Phys102
Term: 131
Online HW-Ch25-Lec02
Q1:
A battery is used to charge a parallel combination of two identical capacitors. If the
potential difference across the battery terminals is V and total charge Q flows through
the battery during the charging process then the charge on the positive plate of each
capacitor and the potential difference across each capacitor are:
A.
B.
C.
D.
E.
Q/2 and V/2, respectively
Q and V, respectively
Q/2 and V, respectively
Q and V/2, respectively
Q and 2V, respectively
Ans:
C
Q2:
Three capacitors are arranged as shown in the figure. C 1 has a capacitance of 5.0 pF,
C 2 has a capacitance of 10 pF, and C 3 has a capacitance of 15 pF. Find the potential
difference between points A and B if the potential difference across C 2 is 30 V (Give
the answer in three significant figures and SI units)
Ans:
V2 = V3 = 30 V (Parallel)
C23 = C2 + C3 = 10 + 15 = 25 pF (Parallel)
Q23 = C23 × V2 = 25 × 10−12 × 30 = 7.5 × 10−10 C
Q1 = Q23 = 7.5 × 10−10 C (Series)
⇒ V1 =
Q1
7.5 × 10−10
=
= 150 V
C1
5.0 × 10−12
⇒ VAB = V1 + V23 = 150 + 30 = 180 V
Q3:
A 100 V battery is connected across a combination of n capacitors connected in
series. The capacitance of each capacitor is 5.00 μF. If the total stored energy is 50
μJ, what is n? (Give the answer in three significant figures and SI units)
Ans:
1
1
1
1
n
C1
= +
+ …
=
⇒ Ceq =
Ceq C1 C2
Cn
C1
n
Utotal =
1
1 C1 2 1 (5 × 10−6 )(100)2
Ceq V 2 =
V =
= 50 × 10−6 ⇒ n = 500
2
2 n
2
n
KFUPM-Physics Department
1