1 Capacitance and Dielectrics Chapter 27 Physics chapter 27

2 Capacitor Any two conductors separated by an insulator Used in electronics Usually have equal but opposite charges, so the net charge is zero If a capacitor has a charge Q, that means +Q on one conductor and -Q on the other Physics chapter 27

3 Capacitance Ratio of the magnitude of the charge on either conductor to the magnitude of the potential difference Units of farad (1 F = 1 C/V) Often use  F (10 -6 ) or pF ( ) Physics chapter 27

4 Circuit diagrams Capacitors represented as Physics chapter 27

5 Parallel plate capacitors Two parallel conducting plates separated by a distance that is small compared to their dimensions. Nearly all the electric field is between them, and it is uniform Physics chapter 27

6 Example The plates of a parallel-plate capacitor are 4.00 mm apart and 1.00 m 2 in area. A potential difference of V is applied across the capacitor. Find The capacitance The charge on the capacitor Physics chapter 27

7 Capacitors in series Charge is the same Voltage is not Equivalent capacitance Physics chapter 27

8 Example Three capacitors, C 1 = 10pF, C 2 = 25 pF, and C 3 = 5  F, are connected in series. Find the equivalent capacitance of the series. Physics chapter 27

9 Capacitors in parallel Voltage is the same Charge is not Equivalent capacitance Physics chapter 27

10 Example Exercise 27-7 Physics chapter 27

11 Potential energy of charged capacitor Physics chapter 27

12 Dielectrics Solid, nonconducting material between capacitor plates Helps maintain plate separation Increases capacitance Physics chapter 27

13 Dielectric constant, K C, V, and  are with the dielectric C 0, V 0, and  0 are without it K is always greater than 1 Physics chapter 27