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Chapter 15

Chapter 15. Magnetic Circuits and Transformers. Forces and Laws of Physics. Lorentz Force The force on a moving charge in a magnetic field is proportional to the charge, velocity, and magnetic field strength. Faraday’s Law Currents can be induced in wires by a changing magnetic fields.

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Chapter 15

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  1. Chapter 15 Magnetic Circuits and Transformers

  2. Forces and Laws of Physics Lorentz Force • The force on a moving charge in a magnetic field is proportional to the charge, velocity, and magnetic field strength. Faraday’s Law • Currents can be induced in wires by a changing magnetic fields. Lenz’s Law • The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them Ampere’s Law • The line integral of the magnetic field intensity around a closed path is equal to the algebraic sum of the currents flowing through the area enclosed by the path.

  3. Magnetic Compass • Desktop Demo • Try to balance a disk magnet on its side in your desk. • Try to suspend a magnet from a thread.

  4. Lorentz Force Motor • Construct the simple motor shown in the figure. • Put the magnet on the head of your nail. • Touch the sharp end of the nail to the negative side of the battery. • Hold the wire to the positive side of the battery. • Lightly touch the wire to the edge of the magnet. • What are things that you can do to change the direction of the motor? • How can you increase the speed of the motor? • Note: The magnets are about 50 cents each and are available at www.rare-earth-magnets.com 1 Short Wire 1 D Cell Battery 1 Nail or Screw 1 Disk Magnet

  5. Chapter 15 Magnetic Circuits and Transformers

  6. Lorentz Force • the force on a move charge in a magnetic field • right hand rule • force on electrons • force on current carrying wires • Desktop Demo • What are two ways that you can change the direction of the Lorentz force on a wire?

  7. Lorentz Force Motor • Construct the motor below.

  8. How do you create a Magnetic Field? Answer: • move electrical charges

  9. Draw the magnetic field lines for the following: • A Current Carrying Wire • A Current Carrying Loop • An atom • A Permanent Magnet • The Earth • A Coil of Wire • a.k.a. an Electromagnet, Solenoid, or Inductor

  10. Chapter 15 Magnetic Circuits and Transformers

  11. Faraday’s Law Examples “Currents can be induced in wires by a changing magnetic fields.” • Horseshoe magnet, rotating coil, crankshaft • Magnet, coil, oscilloscope • Faraday’s Flashlight

  12. How do power plants create AC power? Faraday’s Law “Currents can be induced in wires by a changing magnetic fields.” 1. Move a magnet near a coil of wire 2. Move a coil near a magnet.

  13. How do power plants move the magnet or coil? They are rotated on a shaft by… a water wheel in a hydroelectric dam a large diesel engine or a gas turbine But in most cases, the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor.

  14. How does a transformer work? • An AC current in the primary coil creates a changing magnetic field in the iron core. • This changing magnetic field induces a current in the secondary coil as described by Faraday’s Law.

  15. Transformers • 3D Drawings (Cylinder and Block) • Circuit Symbol • Transformer Makers Equations

  16. What is a step-up transformer? What is a step-down transformer?

  17. Center-tap Transformer

  18. How does a jumping ring work? Lenz’s Law “The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..” Demo - Refrigerator Draw Picture

  19. How does and inductor impede current? Lenz’s Law “The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..”

  20. How do you calculate inductance? L = Inductance in Henries N = Number of Turns = Length of the Core A = Area of the Core m = Permeability of the Core =mr mo • mo = 4p × 10-7 H/m Applications: Stoplight Car Detectors and Metal Detectors

  21. Ampere’s Law Examples Find the magnetic field strength for each of the following: • Example 15.1 - A Long Straight Wire • Example 15.2 - A Toroidal Coil • Solenoid

  22. Did we do these Demos? • Break a Transformer • Cool the Jumping Ring • Open a DC Power Supply • Investigate a Tesla Coil • Dr. T’s Faraday’s Law Demo • 2 springs, 2 magnets, 2 coils • Moving a wire in a magnetic field

  23. Chapter 16 DC Machines

  24. Just the basics… • Electric Motor • Stator • Stationary part • Rotor • Rotation part connect to a shaft • What role does a “brush” play in a motor? • Answer: Changes the direction of the current with the help of a “commutator”.

  25. The Fundamentals of Engineering Examination • Becoming a Professional Engineer (PE) • PE Examination for PE License • One goal of this course is to supply you with enough knowledge to pass the PE Exam. • See Appendix C for an example. • Some of these questions will appear on the final exam in a modified form.

  26. Take Home Demo • Zip Lock Bags • Write Names on Bags • Include Printed Instructions • Disk Magnet • D Cell Battery • Screw • Paperclips • Rubber Bands • 6” Wire with Plastic Insulation • 30cm Wire with Varnish

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