Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley PowerPoint ® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman Lectures by James Pazun Chapter 24 Capacitance and Dielectrics

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Goals for Chapter 24 To consider capacitors and capacitance To study the use of capacitors in series and capacitors in parallel To determine the energy in a capacitor To examine dielectrics and see how different dielectrics lead to differences in capacitance

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Introduction When flash devices made the “big switch” from bulbs and flashcubes to early designs of electronic flash devices, you could use a camera and actually hear a high-pitched whine as the “flash charged up” for your next photo opportunity. The person in the picture on page 815 must have done something worthy of a picture. Just think of all those electrons moving on camera flash capacitors!

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Keep charges apart and you get capacitance Any two charges insulated from each other form a capacitance. Refer to Figure 24.1 below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley How do we build a capacitor? What’s it good for?

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The unit of capacitance, the farad, is very large Commercial capacitors for home electronics are often cylindrical, from the size of a grain of rice to that of a large cigar. Capacitors like those mentioned above and pictured at right are microfarad capacitors.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Some examples of flat, cylindrical, and spherical capacitors See just how large a 1 F capacitor would be. Refer to Example Refer to Example 24.2 to calculate properties of a parallel-plate capacitor. Follow Example 24.3 and Figure 24.5 to consider a spherical capacitor. Follow Example 24.3 and Figure 24.5 to consider a cylindrical capacitor.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Capacitors may be connected one or many at a time Connection “one at a time” in linear fashion is termed “capacitors in series.” This is illustrated in Figure Multiple connections designed to operate simultaneously is termed “capacitors in parallel.” This is illustrated in Figure 24.9.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Calculations regarding capacitance Refer to Problem-Solving Strategy Follow Example Follow Example The problem is illustrated by Figure below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The Z Machine—capacitors storing large amounts of energy This large array of capacitors in parallel can store huge amounts of energy. When directed at a target, the discharge of such a device can generate temperatures on the order of 10 9 K!

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Dielectrics change the potential difference The potential between to parallel plates of a capacitor changes when the material between the plates changes. It does not matter if the plates are rolled into a tube as they are in Figure or if they are flat as shown in Figure

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Table 24.1—Dielectric constants

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Field lines as dielectrics change Moving from part (a) to part (b) of Figure shows the change induced by the dielectric.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Examples to consider, capacitors with and without dielectrics Refer to Problem-Solving Strategy Follow Example to compare values with and without a dielectric. Follow Example to compare energy storage with and without a dielectric. Figure illustrates the example.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Dielectric breakdown A very strong electrical field can exceed the strength of the dielectric to contain it. Table 24.2 at the bottom of the page lists some limits.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Molecular models Figure (at right) and Figure (below) show the effect of an applied field on individual molecules.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Polarization and electric field lines

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Gauss’s Law in dielectrics Refer to Figures and to illustrate Gauss’s Law in dielectrics. Follow Example