Download Exercise 2B_ Capacitor(2)

CK Cheung
Exercise 2_ Capacitor(1)_0506
89'
connected in series to a d.c. voltage
supply as shown. At time t = 0, the
key K is closed.
Which of the
following statements is/are true?
26. An isolated spherical conductor of radius r
is charged to a potential V.
The total
electrical energy stored is
A.
V²/(80 r).
B.
V²/(40 r).
C.
V²/(20 r).
D.
V²  (20 r).
V²  (40 r).
E.
(1) At any time t the sum of the
potential differences across C
and R is a constant.
(2) The capacitor is almost fully
charged after a time t = 5 CR.
(3) The final charge on the capacitor
does not depend on the position
of R in the circuit.
89'
29.
1

1
A
In
the
A.
B.
C.
D.
E.
B
2
above
circuit,
the
(1), (2) and (3)
(1) and (2) only
(2) and (3) only
(1) only
(3) only
85'
equivalent
resistance between A and B is
43.
C1
C2
A.
2/7 .
B.
2/5 .
C.
2/3 .
A charged capacitor C1 is connected to an
D.
2 .
uncharged capacitor C2 as shown.
E.
5 .
If
E1 = the total enrgy stored in C1 and C2
83'
46.
before connection, and
E2 = the total energy stored in C1 and C2
R
C
after connected,
K
d.c.
which of the follwoing is/are correct?
(1)
A capacitor and a resistor are
E1 = E2.
(2) After connection, the p.d. across C1 is
1
CK Cheung
equal to the p.d. across C2.
In the circuit above, the equivalent
resistance across XY is
(3) After connection, the charges stored in
C1 and C2 are in the ratio
capacitance of C1
capacitance of C2
A. 0 .
.
B. 10 .
C. 20 .
D. 40 .
E. 64 .
A.
(1), (2) and (3)
B.
(1) and (2) only
C.
(2) and (3) only
D.
(1) only
87'
E.
(3) only
28.
A
3 F
86'
32.
4 F
C
16 V
1 F
2
B
2
2 F
D
2mH
A voltage of 1200 V is applied across AB in
2 F
the capacitor network shown above.
2
The
voltage across CD is found to be 450 V.
If
after some time, the voltage across CD
When a steady state is reached, the
current delivered by the battery is
suddenly jumps to 600 V, which capacitor(s)
A.
zero.
A.
1 F
B.
C.
D.
4.0 A.
5.0 A.
5.3 A.
B.
2 F
C.
3 F
D.
4 F
E.
8.0 A.
has been shorted?
E.
86'
34.
All 4 capacitors have been shorted.
Further, can you find the charge stored in
each capacitor in the above circuit?
8
8
8
90’
8
8
X
Y
8
8
30.
8
2
CK Cheung
A parallel-plate capacitor is connected to a
battery as shown.
The above figure shows a network of
resistors. If a voltage of 100Vis applied
across terminals A and B, the potential
difference between C and D is 80V. If the
voltage is applied across terminals C and
D instead. What is the potential difference
between A and B ?
What will happen if the
separation of the plates is increased?
capacitance
voltage
charge
A. decreases
decreases
decreases
B. decreases
unchanged
decreases
C. decreases
decreases
increases
A.
80V
D. increases
unchanged
decreases
B.
60V
E. increases
unchanged
increases
C.
40V
D.
20V
E.
90’
It cannot be found as the value
of R is not known.
33.
2000’
3
3
23.
3
I
6V
In the above circuit, the battery has
The figure shows some of the resistors in a
negligible internal resistance.
network of resistors. The magnitudes and
The current
I is
directions of some of the currents are
A.
2/3 A.
B.
1 A.
C.
4/3 A.
D.
2 A.
E.
6 A.
Also find the
equivalent
resistance
across the
battery (1Ω)
marked as shown. Find the magnitude and
direction of the current passing through the
resistor R.
2001’
21.
A.
0.2A from right to left
B.
0.2A from left to right
C.
0.4A from right to left
D.
0.4A from left to right
E.
It cannot be determined as the
value of R is not given.
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CK Cheung
81’IIB(4)
Revision on R-C circuit
81’ IIB
4. A student is asked to perform an experiment to demonstrate that the
charge on a capacitor is proportional to the p.d. across its terminals.
The circuit to be used is shown in Figure 2.
A
10 V
C
S
to oscilloscope y-input;
timebase off;
y-sensitivity set to
d.c. 2 V/cm
Figure 2
The principle of the experiment is to close switch S and charge the capacitor linearly
by continuously adjusting the variable resistor to keep the current in the circuit
constant at its initial value. The p.d. across the capacitor will therefore increase
linearly with time, and a graph of the deflection of the oscilloscope spot in the
y-direction against time will be a straight line.
(a) The student decides that it would be most convenient to start with the variable
resistor set to its maximum value of 100 k and to use an ammeter which he can
maintain at full scale deflection when the switch is closed. What should be the
full scale deflection of the meter?
(b) Having selected his meter, the student considers the deflection of the oscilloscope
spot. He decides that he will be able to time a deflection rate of 0.1 cm/s
satisfactorily. What value of C should he choose?
(c) What will be the value of the variable resistor 10 s after closing the switch S? (8 marks)
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