Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.

In the LCR circuit shown in figure, the ac driving voltage is v = v_m sin ωt.

1. Write down the equation of motion for q (t).
2. At t = t₀, the voltage source stops and R is short circuited. Now write down how much energy is stored in each of L and C.
3. Describe subsequent motion of charges.

The conductivity of a semiconductor increases with increase in temperature because ______.

Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?

Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator while Si and Ge are semiconductors. Why?

Suppose a ‘n’-type wafer is created by doping Si crystal having 5 × 10²⁸ atoms/m3 with 1 ppm concentration of As. On the surface 200 ppm Boron is added to create ‘P’ region in this wafer. Considering n i = 1.5 × 10¹⁶ m^–3, (i) Calculate the densities of the charge carriers in the n and p regions. (ii) Comment which charge carriers would contribute largely for the reverse saturation current when diode is reverse biased.

An iron cored coil is connected in series with an electric bulb with an AC source as shown in figure. When iron piece is taken out of the coil, the brightness of the bulb will ______.

The figure shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Should the value of R be increased or decreased to keep the reading of the ammeter constant, when semiconductor S is heated? Justify your answer

An a.c. source generating a voltage ε = ε₀ sin ωt is connected to a capacitor of capacitance C. Find the expression for the current I flowing through it. Plot a graph of ε and I versus ωt to show that the current is ahead of the voltage by π/2.

Two crystals C₁ and C₂, made of pure silicon, are doped with arsenic and aluminium respectively.

Identify the extrinsic semiconductors so formed.

A ray of light travels from a medium into the water at an angle of incidence of 18°. The refractive index of the medium is more than that of water and the critical angle for the interface between the two media is 20°. Which one of the following figures best represents the correct path of the ray of light?

For which of the following media, with respect to air, the value of critical angle is maximum?

Name the extrinsic semiconductors formed when pure germanium is doped with a trivalent impurity. Draw the energy band diagram of extrinsic semiconductors so formed.

Name the extrinsic semiconductors formed when pure germanium is doped with a Pentavalent impurity. Draw the energy band diagram of extrinsic semiconductors so formed.

A point P lies at a distance x from the midpoint of an electric dipole on its axis. The electric potential at point P is proportional to ______.

Which one of the following metals does not exhibit emission of electrons from its surface when irradiated by visible light?

In an extrinsic semiconductor, the number density of holes is 4 × 10²⁰ m^-3. If the number density of intrinsic carriers is 1.2 × 10¹⁵ m^-3, the number density of electrons in it is ______.

Pieces of copper and of silicon are initially at room temperature. Both are heated to temperature T. The conductivity of ______.

Define relaxation time.

Derive an expression for resistivity of a conductor in terms of the number density of charge carriers in the conductor and relaxation time.