List of top Questions asked in CBSE CLASS XII

An infinitely long straight wire having a charge density \( \lambda \) is kept along the y-axis in the x-y plane. The Coulomb force on a point charge \( q \) at a point \( P(x, 0) \) will be:

An electric dipole of dipole moment \( \vec{p} \) is kept in a uniform electric field \( \vec{E} \). The amount of work done to rotate it from the position of stable equilibrium to that of unstable equilibrium will be:

What is meant by displacement current? How is this current different from the conduction current? A capacitor is being charged by a source of emf. Justify the continuity of current in the circuit.

A thin spherical conducting shell of radius \( R \) has a charge \( q \). A point charge \( Q \) is placed at the center of the shell. Find:
(i) The charge density on the outer surface of the shell.
(ii) The potential at a distance \( R/2 \) from the center of the shell.

Two wires A and B of different metals have their lengths in the ratio 1:2 and their radii in the ratio 2:1 respectively. The I-V graphs for them are shown in the figure. Find the ratio of their:
(i) Resistances \( \frac{R_A}{R_B} \)
(ii) Resistivities \( \frac{\rho_A}{\rho_B} \)

In normal adjustment, for a refracting telescope, the distance between the objective and the eyepiece lens is 1.00 m. If the magnifying power of the telescope is 19, find the focal length of the objective and the eyepiece lens.

An AC source \( V = 282 \sin(100t) \) volt is connected across a \( 1 \mu F \) capacitor. The RMS value of current in the circuit will be (Take \( \sqrt{2} = 1.41 \)):

A particle of mass \( m \) and charge \( q \) is moving with velocity \( \mathbf{v} = v_x \hat{i} + v_y \hat{j} \). If it is subjected to a magnetic field \( \mathbf{B} = B_0 \hat{i} \), it will move in a:

The Coulomb force (\( F \)) versus \( \frac{1}{r^2} \) graphs for two pairs of point charges (\( q_1 \) and \( q_2 \)) and (\( q_2 \) and \( q_3 \)) are shown in the figure. The charge \( q_2 \) is positive and has the least magnitude. Then: 

An electron is revolving around a proton in an orbit of radius r with a speed v. Obtain an expression for the magnetic moment associated with the electron.

With the help of a diagram, briefly explain the construction and working of an AC generator.

Mention the factors on which the resonant frequency of a series LCR circuit depends. Plot a graph showing variation of impedance of a series LCR circuit with the frequency of the applied AC source.

Using Gauss's law, show that the electric field \( E \) at a point due to a uniformly charged infinite plane sheet is given by \( E = \frac{\sigma}{2\epsilon_0} \), where symbols have their usual meanings.

Differentiate between a wavefront and a ray.

Draw a ray diagram for the formation of the image of an object by a convex mirror. Hence, obtain the mirror equation.

A beam of light coming parallel to the principal axis of a convex lens \( L_1 \) of focal length 16 cm is incident on it. Another convex lens \( L_2 \) of focal length 12 cm is placed coaxially at a distance 40 cm from \( L_1 \). The nature and distance of the final image from \( L_2 \) will be:

A thin convex lens \( L \) of focal length 10 cm and a concave mirror \( M \) of focal length 15 cm are placed coaxially 40 cm apart as shown in the figure. A beam of light coming parallel to the principal axis is incident on the lens. The final image will be formed at a distance of: 

The focal length of an equiconvex lens:

A thin converging lens of focal length 20 cm and a thin diverging lens of focal length 15 cm are placed coaxially in contact. The power of the combination is:

An AC voltage \( V = 0.5 \sin (100\pi t) \) volt is applied, in turn, across a half-wave rectifier and a full-wave rectifier. The frequency of the output voltage across them respectively will be:

Initially, during the formation of a p-n junction:

During the formation of a p-n junction:

At a given temperature, the number of intrinsic charge carriers in a semiconductor is \( 2.0 \times 10^{10} \) cm\(^{-3}\). It is doped with pentavalent impurity atoms. As a result, the number of holes in it becomes \( 8 \times 10^3 \) cm\(^{-3}\). The number of electrons in the semiconductor is:

When Ge is doped with a pentavalent impurity, the energy required to free the weakly bound electron from the dopant is about:

What are ferromagnetic materials? Explain ferromagnetism with the help of suitable diagrams, using the concept of magnetic domains.