Chapter 25. Electromagnetic Induction

Transcription

1 Lecture 28 Chapter 25 Electromagnetic Induction Electromagnetic Induction Voltage is induced (produced) when the magnetic field changes near a stationary conducting loop or the conductor moves through different regions of a stationary magnetic field. Voltage induced in loop requires changing magnetic field in loop by moving loop near a magnet, moving a magnet near a loop, or changing the current in a nearby loop.

2 Electric Generator Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction Generators & Alternating Current Generator Opposite of a motor Converts mechanical energy into electrical energy via coil motion Produces alternating voltage and current

3 Induction: No Free Lunch Takes work to turn the generator crank to produce electric current. The faster we turn the crank to produce more current, the more difficult it is to turn. More difficult to push magnet into coil with more loops because the magnetic field of each current loop resists the motion of the magnet. Faraday s Law The induced voltage in a coil is proportional to the product of the number of loops and rate at which the magnetic field changes within the loops. Small Voltage Medium Voltage Large Voltage

4 Faraday s Law CHECK YOURSELF The resistance you feel when pushing a magnet into a coil connected to a closed circuit involves A. repulsion by the magnetic field you produce from coil. B. energy transfer between the magnet and coil. C. Newton s third law. D. all of the above E. none of the above Faraday s Law CHECK YOURSELF The resistance you feel when pushing a magnet into a coil involves A. repulsion by the magnetic field you produce from coil. B. energy transfer between the magnet and coil. C. Newton s third law. D. all of the above E. none of the above

5 Faraday s Law Voltage induced in a wire requires changing magnetic field in the loop by moving the loop in region around a magnet, moving a magnet in a region near a loop, changing the current in a nearby loop. Faraday s Law CHECK YOURSELF More voltage is induced when a magnet is thrust into a coil A. more quickly. B. more slowly. C. Both A and B. D. Neither A nor B.

6 Faraday s Law CHECK YOURSELF More voltage is induced when a magnet is thrust into a coil A. more quickly. B. more slowly. C. Both A and B. D. Neither A nor B. Self-Induction & Lenz s Law When a current is induced by a changing magnetic field, that current itself produces its own magnetic field. This effect is called self-induction. Lenz s Law: A current produced by an induced voltage moves in a direction so that the magnetic field it produces tends to oppose the original change in field. Primary Magnetic Field Self-Induced Magnetic Field Current

7 Problem Solving with Lenz s Law 1. Determine whether the magnetic flux (amount of magnetic field lines) in the loop is increasing, decreasing, or unchanged. 2. The magnetic field due to any induced current in the loop points in the opposite direction to the original field if the flux is increasing; in the same direction if it is decreasing; and is zero if the flux is not changing. 3. Use the right-hand rule to determine the direction of the current. 4. Remember that the external field and the field due to the induced current are different. Demo: Lenz s Law Induced current produces a secondary magnetic field that is always opposed to the primary magnetic field that induced it, an effect called Lenz s law. Metal ring is levitated by self-induced secondary magnetic field Connect to alternating current (AC) Electro- Magnet Oscillating Magnetic Field

8 Check Yourself Does the ring levitate if we connect the electromagnet to direct current (e.g., to a battery)? Does the ring levitate if it has a gap? Electro- Magnet Magnetic Brakes Strong magnet dropped into a copper pipe falls slowly due to secondary magnetic field induced by its motion. Great America s Drop Zone has a 22 story freefall, lasting four seconds, decelerated by magnetic braking.

9 Eddy Currents Changing magnetic field induces eddy currents within any conductor. These internal currents produce self-induced magnetic fields, which by Lenz s law are in opposition of the primary magnetic field. Eddy Currents Primary Magnetic Field Self-Induced Magnetic Field Metal Detectors Can detect the presence of metals by using a transmitter coil to create an oscillating primary magnetic field. This creates a secondary magnetic field due to eddy currents in the metal. Can detect this secondary magnetic field by using a receiver coil.

10 Key Points of Lecture 28 Faraday s Law Induced Current Generators Lenz s Law Eddy Currents and Magnetic Braking Back Voltage Transformers and Power Transmission Field Induction and Electromagnetic Waves (Light) Before Wednesday, read Hewitt Chap. 25. Homework #20 due by 11:00 PM Friday Nov. 5.