CBSE Class 12 Physics — Short Answer Questions

Briefly explain the two important processes that occur during the formation of a p-n junction.

(a) Using Gauss's law, deduce an expression for electric field at a point due to a uniformly charged infinite plane thin sheet. (b) Two large thin plane sheets, each having surface charge density σ, are held close and parallel to each other in air. What is the net electric field at a point (i) inside and (ii) outside, the sheets? OR (a) Obtain the condition of balance of a Wheatstone bridge. (b) Find net resistance of the network of resistors connected between A and B, as shown in figure.

A parallel plate capacitor of capacitance C has a dielectric slab between its plates. It is charged to a potential difference V by connecting it across a battery. The battery is then disconnected. If the dielectric slab is now withdrawn from the capacitor, how will the following be affected? (a) Capacitance of the capacitor, (b) Energy stored in the capacitor, and (c) The potential difference between the plates of the capacitor. Justify your answer in each case.

Figure shows a narrow beam of electrons entering with a velocity of $3 \times 10^7$ m/s, symmetrically through the space between two parallel horizontal plates $P_1 P_1'$ and $P_2 P_2'$ kept 2 cm apart. If each plate is 3 cm long, calculate the potential difference V applied between the plates so that the beam just strikes the end $P_2'$.

An ac voltage $V_i = 12 \sin (100\pi t)$ V is applied between points A and B in a network of two ideal diodes and three resistors as shown in figure. During the positive half-cycle of the input voltage V_i supplied to the network. (a) Identify which of the two diodes will conduct and why? (b) Redraw an equivalent circuit diagram to show the flow of current. (c) Calculate the output voltage drops V_0 across the three resistors when the input voltage attains its peak value.

(a) Draw the ray diagram to show the image formation by a refracting telescope and write the expression for angular magnification for the telescope in normal adjustment. (b) Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.

(a) State the two conditions under which total internal reflection occurs. (b) A transparent container contains layers of three immiscible transparent liquids A, B and C of refractive indices n, $(3/4)n$ and $(2/3)n$, respectively. A laser beam is incident at the interface between A and B at an angle θ as shown in figure. Prove that the beam does not enter region C at all for $\sin \theta \ge 2/3$.