List of top Questions asked in CBSE CLASS XII

A doped semiconductor is electrically neutral.
In a p-n junction under equilibrium, there is no net current.
Which of the following statements about dielectrics is correct?
When a dielectric slab is inserted between the plates of an isolated charged capacitor, the energy stored in it:
State Lenz's Law. In a closed circuit, the induced current opposes the change in magnetic flux that produced it as per the law of conservation of energy. Justify.
A metal rod of length 2 m is rotated with a frequency of 60 rev/s about an axis passing through its centre and perpendicular to its length. A uniform magnetic field of 2T perpendicular to its plane of rotation is switched-on in the region. Calculate the e.m.f. induced between the centre and the end of the rod.
State and explain Ampere’s circuital law.
Two long straight parallel wires separated by 20 cm carry 5 A and 10 A current respectively, in the same direction. Find the magnitude and direction of the net magnetic field at a point midway between them.
(i) Name the parts of the electromagnetic spectrum which are also known as ‘heat waves’.
Name the parts of the electromagnetic spectrum which are absorbed by the ozone layer in the atmosphere.
Write briefly one method each, of the production and detection of these radiations.
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.
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.
In which series of hydrogen spectrum does this line fall?
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 \( = 6.0 \times 10^{24} \, \text{kg}\)).
Write Einstein’s photoelectric equation. How did Millikan prove the validity of this equation?
Define the term 'electric flux' and write its dimensions.
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): 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.
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.
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.
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.
The electromagnetic waves used to purify water are:
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: