List of top Physics Questions asked in CBSE CLASS XII

Draw the circuit diagrams for obtaining the V-I characteristics of a p-n junction diode. Explain briefly the salient features of the V-I characteristics in (i) forward biasing, and (ii) reverse biasing.
The electric field in an electromagnetic wave in vacuum is given by:
\(\vec{E} = 6.3 \, N/C} \left[\cos \left(1.5 \, rad/m} \cdot y + 4.5 \times 10^8 \, rad/s} \cdot t\right)\right] \hat{i}\) % Part (a) Wavelength and Frequency Calculation (a) Find the wavelength and frequency of the wave:
Use the Bohr's first and second postulates to derive an expression for the radius of the nth orbit in a hydrogen atom.
(a) Define atomic mass unit (u).
(b) Calculate the energy required to separate a deuteron into its constituent parts (a proton and a neutron).
\( m_D = 2.014102 \, u} \)
\( m_H = 1.007825 \, u} \)
\( m_n = 1.008665 \, u} \)
Determine the current in branches AB, AC, and BC of the network shown in the figure.
\includegraphics[width=0.5\linewidth]{22.png}
Suppose a pure Si crystal has \(5 \times 10^{28}\) atoms m\(^{-3}\). It is doped by 1 ppm concentration of boron. Calculate the concentration of holes and electrons, given that \(n_i = 1.5 \times 10^{16}\) m\(^{-3}\). Is the doped crystal n-type or p-type?
Two long straight parallel conductors carrying currents exert a force on each other. Why? Derive an expression for the force per unit length between two long straight parallel conductors carrying currents in opposite directions. Explain the nature of the force between these conductors.
A sinusoidal voltage is applied to an electric circuit containing a circuit element `X' in which the current leads the voltage by \(\pi/2\).
(a) Identify the circuit element `X' in the circuit:
(b) Write the formula for its reactance:
(c) Show graphically the variation of this reactance with frequency of ac voltage:
To illustrate the relationship graphically:
The minimum intensity of white light that our eyes can perceive is about \(0.1 \, nWm}^{-2}\). Calculate the number of photons of this light entering our pupil (area \(0.4 \, cm}^2\)) per second.
(Take average wavelength of white light \(= 500 \, nm}\) and Planck’s constant \(= 6.6 \times 10^{-34} \, Js}\).) % Correct answer Correct Answer: To find the number of photons, we first calculate the energy per photon using the equation for photon energy: \[ E = \frac{hc}{\lambda} \] where \( h = 6.6 \times 10^{-34} \, Js} \) (Planck's constant), \( c = 3 \times 10^8 \, m/s} \) (speed of light), and \( \lambda = 500 \, nm} = 500 \times 10^{-9} \, m} \).
For a simple microscope, the angular size of the object equals the angular size of the image. Yet it offers magnification. Explain how this is possible.
Both plane and convex mirrors produce virtual images of objects. Can they produce real images under some circumstances? Explain.
The magnifying power of an astronomical telescope is 24. In normal adjustment, the distance between its two lenses is 150 cm. Find the focal length of the objective lens.
What is meant by 'relaxation time' of free electrons in a conductor? Show that the resistance of a conductor can be expressed by \( R = \frac{mL}{n e^2 \tau A} \), where symbols have their usual meanings.
Draw the circuit diagram of a Wheatstone bridge. Obtain the condition when no current flows through the galvanometer in it.
\includegraphics[width=0.5\linewidth]{17.png}
% Wheatstone Bridge Diagram Explanation Circuit Diagram and Condition:
The Wheatstone bridge consists of a quadrilateral circuit with four resistors, \( R_1, R_2, R_3, \) and \( R_4 \), a galvanometer \( G \), and a battery. The galvanometer is connected between the junctions of \( R_2 \) and \( R_3 \), and \( R_1 \) and \( R_4 \). The battery is connected across the bridge. % Correct answer Correct Answer: The condition for no current to flow through the galvanometer is when the bridge is balanced, given by the equation: \[ \frac{R_1}{R_2} = \frac{R_3}{R_4} \]
Assertion (A): In a semiconductor, the electrons in the conduction band have lesser energy than those in the valence band.
Reason (R): Donor energy level is just above the valence band in a semiconductor.
Assertion (A): A convex lens, when immersed in a liquid, disappears.
Reason (R): The refractive indices of the material of the lens and the liquid are equal.
Assertion (A): Photoelectric effect demonstrates the particle nature of light.
Reason (R): Photoelectric current is proportional to frequency of incident radiation.
Assertion (A): A proton and an electron enter a uniform magnetic field \( \vec{B} \) with the same momentum \( \vec{p} \) such that \( \vec{p} \) is perpendicular to \( \vec{B} \). They describe circular paths of the same radius.
Reason (R): In a magnetic field, orbital radius \( r \) is equal to \( \frac{p}{qB} \).
Which one of the following is the correct graph between the maximum kinetic energy (\( K_m \)) of the emitted photoelectrons and the frequency of incident radiation (\( \nu \)) for a given photosensitive surface?
\includegraphics[width=0.5\linewidth]{10o.png}
An electron makes a transition from the \( n = 2 \) level to the \( n = 1 \) level in the Bohr model of a hydrogen atom. Its period of revolution:
Silicon is doped with a pentavalent element. The energy required to set the additional electron free is about:
Which one of the following has the highest frequency?
A proton and an alpha particle having equal velocities approach a target nucleus. They come momentarily to rest and then reverse their directions. The ratio of the distance of closest approach of the proton to that of the alpha particle will be:
Consider a solenoid of length \( l \) and area of cross-section \( A \) with a fixed number of turns. The self-inductance of the solenoid will increase if:
The current in a coil of \(15 \, mH}\) increases uniformly from zero to \(4 \, A}\) in \(0.004 \, s}\). Calculate the electromotive force (emf) induced in the coil: