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

The gravitational force between a H-atom and another particle of mass m will be given by Newton’s law: `F = G(M.m)/r^2`, where r is in km and ______.

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.

A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to ______.

An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because ______.

1. the size of the required antenna would be at least 5 km which is not convenient.
2. the audio signal can not be transmitted through sky waves.
3. the size of the required antenna would be at least 20 km, which is not convenient.
4. effective power transmitted would be very low, if the size of the antenna is less than 5 km.

A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i).

If the magnetizing field on a ferromagnetic material is increased, its permeability ______.

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 work function for a metal surface is 4.14 eV. The threshold wavelength for this metal surface is ______.

A uniform magnetic field gets modified as shown in figure when two specimens A and B are placed in it.

(a)	(b)

1. Identify the specimen A and B.
2. How is the magnetic susceptibility of specimen A different from that of specimen B?

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.

The graph shows the variation of photocurrent for a photosensitive metal

1. What does X and A on the horizontal axis represent?
2. Draw this graph for three different values of frequencies of incident radiation ʋ₁, ʋ₂ and ʋ₃ (ʋ₃ > ʋ₂ > ʋ₁) for the same intensity.
3. Draw this graph for three different values of intensities of incident radiation I₁, I₂ and I₃ (I₃ > I₂ > I₁) having the same frequency.

Draw equipotential surfaces for (i) an electric dipole and (ii) two identical positive charges placed near each other.

Why it is the frequency and not the intensity of the light source that determines whether the emission of photoelectrons will occur or not? Explain.

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

Identify the extrinsic semiconductors so formed.

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.