List of top Physics Questions asked in CBSE CLASS XII

(i) Name the parts of the electromagnetic spectrum which are also known as ‘heat waves’.
An electron moving with a velocity \( \vec{v} = (1.0 \times 10^7 \, \text{m/s}) \hat{i} + (0.5 \times 10^7 \, \text{m/s}) \hat{j} \) enters a region of uniform magnetic field \( \vec{B} = (0.5 \, \text{mT}) \hat{j} \). Find the radius of the circular path described by it. While rotating, does the electron trace a linear path too? If so, calculate the linear distance covered by it during the period of one revolution.}
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.
State and explain Ampere’s circuital law.
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 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.
Define the term 'electric flux' and write its dimensions.
Explain the existence of threshold frequency of incident radiation for photoelectric emission from a given surface.
Write Einstein’s photoelectric equation. How did Millikan prove the validity of this equation?
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 \( = 6.0 \times 10^{24} \, \text{kg}\)).
In which series of hydrogen spectrum does this line fall?
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.
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.
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.
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): 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): 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.
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:
The potential energy between two nucleons inside a nucleus is minimum at a distance of about:
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:
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
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: