List of top Physics Questions asked in CBSE CLASS XII

A cell of emf $E$ and internal resistance $r$ is connected across a resistor of variable resistance $R$. Draw a plot showing the variation of (i) the current ($I$) in the circuit with $R$, and (ii) the terminal potential difference of the cell with $R$.

A potential difference $V$ is applied across the ends of a cylindrical conductor of length $l$, area of cross-section $A$ and resistance $R$. It is gradually stretched till its length is tripled. How will (i) the drift speed of electrons in the conductor, and (ii) the resistance of the conductor be affected? Justify your answers.

Two charges of $8\,\mu\text{C}$ and $-4\,\mu\text{C}$ are located at points $(-18\ \text{cm}, 0, 0)$ and $(18\ \text{cm}, 0, 0)$ respectively.

(i) Calculate the electrostatic potential energy of the system of the two charges.

(ii) The same system of charges is placed in a region where electrostatic potential $V(r) = \dfrac{A}{r}$, where $A = 9 \times 10^4$ V.m. Calculate the potential energy of the system.

A charged particle moving with velocity $\vec{v}$ enters a region of uniform magnetic field $\vec{B}$. The force acting on it is non-zero. Would the particle gain any energy? Explain.

Draw a labelled ray diagram of a refracting telescope in normal adjustment. Briefly explain its working.

Why is the eyepiece of a telescope of short focal length, while the objective is of large focal length? Explain.

Derive the expression for the magnetic field (near its centre) due to an air-filled long solenoid carrying a current $I$ and having $n$ number of turns per unit length.

Derive an expression for refractive index of an equilateral prism in terms of the angle of minimum deviation and the angle of the prism. Plot a graph to show the variation of the angle of deviation as a function of angle of incidence.

A capacitor, of capacitance $C$, plate area $A$ and plate separation $d$ is charged to $V$ volts by a battery. The energy stored in it is $U$. After some time, the battery is disconnected and a slab of dielectric constant $K$, is inserted to fill the space between the plates of the capacitor. Let $V'$ and $U'$ now be the potential difference across the plates of the capacitor and the energy stored in it respectively. Find $\left(\dfrac{V'}{V} ight)$ and $\left(\dfrac{U'}{U} ight)$.

(i) The lower half of a concave mirror's reflecting surface is covered with an opaque non-reflecting material. What effect will this have on the image of an object placed in front of the mirror ?

(ii) A lens made of glass of refractive index $1.5$ disappears, when immersed in a trough of liquid. What is the refractive index of the liquid ?

Give reasons for your answer in each case.

Briefly explain the process of charging a parallel plate capacitor, when it is connected across a dc source.

Draw a plot showing the variation of potential energy of a pair of nucleons as a function of their separation. With the help of the plot, explain why the nuclear force is attractive for $r > r_0$ and repulsive for $r < r_0$.

Two identical circular coils A and B of radius $10$ cm each are placed concentrically and mutually perpendicular in XY and YZ planes respectively. Currents of $1$ A and $\sqrt{3}$ A flow anticlockwise in the coils as seen from YZ plane. Find the magnitude of the net magnetic field at the common centre of the coils.

Assertion (A): The ratio of the speeds of the electron in the second to that in the third orbit in Bohr's hydrogen atom is $\dfrac{3}{2}$.

Reason (R): The speed of the electron in $n^{th}$ permitted orbit in hydrogen atom varies proportional to $\dfrac{1}{n}$ i.e. $v_n \propto \dfrac{1}{n}$.

Calculate the kinetic energy of an electron which has the de Broglie wavelength of $600$ nm associated with it.

Assertion (A) : Electron emission from surface of zinc occurs when yellow light is incident on it.

Reason (R) : Energy of a photon of yellow light is more than the work function of zinc.

A monochromatic beam of light is travelling from a rarer to a denser medium. Giving reasons, explain the following :

(i) Is the frequency of the reflected and the refracted light the same as the frequency of incident light ?

(ii) Does the decrease in speed of light in denser medium imply a reduction in the energy carried by the light wave ?

Assertion (A) : A free neutron is not a stable particle.

Reason (R) : It decays into a proton and an electron.

The energy of an electron in an orbit in a hydrogen atom (Bohr model) is $-3.4$ eV. Its angular momentum in this orbit is:

A ray of light is incident at an angle of $60^\circ$ on face AB of prism ABC with $\angle A = 30^\circ$. The ray emerging out of the prism makes an angle of $30^\circ$ with the incident ray. The angle of emergence ($\angle e$) is equal to: