List of top Questions asked in CBSE CLASS XII

A plane surface, in the shape of a square of side 1 cm, is placed in an electric field \( \vec{E} = (100 \, \text{N/C}) \hat{i \) such that the unit vector normal to the surface is given by \( \hat{n} = 0.8 \hat{i} + 0.6 \hat{k} \). Find the electric flux through the surface.}
The Earth revolves around the Sun in an orbit of radius \(1.5 \times 10^{11} \, \text{m}\) with orbital speed \(30 \, \text{km/s}\). Find the quantum number that characterizes its revolution using Bohr’s model in this case (mass of Earth \( = 6.0 \times 10^{24} \, \text{kg}\)).
Derive an expression for magnetic force \( \vec{F} \) acting on a straight conductor of length \( L \) carrying current \( I \) in an external magnetic field \( \vec{B} \). Is it valid when the conductor is in zig-zag form? Justify.
A telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm. Calculate its magnifying power in normal adjustment and the distance of the image formed by the objective.
Write Einstein’s photoelectric equation. How did Millikan prove the validity of this equation?
In which series of hydrogen spectrum does this line fall?
Explain the existence of threshold frequency of incident radiation for photoelectric emission from a given surface.
Define the term 'electric flux' and write its dimensions.
Two energy levels of an electron in a hydrogen atom are separated by 2.55 eV. Find the wavelength of radiation emitted when the electron makes a transition from the higher energy level to the lower energy level.
Four point charges of 1 \(\mu C\), -2 \(\mu C\), 1 \(\mu C\), and -2 \(\mu C\) are placed at the corners A, B, C, and D respectively, of a square of side 30 cm. Find the net force acting on a charge of 4 \(\mu C\) placed at the center of the square.
Assertion (A): An alpha particle is moving towards a gold nucleus. The impact parameter is maximum for the scattering angle of 180°. % Reason (R) Reason (R): The impact parameter in an alpha particle scattering experiment does not depend upon the atomic number of the target nucleus.
Assertion (A): Equal amount of positive and negative charges are distributed uniformly on two halves of a thin circular ring as shown in figure. The resultant electric field at the centre O of the ring is along OC.
Reason (R): It is so because the net potential at O is not zero. 
Equal amount of positive and negative charges
Assertion (A): In a Young's double-slit experiment, interference pattern is not observed when two coherent sources are infinitely close to each other. % Reason (R) Reason (R): The fringe width is proportional to the separation between the two sources.
Three point charges, 1 pC each, are kept at the vertices of an equilateral triangle of side 10 cm. Find the net electric field at the centroid of the triangle.
Assertion (A): The energy of a charged particle moving in a magnetic field does not change. % Reason (R) Reason (R): It is because the work done by the magnetic force on the charge moving in a magnetic field is zero.
The focal lengths of the objective and the eyepiece of a compound microscope are 1 cm and 2 cm respectively. If the tube length of the microscope is 10 cm, the magnification obtained by the microscope for most suitable viewing by relaxed eye is:
The electromagnetic waves used to purify water are:
The variation of the stopping potential \( V_0 \) with the frequency \( \nu \) of the incident radiation for four metals A, B, C, and D is shown in the figure. For the same frequency of incident radiation producing photo-electrons in all metals, the kinetic energy of photo-electrons will be maximum for which metal?
variation of the stopping potential
A Young’s double-slit experimental setup is kept in a medium of refractive index \( \frac{4}{3} \). Which maximum in this case will coincide with the 6th maximum obtained if the medium is replaced by air?
The energy of an electron in the ground state of hydrogen atom is -13.6 eV. The kinetic and potential energy of the electron in the first excited state will be:
A pure Si crystal having \( 5 \times 10^{28} \) atoms m\(^{-3}\) is doped with 1 ppm concentration of antimony. If the concentration of holes in the doped crystal is found to be \( 4.5 \times 10^9 \) m\(^{-3}\), the concentration (in m\(^{-3}\)) of intrinsic charge carriers in the Si crystal is about:
Which of the following pairs is that of paramagnetic materials?
A galvanometer of resistance 50 \( \Omega \) is converted into a voltmeter of range (0 — 2V) using a resistor of 1.0 k\( \Omega \). If it is to be converted into a voltmeter of range (0 — 10 V), the resistance required will be:
Two coils are placed near each other. When the current in one coil is changed at the rate of 5 A/s, an emf of 2 mV is induced in the other. The mutual inductance of the two coils is:
A particle of mass \( m \) and charge \( q \) describes a circular path of radius \( R \) in a magnetic field. If its mass and charge were \( 2m \) and \( \frac{q}{2} \) respectively, the radius of its path would be: