Lecture 17-18 Ch 30

Lecture 17-18 Ch 30 - PH 222-3A Spring 2007 Induction and...

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PH 222-3A Spring 2007 duction and inductance Induction and inductance Lecture 17-18 Chapter 30 (Halliday/Resnick/Walker, Fundamentals of Physics 8 th edition) 1
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Chapter 30 Induction and Inductance In this chapter we will study the following topics: -Faraday’s law of induction -Lenz’s rule -Electric field induced by a changing magnetic field -Inductance and mutual inductance - RL circuits -Energy stored in a magnetic field 2
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In a series of experiments, Michael Faraday in England d Joseph Henry in the U S were able t generate Faraday's Experiments and Joseph Henry in the U.S. were able to generate electric currents without the use of batteries. Below we describe some of these experiments that helped formulate what is known as "Faraday's law of induction." The circuit shown in the figure consists of a wire loop connected to a sensitive ammeter (known as a "galvanometer"). If we approach the loop with a permanent agnet we see a current being registered y the galvanometer. The results can be magnet we see a current being registered by the galvanometer. The results can be summarized as follows: A current appears only if there is relative motion between the magnet and the loop. 1. Faster motion results in a larger current. If 2. 3. we reverse the direction of motion or the polarity of the magnet, the current reverses sign and flows in the opposite direction. The current generated is known as " "; the emf that appear induced current s is known as " "; the whole effect is called " " induced emf induction. 3
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In the figure we show a second type of experiment in which current is induced in loop 2 when the itchSinloop1iseitherclosedoro ened When loop 1 switch S in loop 1 is either closed or opened. When the current in loop 1 is constant no induced current is observed in loop 2. The conclusion is that the loop 2 magnetic field in an induction experiment can be generated either by a permanent magnet or by an ectric current in a coil. electric current in a coil. Faraday summarized the results of his experiments in what is known as "" Faraday's law of induction. An emf is induced in a loop when the number of magnetic field lines that pass through the loop is changing. Faraday's law is not an explanation of induction but merely a description of what induction is. It is one of the four " of electromagnetism," Maxwell's equations all of which are statements of experimental results. We have already encountered Gauss' law for the electric field, and Ampere's law (in its incomplete form). 4
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B G ˆ n The magnetic flux through a surface that borders loop is determined as follows: Magnetic Flux Φ B B B dA Φ= G G φ dA a loop is determined as follows: 1 We divide the surface that has the loop as its border into elements of area . dA . For each element we calculate the magnetic flux through it: cos .
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Lecture 17-18 Ch 30 - PH 222-3A Spring 2007 Induction and...

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