CAPACITOR-Conceptual and NCERT Level Questions BM Sharma Notes | EduRev

: CAPACITOR-Conceptual and NCERT Level Questions BM Sharma Notes | EduRev

 Page 1


 
 
1 
CAPACITOR 
2CA-01 
Conceptual and NCERT Level 
1. Suppose the two plates of a capacitor have 
different areas. When the capacitor is 
charged by connecting it to a battery, do the 
charges on the two plates have equal 
magnitude, or may they be different? Explain 
your reasoning.  
2. The charged plates of a capacitor attract each 
other, so to pull the plates farther apart 
requires work by some external force. What 
becomes of the energy added by this work ? 
Explain your reasoning.  
3. The two plates of a capacitor are given 
charges ±Q. The capacitor is then 
disconnected from the charging device so 
that the charges on the plates can’t change, 
and the capacitor is immersed in a tank of 
oil. Does the electric field between the plates 
increase, decrease or stay the same ? Explain 
your reasoning. How can this field be 
measured ? 
4. At which of the two 
points, 1 or 2, of a 
charged capacitor with 
nonparallel plates is the 
surface charge density 
greater ? 
5. In the figure shown, the 
plate A has 100µ charge, 
while the plate B has 60 
µC charge. 
(a) When both 
switches are open, then 
 q
1
 = _____________ 
 q
2
 = _____________ 
 q
3
 = _____________ 
 q
4
 = _____________ 
(b) When switch S
1
 is closed only then 
 q
1
 = ____________ q
2
 = _____________ 
 q
3
 = ____________ q
4
 = _____________ 
(c) When switch S
2
 is also closed, then 
 q
1
 = ____________ q
2
 = _____________ 
 q
3
 = ____________ q
4
 = _____________ 
6. A capacitor of capacitance C is charged to a 
potential difference V from a cell and then 
disconnected from it. A charge +Q is now 
given to its positive plate. Find the potential 
difference across the capacitor. 
7. A parallel-plate vacuum capacitor with plate 
area A and separation x has charges +Q and -
Q on its plates. The capacitor is disconnected 
from the source of charge, so the charge on 
each plate remains fixed.  
 (a) What is the total energy stored in the 
 capacitor ? 
 (b) The plates are pulled apart an additional 
 distance dx. What is the change in the 
 stored energy? 
 (c) If F is the force with which the plates 
 attract each other, then the change in 
 the stored energy must equal the  work 
 dW = F dx done in pulling the plates 
 apart. Find an expression for F.  
 (d) Explain why F is not equal to QE, 
 where E is the electric field between 
 the plates.  
8. (a)  How many excess electrons must be 
 added to one plate and removed from 
 the other to give a 5.00 nF parallel 
 plate capacitor 25.0 J of stored  energy?  
 (b)  How could you modify the geometry 
 of this capacitor to get it to store 50.0 
 J of energy without changing the  charge 
 on its plates? 
9.    A capacitor is connected across a battery. 
 (a) Why does each plate receive a charge 
 of exactly the same magnitude? 
 (b) Is this true even if the plates are of 
 different size? (Yes/No) 
+
2
1
?
s
1
q
1
q
2
A
s
2
q
3
q
4
B
Page 2


 
 
1 
CAPACITOR 
2CA-01 
Conceptual and NCERT Level 
1. Suppose the two plates of a capacitor have 
different areas. When the capacitor is 
charged by connecting it to a battery, do the 
charges on the two plates have equal 
magnitude, or may they be different? Explain 
your reasoning.  
2. The charged plates of a capacitor attract each 
other, so to pull the plates farther apart 
requires work by some external force. What 
becomes of the energy added by this work ? 
Explain your reasoning.  
3. The two plates of a capacitor are given 
charges ±Q. The capacitor is then 
disconnected from the charging device so 
that the charges on the plates can’t change, 
and the capacitor is immersed in a tank of 
oil. Does the electric field between the plates 
increase, decrease or stay the same ? Explain 
your reasoning. How can this field be 
measured ? 
4. At which of the two 
points, 1 or 2, of a 
charged capacitor with 
nonparallel plates is the 
surface charge density 
greater ? 
5. In the figure shown, the 
plate A has 100µ charge, 
while the plate B has 60 
µC charge. 
(a) When both 
switches are open, then 
 q
1
 = _____________ 
 q
2
 = _____________ 
 q
3
 = _____________ 
 q
4
 = _____________ 
(b) When switch S
1
 is closed only then 
 q
1
 = ____________ q
2
 = _____________ 
 q
3
 = ____________ q
4
 = _____________ 
(c) When switch S
2
 is also closed, then 
 q
1
 = ____________ q
2
 = _____________ 
 q
3
 = ____________ q
4
 = _____________ 
6. A capacitor of capacitance C is charged to a 
potential difference V from a cell and then 
disconnected from it. A charge +Q is now 
given to its positive plate. Find the potential 
difference across the capacitor. 
7. A parallel-plate vacuum capacitor with plate 
area A and separation x has charges +Q and -
Q on its plates. The capacitor is disconnected 
from the source of charge, so the charge on 
each plate remains fixed.  
 (a) What is the total energy stored in the 
 capacitor ? 
 (b) The plates are pulled apart an additional 
 distance dx. What is the change in the 
 stored energy? 
 (c) If F is the force with which the plates 
 attract each other, then the change in 
 the stored energy must equal the  work 
 dW = F dx done in pulling the plates 
 apart. Find an expression for F.  
 (d) Explain why F is not equal to QE, 
 where E is the electric field between 
 the plates.  
8. (a)  How many excess electrons must be 
 added to one plate and removed from 
 the other to give a 5.00 nF parallel 
 plate capacitor 25.0 J of stored  energy?  
 (b)  How could you modify the geometry 
 of this capacitor to get it to store 50.0 
 J of energy without changing the  charge 
 on its plates? 
9.    A capacitor is connected across a battery. 
 (a) Why does each plate receive a charge 
 of exactly the same magnitude? 
 (b) Is this true even if the plates are of 
 different size? (Yes/No) 
+
2
1
?
s
1
q
1
q
2
A
s
2
q
3
q
4
B
 
2 
2CA-01 
CAPACITOR 
Conceptual and NCERT Level 
10. The plates of a plane capacitor are drawn 
apart keeping them connected to a battery. 
Next the same plates are drawn apart from 
the same initial condition keeping battery 
disconnected. In which case is more work 
done ? 
11. If a small charge q is moved along a closed 
path in the field between the plates of a 
parallel - plate capacitor, will any work be 
done by the agent who moves the charge? 
(Yes / No) 
12. Which of the following changes to an ideal 
parallel - plate capacitor connected to an 
ideal battery will result in an increase in the 
charge on the capacitor ? 
 (a) Decreasing the potential difference 
 across the plates 
 (b) Decreasing the area of the plates 
 (c) Decreasing the separation of the 
 plates 
13. A parallel - plate capacitor is connected to an 
ideal battery, which provides a fixed 
potential difference. Originally the energy 
stored in the capacitor is U
0
. If the distance 
between the plates is doubled, then the new 
energy stored in the capacitor will be…….. 
14. A capacitor has 
square plates of side 
a making an angle q 
with each other as 
shown in figure.  
Show that the capacitance of this 
arrangement is given by
2
0
1,
2
a a
C
tt
? ?
??
??
??
??
  
where d is the minimum separation between 
the plates at one end. q is very small. 
15. Each plate of a parallel plate air capacitor 
has area S = 5 × 10
-3
 m
2
 and are d = 8.80 mm 
apart. Plate A has positive change q
1
 = +10
-10
 
C and plate B and change q
2
 = +2 × 10
-10
C.  
Calculate the energy supplied by a battery of 
emf E = 10V When its positive terminal is 
connected to plate A and negative terminal to 
plate B. (Given ?
0
 = 8.8 × 10
-12
) 
16. A circuit has a section AB shown in figure.  
5V
A B
C
1
=1 F ? C
2
=2 F ?
10V
  
The emf of the cell is 10 V and the capacitors 
have capacitances C
1
 = 1µF and C
2
 = 2µF. 
The potential difference V
AB
 = 5V. Find the 
charges on the capacitors. 
17. Four identical 
capacitor plates, 
each of area A, 
are arranged 
such that 
adjacent plates 
are a distance d apart. The plates are 
connected to a source of emf V as shown in 
the figure. Find the charge on plate 1 and 
that on Plate 4. 
18. N identical capacitors are connected in 
parallel and then a potential difference of V 
is applied to them. The potential difference 
when these capacitors are reconnected in 
series, their charges being left undisturbed ? 
19. In the arrangement 
shown, plate A & C are 
connected together by a 
conducting wire & plate 
B is given a charge 
equal to 60 µC. The ratio d
1
 / d
2
 = 2. 
 q
1
 = __________________  
 q
2
 = __________________  
 q
3
 = __________________  
q
4
 = __________________  
q
5
 = __________________  
q
6
 = __________________ 
?
a
a
t
+
V 1 2 3 4 5
A B C
q
1
q
2
q
3
q
4
q
5
d
1
d
2
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