Phys102
Term: 131
Online HW-Ch27-Lec03
Q1:
In the circuit shown, both resistors have the same value R. Suppose switch S is
initially closed. When it is then opened, the circuit has a time constant 𝜏𝑎 .
Conversely, suppose S is initially open. When it is then closed, the circuit has a time
constant 𝜏𝑏 . The ratio 𝜏𝑎 / 𝜏𝑏 is:
A.
B.
C.
D.
E.
1
2
0.5
0.667
1.5
Ans:
B
Q2:
A certain capacitor, in series with a resistor, is being charged. At the end of 10ms its
charge is half the final value. The time constant for the process is about: (Give the
answer in three significant figures and SI units)
Ans:
τ = RC
q = q 0 e−t/RC
q1 = q 0 e−t1/RC
t1
RC
1 t1
t1
⇒R= −
⇒ RC = −
Ln (x)
C Ln(x)
(x)q 0 = q 0 e−t1 /RC ⇒ Ln(x) = −
1
10 × 10−3
R = − −9 ×
= 14.4 × 106 Ω
1
10
Ln(2)
Q3:
A 1.0 μF capacitor with an initial stored energy of 0.50 J is discharged through 1.0 MΩ
resistor. Find the charge on the capacitor at t = 0.40 s. (Give the answer in three
significant figures and SI units)
Ans:
Q = Q0 e−t/RC
1 Q0 2
U0 =
⇒ Q0 = �2CU0
2 C
Q = �2CU0 e−t/RC = 6.7 × 10−4 C
KFUPM-Physics Department
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