Midterm Exam 2 for PHY225 Sec152L, Spring 2018 Borough of Manhattan Community Co

Question

Midterm Exam 2 for PHY225 Sec152L, Spring 2018 Borough of Manhattan Community College, Science Department Professor Chiaki Yanagisawa Solve 5 out of 7 problems. No extra credit for solving 7 problems will be given The first 5 problems will be graded. 1. Initially for the circuit in Figure the switch S is open and the capacitor is uncharged. The switch S is closed at time t-0. (20 points) a) The moment when the switch is closed, how 70 vt.w T 20 JuF much is the current through the resistor? (4 points) b) The moment when the switch is closed, how c) When the switch has been closed for a long time, how much is the current through the d) When the switch has been closed for a long time, how much is the charge on the e) when the charge on the capacitor C is 700 ?C, calculate the current I in the circuit 0.40 MO much charge is on the capacitor? (2 points) resistor? (2 points) capacitor? (4 points) numerically. Show your work by using symbols R for the resistance, ? for the emf, C for the capacitance, and I for the current. And then find the numerical value for the current I (8 points).

Explanation / Answer

1. at t = 0 then Vc = 0

I = (70 V) / (0.40 x 10^6 ohm)

I = 1.75 x 10^-4 A  

(B) Q = C Vc

at t = 0 , Vc = 0

then Q = 0

(C) after long time , I = 0

(D) Ic= 0 then Vc = 70 Volt

Q = (20 x 10^-6)(70) = 1.4 x 10^-3 C

(E) Q = C V

700 uC = (20 uF)(Vc)

Vc = 35 Volt

e -Vc - I R = 0

I = (e - Vc) / R

I = (70 - 35) / (0.40 x 10^6)

I = 8.75 x 10^-5 A

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