Chapter 24 Capacitance and Dielectrics PowerPoint Lectures for University Physics, Twelfth Edition Hugh D. Young and Roger A. Freedman Lectures by James Pazun
Main Points 1. Equipotential ti regions (lines, surfaces, volumes)/electrostatic l t ti shielding(polarization l i field completely l cancels external field). Examples = conductors since E=0. 2. A Capacitor is any (geometric) setup or device used to store dipolar charge. The regions that stores ve charge is separated/insulated from the one that stores +ve charge. 3. The capacitance C (unit=f=farad=coulomb/volt) of a capacitor is the amount of (dipolar) charge that can accumulate before a maximum potential difference of 1V can be created in the capacitor. C depends on the geometry (charge distribution) and separation/insulation in the capacitor but not on the amount of charge. It is the charge storage efficiency of the capacitor. If the maximum potential difference between the two regions that store the ve and +ve charges is V, then Q =C V. 4. Effective capacitance for capacitors in series [voltages add up] and in parallel [charges add up]. 5. Potential Energy [density] stored in a Capacitor/Electric Field [work done to charge the capacitor]. 6. Partial Polarization effect, of a dielectric (insulator), on capacitance (dielectric constant K:=C/C_0 >1, permittivity ε=k ε_0). Dielectric reduces potential difference V (and E-field) and so increases C. 7. Gauss s law in dielectrics (Free/Bound charges. Bound [polarization] charges reduce enclosed charge). 8. Examples: Plates, spheres, cylinders,
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
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!
Keep charges apart and you get capacitance Any two charges insulated from each other form a capacitance. Refer to Figure 24.1 241below.
How do we build a capacitor? What s it good for?
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.
Some examples of flat, cylindrical, and spherical capacitors See just how large a 1 F capacitor would be. Refer to Example 24.1. 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.
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 24.8. Multiple connections designed to operate simultaneously is termed capacitors in parallel. This is illustrated in Figure 24.9.
Calculations regarding capacitance Refer to Problem-Solving Strategy 24.1. Follow Example 24.5. Follow Example 24.6. The problem is illustrated t by Figure 24.10 below.
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!
Dielectrics change the potential difference The potential between to parallel plates of a capacitor changes when the material bt between the plates lt changes. It does not matter if the plates are rolled into a tube as they are in Figure 24.13 or if they are flat as shown in Figure 24.14.
Table 24.1 Dielectric constants
Field lines as dielectrics change Moving from part (a) to part (b) of Figure 24.15 shows the change induced by the dielectric.
Examples to consider, capacitors with and without dielectrics Refer to Problem-Solving Strategy 24.2. Follow Example 24.10 to compare values with and without a dielectric. Follow Example 24.11 to compare energy storage with and without a dielectric. Figure 24.16 illustrates the example.
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.
Molecular models Figure 24.18 (at right) and Figure 24.19 (below) show the effect of an applied field on individual molecules.
Polarization and electric field lines
Gauss s Law in dielectrics Refer to Figures 24.22 and 24.23 to illustrate Gauss ss Lawin dielectrics. Follow Example 24.12.