CBSE 2024 · Region 5 · Set 2 · Q30 · 4 marks

Dielectrics play an important role in design of capacitors. The molecules of a dielectric may be polar or non-polar. When a dielectric slab is placed in an external electric field, opposite charges appear on the two surfaces of the slab perpendicular to electric field. Due to this an electric field is established inside the dielectric. The capacitance of a capacitor is determined by the dielectric constant of the material that fills the space between the plates. Consequently, the energy storage capacity of a capacitor is also affected. Like resistors, capacitors can also be arranged in series and/or parallel.

(i)

Which of the following is a polar molecule?

(A)

$\displaystyle \mathrm{O}_{2}$

(B)

$\displaystyle \mathrm{H}_{2}$

(C)

$\displaystyle \mathrm{N}_{2}$

(D)

HCl

(ii)

Which of the following statements about dielectrics is correct?

(A)

A polar dielectric has a net dipole moment in absence of an external electric field which gets modified due to the induced dipoles.

(B)

The net dipole moments of induced dipoles is along the direction of the applied electric field.

(C)

Dielectrics contain free charges.

(D)

The electric field produced due to induced surface charges inside a dielectric is along the external electric field.

(iii)

When a dielectric slab is inserted between the plates of an isolated charged capacitor, the energy stored in it :

(A)

increases and the electric field inside it also increases.

(B)

decreases and the electric field also decreases.

(C)

decreases and the electric field increases.

(D)

increases and the electric field decreases.

(iv)

(a)OR(b)

An air-filled capacitor with plate area A and plate separation d has capacitance $\displaystyle \mathrm{C}_{0}$. A slab of dielectric constant $\displaystyle \mathrm{K}$, area $\displaystyle A$ and thickness $\displaystyle \left(\frac{\mathrm{d}}{5}\right)$ is inserted between the plates. The capacitance of the capacitor will become

(A)

$\displaystyle \left[\frac{4 \mathrm{~K}}{5 \mathrm{~K}+1}\right] \mathrm{C}_{0}$

(B)

$\displaystyle \left[\frac{\mathrm{K}+5}{4}\right] \mathrm{C}_{0}$

(C)

$\displaystyle \left[\frac{5 \mathrm{~K}}{4 \mathrm{~K}+1}\right] \mathrm{C}_{0}$

(D)

$\displaystyle \left[\frac{\mathrm{K}+4}{5 \mathrm{~K}}\right] \mathrm{C}_{0}$

CBSE Class 12 Physics past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.