List of top Questions asked in CBSE CLASS XII

Assertion : In an ideal step-down transformer, the electrical energy is not lost. Reason (R): In a step-down transformer, voltage decreases but the current increases.

Assertion : In Bohr model of hydrogen atom, the angular momentum of an electron in \( n \)th orbit is proportional to the square root of its orbit radius \( r_n \). Reason (R): According to Bohr model, electron can jump to its nearest orbits only.

Assertion : In a semiconductor diode, the thickness of the depletion layer is not fixed. Reason (R): Thickness of depletion layer in a semiconductor device depends upon many factors such as biasing of the semiconductor.

The threshold voltage of a silicon diode is 0.7 V. It is operated at this point by connecting the diode in series with a battery of \( V \) volt and a resistor of 1000 \( \Omega \). Find the value of \( V \) when the current drawn is 15 mA.

Show the refraction of light wave at a plane interface using Huygens' principle and prove Snell's law.

Two convex lenses A and B, each of focal length 10.0 cm, are mounted on an optical bench at 50.0 cm and 70.0 cm respectively. An object is mounted at 20.0 cm. Find the nature and position of the final image formed by the combination.

Radiations of two frequencies are incident on a metal surface of work function 2.0 eV one by one. The energies of their photons are 2.5 eV and 4.5 eV respectively. Find the ratio of the maximum speed of the electrons emitted in the two cases.

Two wires of the same material and the same radius have their lengths in the ratio 2:3. They are connected in parallel to a battery which supplies a current of 15 A. Find the current through the wires.

In the circuit, three ideal cells of e.m.f. \( V \), \( V \), and \( 2V \) are connected to a resistor of resistance \( R \), a capacitor of capacitance \( C \), and another resistor of resistance \( 2R \) as shown in the figure. In the steady state, find (i) the potential difference between P and Q, (ii) the potential difference across capacitor C.

Define resistivity of a conductor. Discuss its dependence on temperature of the conductor and draw a plot of resistivity of copper as a function of temperature.

"A low voltage battery from which high current is required must have low internal resistance." Justify.

"A high voltage battery must have a large internal resistance." Justify.

When a parallel beam of light enters water surface obliquely at some angle, what is the effect on the width of the beam?

With the help of a ray diagram, show that a straw appears bent when it is partly dipped in water and explain it.

Differentiate between the peak value and root mean square value of an alternating current. Derive the expression for the root mean square value of alternating current, in terms of its peak value.

How is an electromagnetic wave produced?

An electromagnetic wave is travelling in vertically upward direction. At an instant, its electric field vector points in west direction. In which direction does the magnetic field vector point at that instant?

Estimate the ratio of shortest wavelength of radio waves to the longest wavelength of gamma waves.

In a region of a uniform electric field \( \mathbf{E} \), a negatively charged particle is moving with a constant velocity \( \mathbf{v} = -v_0 \hat{i} \) near a long straight conductor coinciding with XX' axis and carrying current \( I \) towards -X axis. The particle remains at a distance \( d \) from the conductor.

What are the various forces acting on the charged particle?

Find the value of \( v_0 \) in terms of \( E \), \( d \), and \( I \).

Two infinitely long conductors kept along XX' and YY' axes are carrying current \( I_1 \) and \( I_2 \) along -X axis and -Y axis respectively. Find the magnitude and direction of the net magnetic field produced at point P(X, Y).

A p-n junction is forward biased. Describe the movement of the charge carriers which produce current in it.

The graph shows the variation of current with voltage for a p-n junction diode. Estimate the dynamic resistance of the diode at \( V = -0.6 \) V.

Show the variation of binding energy per nucleon with mass number. Write the significance of the binding energy curve.