Assignment 5 Due: 11:59pm on Thursday, June 23, 2011 Note: To understand how points are awarded, read your instructor's Grading Policy . [Switch to Standard Assignment View ] Charged Capacitor and Resistor Learning Goal: To study the behavior of a circuit containing a resistor and a charged capacitor when the capacitor begins to discharge. A capacitor with capacitance is initially charged with charge . At time , a switch is thrown to close the circuit connecting the capacitor in series with a resistor of resistance . Part A What happens to the charge on the capacitor immediately after the switch is thrown? ANSWER: The electrons on the negative plate of the capacitor are held inside the capacitor by the positive charge on the other plate. Only the surface charge is held in the capacitor; the charge inside the metal plates flows through the resistor. The electrons on the negative plate immediately pass through the resistor and neutralize the charge on the positive plate. The electrons on the negative plate eventually pass through the resistor and neutralize the charge on the positive plate. Correct Part B What is the current that flows through the resistor immediately after the switch is thrown? Hint B.1 How to approach the problem [ Print ]
Hint not displayed Hint B.2 Find the voltage at Hint not displayed Express your answer in terms of any or all of the quantities , , and . ANSWER: = Correct Note that since current is charge per time, the preceeding formula shows that the units of must be time. The combination of variables is called the time constant . It will occur frequently in problems involving a resistor and a capacitor. Charging a Capacitor in an R-C Circuit Learning Goal: To understand the dynamics of a series R-C circuit. Consider a series circuit containing a resistor of resistance and a capacitor of capacitance connected to a source of EMF with negligible internal resistance. The wires are also assumed to have zero resistance. Initially, the switch is open and the capacitor discharged. Let us try to understand the processes that take place after the switch is closed. The charge of the capacitor, the current in the circuit, and, correspondingly, the voltages across the resistor and the capacitor will be changing. Note that at any moment in time during the life of our circuit, Kirchhoff's loop rule holds and, indeed, it is helpful: , where is the voltage across the resistor and is the voltage across the capacitor. Part A Immediately after the switch is closed, what is the voltage across the capacitor? ANSWER: zero Correct
Part B Immediately after the switch is closed, what is the voltage across the resistor? ANSWER: zero Correct Part C Immediately after the switch is closed, what is the direction of the current in the circuit? ANSWER:
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