Induction. Chapter 29. PowerPoint Lectures for University Physics, Twelfth Edition Hugh D. Young and Roger A. Freedman. Lectures by James Pazun

Transcription

1 Chapter 29 Electromagnetic Induction PowerPoint Lectures for University Physics, Twelfth Edition Hugh D. Young and Roger A. Freedman Lectures by James Pazun

2 29. Electromagnetic induction 1. Magnetic flux/faraday s law: A wire loop develops a current (called induced current) through it whenever the magnetic flux through h it changes. The corresponding emf that t drives the induced d current through h 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 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 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 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 material is cooled, motion of electrons and nuclei becomes less random. Electric/magnetic i shielding occur separately.

3 Goals for Chapter 29 To consider Faraday s Law To consider Lenz s Law To study motional emf To explore induced electric fields To summarize Maxwell s equations and see their application to displacement current

4 Introduction The motion of a magnet can induce current in practical ways. If a credit card has a magnet strip on its back, swiping the card can generate tiny currents that send information to cash registers. A coil of wire and magnets set into motion around each other will generate currents in the wire. A source of mechanical energy can drive the rotation and make a waterfall a into an electrical ect ca power station.

5 Induced current 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 motion relative to each other. See Figure 29.1 below.

6 Knowing the magnetic flux Regardless of what moves, knowing the magnetic flux around a conducting entity will allow dt determination ti of current tid induced. d See Figure 29.3 at right and Figure 29.4 below.

7 Emf and the current induced in a loop Follow Example Figure 29.5 illustrates the example.

8 Find the direction of an induced emf Consult Figure 29.6 and the text on the bottom of page 997 and top of page 998 and solve.

9 Faraday s Law Follow Problem-Solving Strategy Refer to Example Figure 29.7 illustrates t Example

10 The search coil, generator example I Follow Conceptual Example Consider Example 29.4 and Figures 29.8 and 29.9.

11 Generator example II The example considers a DC generator and back emf in a motor.

12 Generator example III Follow Example Figure illustrates Example 29.6.

13 Work and power in a slidewire generator Follow Example Figure illustrates Example 29.7.

14 Lenz s Law The direction of any magnetic induction effect is such as to oppose the cause of the effect. Follow Conceptual Example Follow Example 29.9; Figure illustrates the example.

15 A conducting rod moving in a uniform magnetic field The rod, velocity, and current are mutually perpendicular. See the direction of the induced current in the circuit below. Follow Example

16 The Faraday Disk dynamo Follow Example Figure illustrates Example

17 Induced electric fields I The windings of a long solenoid carrying a current I

18 Induced electric fields II Refer to Figure below. Follow Example

19 Eddy currents Consider Figure at right. Consider Figure below.

20 Displacement current and Maxwell s equations A varying electric field will give rise to a magnetic field. Explains electromagnetic waves.

21 Superconductivity and the Meissner effect When cooled below the critical temperature, superconductive materials lose all resistance to electrical current. Refer to Figures and at right.