Electromagnetic Induction - Jinkserjinkser.com/physics22info/review/ph22ch29.pdf · 2014-05-19 ·...

Chapter 29p

ElectromagneticElectromagnetic Induction

PowerPoint® Lectures forUniversity Physics, Twelfth Edition

– Hugh D. Young and Roger A. Freedman

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

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Lectures by James Pazun

29. Electromagnetic induction1. Magnetic flux/Faraday’s law: A wire loop develops a current (called induced current) through it whenever the

ti fl th h it h Th di f th t d i th i d d t th h th l i ll dmagnetic flux through it changes. The corresponding emf that drives the induced current through the loop is called induced emf.

2. Lenz’s Law: The sense of the induced current is such that the flux of the induced magnetic field (magnetic field of the induced current) opposes the magnetic flux change of the magnetic field that produced it.

3 Electric generators work by electromagnetic induction3. Electric generators work by electromagnetic induction.

4. Example: Motional emf . For a piece of conductor moving towards the right and perpendicular to its length, the B-flux on the left increases while that on the left decreases. Therefore, by Lenz’s rule, the induced B-field will oppose the field on the right and aid the field on the right.

5 Induced electric fields: The electric field associated with the induced emf by the work energy relation is called induced5. Induced electric fields: The electric field associated with the induced emf by the work energy relation is called induced electric field. It follows that locally (at any spatial point), a changing magnetic field is always accompanied by an induced electric field in that locality and vice versa.

6. Displacement current/Maxwell’s equations/Lorentz force [Stoke’s theorem useful]: Ampere’s law will not apply to a circuit loop containing a capacitor, and similar situations, unless the right hand side of the Law is modified to take account of the unwired space between the capacitor plates. This follows from Stokes’ theorem which expresses a closed line integral in terms of a surface integral. Just as we associated the energy stored in the capacitor with the energy stored in the electric field, we can also associate the charge deposited on the capacitor plates with the electric flux between the capacitor plates. This rate of change of electric flux is called displacement current. In the region between the capacitor plates, a magnetic field exist and can be seen as the field produced by the displacement current confirming that the displacement current behaves just like an actual current Maxwell’s equations are a list ofcurrent, confirming that the displacement current behaves just like an actual current. Maxwell s equations are a list of the most fundamental (general) equations of electricity and magnetism.

7. Superconductors/Meissner effect: A superconductor is a conductor in a zero electrical resistance phase. Superconductors are to [external] magnetic fields as conductors are to [external] electric fields. That is, a superconductor has perfect diamagnetism just as the conductor has perfect electrical polarization. As the conductor

i l i l d i f l d l i b l d El i / i hi ldi l


material is cooled, motion of electrons and nuclei becomes less random. Electric/magnetic shielding occur separately.

Goals for Chapter 29

• To consider Faraday’s Law• To consider Faraday s Law

• To consider Lenz’s Law

• To study motional emf

• To explore induced electric fields

• To summarize Maxwell’s equationsand see their application to ppdisplacement current


Introduction

• The motion of a magnet can induce• The motion of a magnet can induce current in practical ways. If a credit card has a magnet strip on its back, g p ,“swiping” the card can generate tiny currents that send information t h i tto cash registers.

• A coil of wire and magnets set into motion around each other will generate currents in the wire. A so rce of mechanical energ cansource of mechanical energy can drive the rotation and make a waterfall into an electrical power


wate a to a e ect ca powestation.

Induced current

• Joseph Henry worked in the United States and Michael• Joseph Henry worked in the United States and Michael Faraday worked in England to discern the details of current generated in wire and permanent magnets in motionrelative to each other.

• See Figure 29.1 below.g


Knowing the magnetic flux• Regardless of what moves knowingRegardless of what moves, knowing

the magnetic flux around a conducting entity will allow d t i ti f t i d ddetermination of current induced.

• See Figure 29.3 at right and Figure 29 4 b l29.4 below.


Emf and the current induced in a loop • Follow Example 29.1.Follow Example 29.1.

• Figure 29.5 illustrates the example.


Find the direction of an induced emf

• Consult Figure 29 6 and the text on the bottom of page 997 andConsult Figure 29.6 and the text on the bottom of page 997 and top of page 998 and solve.


Faraday’s Law• Follow Problem-Solving Strategy 29.1.Follow Problem Solving Strategy 29.1.

• Refer to Example 29.2.

Fi 29 7 ill t t E l 29 2• Figure 29.7 illustrates Example 29.2.


The search coil, generator example I

• Follow Conceptual Example 29 3• Follow Conceptual Example 29.3.

• Consider Example 29.4 and Figures 29.8 and 29.9.


Generator example II

• The example considers a DC generator and back emf in a motorThe example considers a DC generator and back emf in a motor.


Generator example III

• Follow Example 29 6• Follow Example 29.6.

• Figure 29.11 illustrates Example 29.6.


Work and power in a slidewire generator

• Follow Example 29 7Follow Example 29.7.



Lenz’s Law

• The direction of any magnetic induction effect is such as toThe direction of any magnetic induction effect is such as to oppose the cause of the effect.

• Follow Conceptual Example 29.8.p p

• Follow Example 29.9; Figure 29.14 illustrates the example.


A conducting rod moving in a uniform magnetic field • The rod, velocity, and current are mutually perpendicular.The rod, velocity, and current are mutually perpendicular.

• See the direction of the induced current in the circuit below.


• Follow Example 29.10.

The Faraday Disk dynamo

• Follow Example 29 11Follow Example 29.11.



Induced electric fields I

• The windings of a long solenoid carrying a current IThe windings of a long solenoid carrying a current I


Induced electric fields II • Refer to Figure 29.18 below.Refer to Figure 29.18 below.

• Follow Example 29.12.


Eddy currents

• Consider Figure 29 19• Consider Figure 29.19 at right.

• Consider Figure 29.20Consider Figure 29.20 below.


Displacement current and Maxwell’s equations

• A varying electric field will give rise to a magnetic field• A varying electric field will give rise to a magnetic field.

• Explains electromagnetic waves.


Superconductivity and the Meissner effect

• When cooled below• When cooled below the critical temperature, superconductivesuperconductive materials lose all resistance to electrical currentelectrical current.

• Refer to Figures 29.24 and 29.25 at29.24 and 29.25 at right.


Electromagnetic Induction - Jinkserjinkser.com/physics22info/review/ph22ch29.pdf · 2014-05-19 ·...

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