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

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.

A resistor of 50 Ω, a capacitor of `(25/pi)` µF and an inductor of `(4/pi)` H are connected in series across an ac source whose voltage (in volts) is given by V = 70 sin (100 πt). Calculate:

1. the net reactance of the circuit
2. the impedance of the circuit
3. the effective value of current in the circuit.

The potential difference applied across a given conductor is doubled. How will this affect (i) the mobility of electrons and (ii) the current density in the conductor? Justify your answers.

Explain how free electrons in a metal at constant temperature attain an average velocity under the action of an electric field. Hence, obtain an expression for it.

Consider two conducting wires A and B of the same diameter but made of different materials joined in series across a battery. The number density of electrons in A is 1.5 times that in B. Find the ratio of the drift velocity of electrons in wire A to that in wire B.

A potential difference (V) is applied across a conductor of length 'L' and cross-sectional area 'A'.

How will the drift velocity of electrons and the current density be affected if another identical conductor of the same material were connected in series with the first conductor? Justify your answers.

Two conductors, made of the same material have equal lengths but different cross-sectional areas A₁ and A₂ (A₁ > A₂). They are connected in parallel across a cell. Show that the drift velocities of electrons in two conductors are equal.

• Assertion (A): Putting the p-type semiconductor slab directly in physical contact with the n-type semiconductor slab cannot form the pn junction.
• Reason (R): The roughness at contact will be much more than inter atomic crystal spacing and continuous flow of charge carriers is not possible.

Why are microwaves considered suitable for radar systems used in aircraft navigation?

Define 'quality factor' of resonance in a series LCR circuit. What is its SI unit?