1. Lecture 10: Electromagnetic Induction (Ch. 22) contd. 04-17-07

2. Faraday’s Law of Electromagnetic Induction M. Faraday, 1791-1867 J. Henry, 1797-1878 Whenever there is a change in flux through a loop of wire, an emf is induced in the loop. Change in flux with respect to time. The magnitude of the induced emf is equal to the time rate of change of the magnetic flux. (we saw this before) What about N loops and same flux passes through each loop? Utilizes falling water to turn a turbine, which generates electricity.

3. Faraday’s Law of Electromagnetic Induction The average emf E induced in a coil of N loops is where ΔΦ is the change in magnetic flux through one loop and Δt is the time interval during which the change occurs. The term ΔΦ/Δt is the average time rate of change of the flux that passes through one loop. SI Unit of Induced EMF: Volt (V) Examples 5 and 6 Ground fault interrupter Induction Stove

4. Lenz’s Law Heinrich Lenz (1804-1865) Keep in mind that the net magnetic field penetrating a coil of wire results from two contributions. One is the original magnetic field that produces the changing flux that leads to the induced emf. The other arises because of the induced current, which, like any current, creates its own magnetic field. The field created by the induced current is called the induced magnetic field. What about the Polarity of the induced EMF? The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change. Determining the Polarity of the Induced Emf 1. Determine whether the magnetic flux that penetrates a coil is increasing or decreasing. 2. Find what the direction of the induced magnetic field must be so that it can oppose the change in flux by adding to or subtracting from the original field. 3. Having found the direction of the induced magnetic field, use RHR-2 (see Section 21.7) to determine the direction of the induced current. Then the polarity of the induced emf can be assigned because conventional current is directed out of the positive terminal, through the external circuit, and into the negative terminal.

5. Lenz’s Law the induced field is not always opposite to the external field, because Lenz’s law requires only that it must oppose the change in the flux that generates the emf.

6. Applications of Electromagnetic Induction

7. The Electric Generator

8. The Electric Generator & Counter Torque

9. Mutual Inductance M: Mutual Inductance unit for the mutual inductance M is V·s/A, which is called a henry (H) in honor of Joseph Henry: 1 V·s/A = 1 H. In most situations, values of M are less than 1 H and are often on the order of millihenries (1 mH = 1 × 10 −3 H) or microhenries

10. Self-Inductance L: self-inductance

11. Transformers