Ch 21 1 Chapter 21 Electromagnetic Induction Faraday’s Law AC Circuits © 2006, B.J. Lieb Some figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Giancoli, PHYSICS,6/E © 2004.

Ch 21 2 Induction Discovered in 1820 by Michael Faraday and Joseph Henry Magnetic field causes a current, but only when the magnetic flux is changing.

Ch 21 3 Magnetic Flux Necessary to explain induction In the figure below A is the area of a surface-usually the area inside of a coil of wire. In a properly drawn magnetic field, flux is proportional to the total number of lines passing through a point.

Ch 21 4 Faraday’s Law of induction the flux  B can change because B is changing A is changing or moving A is rotating 

Ch 21 5 Lenz’s Law An induced emf always gives rise to a current whose magnetic field opposes the original change in flux. In the figure below: (a)Original state is no flux, so current flows in clockwise direction which would give a downward magnetic flux (b)Original state is upward flux so current flows counterclockwise to maintain that state. (c)No flux change, so no current.

Ch 21 6 Example In the figures below, a circular loop of wire and a straight wire carrying a current lie on the surface of a table. The straight wire is shown as an arrow in the current direction. Determine the direction of the current induced in the loop of wire due to the changing current in the straight wire for each figure. I increasing I decreasing I constantI increasing

Ch 21 7 Example 21-2 The loop shown is a 10-turn coil of wire of radius 12 cm and is in a magnetic field of strength 0.15 T. Calculate the average EMF induced if the coil rotates 90 0 about an axis perpendicular to the field in 0.20 seconds.

Ch 21 8 Example 21-1

Ch 21 9 Electric Generators A coil of wire rotating in a magnetic field experiences an sinusoidal EMF:  This is the basis of electric generator.

Ch Transformers Changing current in the primary creates a changing flux in the secondary coil If the flux change is equal In an ideal transformer: Power Out = Power In

Ch Example 21-3

Ch Power Grid Power is transmitted at the highest possible voltage in order to minimize losses.