s Law of Induction (Lecture II) PHY 1161

  2. Outline • Lenz’s law • Several examples • Motional EMF • A qualitative discussion • A quantitative discussion • Mechanical work/electrical energy • An example • Real world applications • Electric generators and motors PHY 1161

  3. Lenz’s Law • Lenz’s Law: An induced current always flows in a direction that opposes the change that caused it. • Example 1: consider a bar magnet that is moved toward or away from a conducting ring. PHY 1161

  4. More Examples of Lenz’s Law • Example 2 (Conceptual checkpoint 23-2): As the magnets drop toward the rings, does the magnet on the left have an acceleration that is (a) more than, (b) less than, or (c) the same as that of the magnet on the right? • Example 3: Suppose that a magnetic field is decreased with time. What is the direction of the induced current in the ring? PHY 1161

  5. More Examples of Lenz’s Law (Cont.) • Example 4 (conceptual checkpoint 23-3): Consider a system in which a metal ring is falling out of a region with a magnetic field and into a field-free region. According to Lenz’s law, is the induced current in the ring (a) clockwise or (b) counterclockwise? PHY 1161

  6. Motional EMF: Qualitative • Motional emf is created in this system as the rod falls. The result is an induced current, which causes the light to shine. • What is the direction of the induced current when the rod is released from rest and allowed to fall? • Connections between mechanical work and electrical energy. PHY 1161

  7. Motional EMF: Quantitative • Example: Consider a rod that slides horizontally without friction. A constant magnetic field points out of the page, and the rod is pushed by an external agent so that it moves to the right with a constant speed v. • Find the following: • The increase in magnetic flux, . • The magnitude of the induced emf, ||. • The electric filed caused by the motion of the rod, E. PHY 1161

  8. Mechanical Work/Electrical Energy • Find the following: • The magnetic force acting on the rod, F. • The mechanical power delivered by the external force Pmechanical. • The electrical power converted to light and heat in the light bulb Pelectrical. • With the aid of a magnetic field, we can convert mechanical power directly to electrical power. PHY 1161

  9. Example 23-3 Light Power • The light bulb in the circuit shown has a resistance of 12  and consumes 5.0 W of power; the rod is 1.25 m long and moves to the left with a constant speed of 3.1 m/s. • (a) What is the strength of the magnetic field? • (b) What external force is required to maintain the rod’s constant speed? PHY 1161

  10. Electric Generators and Motors (a) An Electric Generator (b) An Electric Motor PHY 1161

  11. Homework #9 • Chapter 23, P. 830-831, Problems: #29, 32, 38 (Physics, Walker, 4th edition). PHY 1161

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