Capacitance(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

CAPACITANCE MCQs

Total Questions : 26 | Page 2 of 3 pages

Question 11. A capacitor 1mF withstands a maximum voltage of 6KV, while another capacitor 2mF withstands a maximum voltage of 4KV. If the capacitors are connected in series, the system will withstand a maximum voltage of?

2 KV
4 KV
6 KV
9 KV

Discuss Question

Answer: Option D. -> 9 KV
:
D

q_{1} = C_{1} V_{1} = 10^{- 3} \times 6 \times 10^{3} = 6 C

q_{1} = C_{2} V_{2} = 2 \times 10^{- 3} \times 4 \times 10^{3} = 8 C

Q = q_{1} = 6 C Q = C V

6 = (\frac{C_{1} C_{2}}{C_{1} + C_{2}}) V

6 = \frac{2}{3} \times 10^{- 3} V

V = 9 \times 10^{3} V

= 9 K V

Question 12. The radii of two charged metal spheres are 5 cm and 10 cm both having same charge 75 mC. If they are connected by a wire, the quantity of charge transferred through the wire is

25 mC
50 mC
75 mC
Zero

Discuss Question

Answer: Option A. -> 25 mC
:
A

V = \frac{q_{1} + q_{2}}{C_{1} + C_{2}} = \frac{q_{1} + q_{2}}{4 π ϵ_{0} [r_{1} + r_{2}]} = \frac{150 \times 10^{- 3} \times 9 \times 10^{9}}{15 \times 10^{- 2}}

V = 9 \times 10^{9} V

q_{1}^{1} = C_{1} V = 4 π ϵ_{0} r_{1} \times 9 \times 10^{9} = \frac{1}{9 \times 10^{9}} \times 5 \times 10^{- 2} \times 9 \times 10^{9} = 5 \times 10^{- 2} C

q_{1}^{1} = 50 m c

q_{1} - q_{1}^{1} = 75 - 50 = 25 m C

Question 13. A certain number of identical condensers are first connected in series and then in parallel. If the equivalent capacities in the two cases are found to be in the ratio 1: 9, what is the number of condensers?

Discuss Question

Answer: Option B. -> 3
:
B

\frac{C_{s} e r i e s}{C_{p} a r a l l e l} = \frac{(\frac{C}{n})}{n c} = \frac{1}{n^{2}}

\frac{1}{9} = \frac{1}{n^{2}}

n = 3

Question 14. If two conducting spheres are separately charged and then brought in contact, then

The total energy of the two spheres is conserved
The total charge on the two spheres is conserved
Both the total energy and charge are conserved
The final potential is always the mean of the original potentials of the two spheres

Discuss Question

Answer: Option B. -> The total charge on the two spheres is conserved
:
B
The total charge on the spheres will be conserved

Question 15. A fully charged capacitor has a capacitance C. It is discharged through a small coil of resistance wire embedded in a thermally insulated block of specific heat capacity s and mass m. If the temperature of the block is raised by

Δ

T, the potential difference V across the capacitance was?

√2mCΔTs
mCΔTs
msCΔTC
√2msΔTC

Discuss Question

Answer: Option D. -> √2msΔTC
:
D

\frac{1}{2} C V^{2} = m S Δ T

V^{2} = \frac{2 m S Δ T}{C}

V = \sqrt{\frac{2 m s Δ T}{C}}

Question 16. In a charged capacitor, the energy resides as

Positive charges
Both the positive and negative charges
The field between the plates
Around the edge of the capacitor plates

Discuss Question

Answer: Option C. -> The field between the plates
:
C
In a charged capacitor energy is stored in the form electric field

Question 17. The insulated metal spheres of radii R₁ and R₂ having charges Q₁ and Q₂ respectively are connected to each other. There is

No change in the energy of the system
An increase in the energy of the system
Always a decrease in the energy of the system
A decrease in the energy of the system unless Q1R2=Q2R1

Discuss Question

Answer: Option D. -> A decrease in the energy of the system unless Q1R2=Q2R1
:
D
WhenPotentials of spheres are unequal, current will flow in connecting wire so that energy decreases in the form of heat through the connecting wire.

Question 18. A parallel plate capacitor is charged upto 100V. A 2mm thick insulator sheet is inserted between the plates. Then to maintain the same potential difference, the distance between the capacitor plates is increased by 1.6mm. The dielectric constant of the insulator is:

Discuss Question

Answer: Option C. -> 5
:
C

x = 1 = [1 - \frac{1}{k}]

1.6 = 2 [1 - \frac{1}{k}]

\frac{1}{k} = 1 - 0.8

= 0.2

k = \frac{1}{0.2} = 5

Question 19. A battery of 10 V is connected to a condenser of capacity 100 mF. The battery is removed and the condenser is connected to an identical uncharged condenser. It is found that the charge distributes equally on these two condensers. The common potential of the system is?

10V
5V
20V
2.5V

Discuss Question

Answer: Option B. -> 5V
:
B
Common potential

V = \frac{C_{1} V_{1} + C_{2} V_{2}}{C_{1} + C_{2}} V = \frac{(100 \times 10 + 0)}{2 C_{1}} 10^{- 3} = \frac{1}{2 \times 100 \times 10^{- 3}} = \frac{1000}{200} = 5 V

Question 20. A conducting sphere of radius 10 cm is charged to

10 μ C

. Another uncharged sphere of radius 20 cm is allowed to touch it for some time. After that if the spheres are separated then surface density of charges, on the spheres will be in the ratio of: