List of top Physics Questions asked in CBSE CLASS XII

(a) A particle of charge $q$ and mass $m$ is accelerated through a potential difference $V$. Deduce an expression for the de Broglie wavelength associated with the particle.

(b) Plot a graph for a particle of charge $q$ and mass $m$ showing variation of de Broglie wavelength $\lambda$ with $\dfrac{1}{\sqrt{V}}$, where $V$ is the potential difference through which the particle is accelerated.

The equivalent mass of a photon of wavelength $100\ \text{nm}$ is:

The electric field and the magnetic field components of an electromagnetic wave travelling in vacuum are represented as $E = m\,\sin(\alpha z - \beta t)$ and $B = n\,\sin(\alpha z - \beta t)$, where $m$, $n$, $\alpha$ and $\beta$ are constants characterising the wave. Then:

Which one of the following materials has magnetic susceptibility $\chi$ in the range $-1 \leq \chi < 0$?

(a) An electric field $\vec{E}$ is established across the ends of a cylindrical conductor of length $L$ and area of cross-section $A$. Discuss how electrons attain an average velocity, independent of time. Hence, obtain a relation between current in the conductor and this ‘average velocity’ of electrons.

(b) (i) This ‘average velocity’ is found to be a few $\text{mm}\,\text{s}^{-1}$ for currents in range of a few amperes. How then is current established almost the instant a circuit is closed?

(ii) Two copper wires having their radii in the ratio of $3:2$ are connected in series across a battery. Find the ratio of the drift velocities of the electrons in the wires.

(i) With the help of a labelled diagram, explain the principle, construction and working of an a.c. generator.

(ii) Deduce an expression for the induced emf in the coil of the generator.

(iii) If $T$ is the time period of the rotation of the coil, at what values of $T$ in a cycle, the emf generated is maximum?

A photosensitive surface is illuminated by radiations of wavelength $\lambda_{1}$, $\lambda_{2}\;(>\lambda_{1})$ and $\lambda_{3}$ one by one and photoemission is observed in each case. $\lambda_{1}$ and $\lambda_{2}$ lie in UV range and $\lambda_{3}$ in visible range. If $V_{1}$, $V_{2}$ and $V_{3}$ are stopping potentials in these cases respectively, then:

(i) With the help of a labelled diagram, explain the principle, construction and working of an a.c. generator.

(ii) Deduce an expression for the induced emf in the coil of the generator.

(iii) If $T$ is the time period of the rotation of the coil, at what values of $T$ in a cycle, the emf generated is maximum?

(a) An electric field $\vec{E}$ is established across the ends of a cylindrical conductor of length $L$ and area of cross-section $A$. Discuss how electrons attain an average velocity, independent of time. Hence, obtain a relation between current in the conductor and this ‘average velocity’ of electrons.

(b) (i) This ‘average velocity’ is found to be a few $\text{mm}\,\text{s}^{-1}$ for currents in range of a few amperes. How then is current established almost the instant a circuit is closed?

(ii) Two copper wires having their radii in the ratio of $3:2$ are connected in series across a battery. Find the ratio of the drift velocities of the electrons in the wires.

In Bohr model of hydrogen atom, the electron makes a transition from $n=5$ to $n=1$ state. As a result, a photon of wavelength $\lambda$ is emitted in the process. The wavelength of the photon emitted when an electron makes a transition from energy level $n=5$ to $n=2$ will be:

In an electron microscope, accelerated electrons have wavelength of $0.011$ nm. Calculate the voltage through which electrons were accelerated to attain this wavelength.

(Take $e = 1.6 \times 10^{-19}$ C, $m_e = 9 \times 10^{-31}$ kg, $h = 6.6 \times 10^{-34}$ J.s)

A series LCR circuit with $R = 3\,\Omega$, $X_C = 4\,\Omega$ and $X_L = 8\,\Omega$ is connected to a 220 V, 50 Hz ac source. The power factor for the circuit is:

A current of 1.5 A is maintained in a copper wire of length 1 m with area of cross-section $1.7 \times 10^{-7}$ m$^2$. The magnitude of electric field in the wire is $[\rho_{Cu} = 1.7 \times 10^{-8}\,\Omega\,\text{m}]$:

A semiconductor has equal electron and hole concentration of $3 \times 10^8$ m$^{-3}$. On doping with a certain impurity, the hole concentration increases to $6 \times 10^{10}$ m$^{-3}$.

(a) What type of semiconductor is obtained on doping?
(b) Calculate the new electron concentration of the semiconductor.
(c) How does the energy gap of semiconductor change with doping? Draw the energy band diagram for it.

A concave lens of focal length 40 cm is coaxially in contact with two convex lenses, each of focal length 20 cm, on each side. The focal length of the combination is:

Two thin converging lenses of focal lengths $f_1$ and $f_2$ are placed coaxially in contact with each other. Draw a ray diagram to show the image formation by the combination. Hence, derive the expression for the focal length of the combination.

Two converging lenses $L_1$ and $L_2$, of focal lengths $144\,\text{cm}$ and $6\,\text{cm}$ respectively, are arranged coaxially in a tube $147\,\text{cm}$ apart. The arrangement is used to view a far off tree, keeping the eye close to $L_2$.

(i) Draw the ray diagram depicting image formation by the combination.

(ii) How far from $L_2$ will the final image be formed?

(iii) Calculate the magnification produced by the combination.

Find the ratio of energies of photons emitted during the transition of an electron in hydrogen atom from its:

(i) second permitted energy level to the first permitted energy level, and

(ii) the highest permitted energy level to the first permitted energy level.

The number of free electrons per unit volume ($\text{m}^3$) in a wire made of copper (resistivity $1.72 \times 10^{-8}\,\Omega\,\text{m}$) is $8.5 \times 10^{28}$. Calculate the average number of collisions made by an electron in one second when an electric field is maintained in the wire.

When the distance between the slit and screen is doubled, the angular separation between two fringes in single-slit diffraction experiment will: