Q.

A container has a base of 50 cm×5 cm and height 50 cm, as shown in the figure. It has two parallel electrically conducting walls each of area 50 cm×50 cm. The remaining walls of the container are thin and non-conducting. The container is being filled with a liquid of dielectric constant 3 at a uniform rate of 250 cm3s-1.     

What is the value of the capacitance of the container after 10 seconds

[Given: Permittivity of free space ε0=9×10-12 C2N-1m-2, the effects of the non-conducting walls on the capacitance are negligible]         [2023]

1 27 pF  
2 63 pF  
3 81 pF  
4 135 pF  

Ans.

(2)

In t=10 s, volume of liquid

V=250×10=2500 cm3

  h=VA=250050×5=10 cm

Capacitance of the capacitor when filled with liquid of dielectric constant K=3

Cd=Adε0Kd=50×10-2×10×10-2×ε0×35×10-2=3ε0

Capacitance of the air-filled capacitor

Ca=Aaε0d=50×10-2×40×10-2×ε05×10-2=4ε0

Equivalent capacitance, C=Ca+Cd=7ε0

=7×9×10-12=63 pF