Concept Question: Capacitors

Transcription

1 oncept Question: apacitors Three identical capacitors are connected to a battery. The battery is then disconnected. A How do the charge on A, B & compare before and after the battery B is removed? BEFOE; AFTE 1. Q A = Q B ; No hange 2. Q A = Q B ; Q A > Q B 3. Q A = Q B ; Q A < Q B 4. Q A > Q B ; No hange 5. Q A > Q B ; Q A = Q B 6. Q A < Q B ; No hange 7. Q A < Q B ; Q A = Q B 1

2 oncept Question Answer: apacitors Answer: 4. Q A > Q B ; No hange A B Initially: Potential across A is sum of drops across B &. By symmetry V B =V V A > V B =V Q = V Q A > Q B When battery is disconnected There is no reason for the potential to change or charge to flow so it doesn t. 2

3 oncept Question: ircuit An uncharged capacitor is connected to a battery, resistor and switch. The switch is initially open but at t = 0 it is closed. A very long time after the switch is closed, the current in the circuit is 1. Nearly zero 2. At a maximum and decreasing 3. Nearly constant but non-zero 4. I don t know 3

4 oncept Question Answer: ircuit Answer: 1. After a long time the current is 0 Eventually the capacitor gets full the voltage increase provided by the battery is equal to the voltage drop across the capacitor. The voltage drop across the resistor at this point is 0 no current is flowing. 4

5 oncept Question: ircuit onsider the circuit at right, with an initially uncharged capacitor and two identical resistors. At the instant the switch is closed: 1. I = I = 0 2. I = /2; I = 0 3. I = 0; I = / 4. I = /2; I = / 5. I don t know I I 5

6 oncept Question Answer: ircuit Answer: 3. I = 0; I = / Initially there is no charge on the capacitor and hence no voltage drop across it it looks like a short. Thus all current will flow through it rather than through the bottom resistor. o the circuit looks like: I 6

7 oncept Question: ircuit Now, after the switch has been closed for a very long time, it is opened. What happens to the current through the lower resistor? I I 1. It stays the same 2. ame magnitude, flips direction 3. It is cut in half, same direction 4. It is cut in half, flips direction 5. It doubles, same direction 6. It doubles, flips direction 7. None of the above 7

8 oncept Question Answer: ircuit Answer: 1. It stays the same The capacitor has been charged to a potential of /2, so when it is responsible for pushing current through the lower resistor it pushes a current of /2, in the same direction as before (its positive terminal is also on the left) 8

9 oncept Question: urrent Thru apacitor In the circuit at right the switch is closed at t = 0. At t = (long after) the current through the capacitor will be: I I I = 0 I = I = 2 4. I don t know 9

10 oncept Question Answer: I Thru apacitor Answer: 1. I = 0 After a long time the capacitor becomes fully charged. No more current flows into it. I I 10

11 oncept Question: urrent Thru esistor In the circuit at right the switch is closed at t = 0. At t = (long after) the current through the lower resistor will be: I = 0 I = I = 2 4. I don t know I I 11

12 oncept Question Answer: I Thru esistor Answer: 3. I = 2 ince the capacitor is full we can remove it from the circuit, and all that is left is the battery and two resistors. o the current is /2. I I 12

13 oncept Question: Opening witch in ircuit Now, after the switch has been closed for a very long time, it is opened. What happens to the current through the lower resistor? I I 1. It stays the same 2. ame magnitude, flips direction 3. It is cut in half, same direction 4. It is cut in half, flips direction 5. It doubles, same direction 6. It doubles, flips direction 7. None of the above. 13

14 oncept Question Answer: Opening witch Answer: 1. It stays the same The capacitor has been charged to a potential of /2, so when it is responsible for pushing current through the lower resistor it pushes a current of /2, in the same direction as before (its positive terminal is also on the left) 14

15 oncept Question: Voltage/urrent in tarting from a point in time where the voltage across the battery (V B ) & across the capacitor (V ) as well as the current (I) are all zero, what happens when the battery is turned on? 1. I jumps up then decays as V rises 2. V jumps up then decays as I rises 3. I & V both jump up then decay 4. I & V both gradually rise 5. I don t know 15

16 oncept Question Answer: V/I in Answer: 1. I jumps up, then decays as V rises I is proportional to the voltage across the resistor (V = I ). V + V = V B. o as V increases V must decay, meaning that I must decay. But why does V take time to rise up, while V, V B can jump up quickly? V B V V =I 16

17 MIT OpenourseWare Physics II: Electricity and Magnetism Fall 2010 For information about citing these materials or our Terms of Use, visit: