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Dielectrics and Capacitance Questions and Answers

81.

To obtain a high value of capacitance, the permittivity of the dielectric medium should be :

Exam Year(s)

RSMSSB JE - 2025, RSMSSB - 2025,

Answer: B

The permittivity should be high.

For a parallel-plate capacitor,

\[ C = \frac{\varepsilon_{0}\varepsilon_{r}A}{d} \]

so the capacitance is directly proportional to the permittivity of the dielectric. For a given plate area and spacing, a dielectric with a high relative permittivity, such as a ceramic, gives a much larger capacitance than air.

A low or medium permittivity, or the permittivity of air (\(\varepsilon_{r} \approx 1\)), gives a smaller capacitance, so none of those options achieves a high value.

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82.

Ohm’s law when applied to electromagnetic phenomenon gives the following expression :

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The field form of Ohm's law is \(\overline{J} = \sigma\overline{E}\).

In electromagnetic theory, Ohm's law is written at each point of a conductor: the current density is proportional to the electric field, with the conductivity \(\sigma\) as the constant of proportionality. Integrating it over a uniform wire of length L and cross-section A gives the circuit form \(V = IR\), with \(R = L/\sigma A\).

V = IR is the circuit law, not the expression for electromagnetic fields the question asks for. \(\sigma = \overline{J}\overline{E}\) multiplies where it should divide, and \(\overline{E} = \overline{J}\sigma\) puts the conductivity where the resistivity \(1/\sigma\) belongs.

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