CHAPTER 18
Discussion Questions
18. Envision a parallel plate capacitor across the terminals of a battery. When a dielectric is inserted between the plates, the capacitance increases, as does the charge, and more energy is stored by the device. Where is that additional energy stored?
20. The two identical capacitors in Fig. Q20 are charged at different voltages such that Q(1) > Q(2). What will be the charge on each capacitor after the two switches are closed? Explain your answer completely.
Problems
25. A 100 pico-Farad capacitor is charged by putting it across a 1.5 Volt battery. What is the charge on its plates?
28. Estimate the capacitance of the Earth. Its radius is 6,371 km. GIve the answer to 2 significant digits.
32. What is the capacitance of 2 parallel metal plates each with an area of 100 square centimeters separated by 1 mm of air?
40. What is the equivalent capacitance of the circuit between the terminals A and B, indicated in Fig. P40?
CHAPTER 20
Discussion Questions
5. The circuit of Fig. Q5 contains a gas-filled lamp--the sort of flashing light often used at construction sites. At low voltages, the lamp has nearly infinite resistance, but at a certain breakdown voltage, which is less than the terminal voltage of the battery, it becomes a very good conductor. A blast of charge through the lamp causes it to flash brightly. What happens once the switch is closed?
7. R-C circuits are often used to change the shape of a signal. The arrangement of Fig. Q7 shows a network being fed a square wave signal comprising a series of rectangular dc voltage pulses rising to +V and falling to 0 V. Part (b) shows the corresponding voltage across the capacitor. Explain its shape. What can you say about the value of RC as compared to the width of each pulse? Draw a curve of the output signal as seen across the resistor and explain its features. Write a general expression relating V, V(B), and V(C).
Problems
27. An electronic flash fires a blast of energy from an 800 micro-Farad capacitor into a Xenon lamp. It recharges through a series resistor of R = 5 kilo-ohms. If the switch is closed, how long will it take for the charge on the capacitor to recharge to 63 % of its maximum charge?
28. A charged 600 micro-Farad capacitor is in series with an open switch and a 4 kilo-ohm resistor and 1 12 Volt battery (with negligible internal resistance). The switch is closed at t = 0 sec and a current I immediately appears. Determine I. How long will it take for the current in the circuit to drop to 37 % of its original value?
59. The switch in the circuit P59 is closed and a steady-state is established. What is the charge on the capacitor?
65. With only switch S(1) closed in Fig. P65, (a) what is the steady-state reading of the volmeter? (b) What is the charge on the 3 micro-Farad capacitor?