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

A loop, made of straight edges has six corners at A(0, 0, 0), B(L, O, 0) C(L, L, 0), D(0, L, 0) E(0, L, L) and F(0, 0, L). A magnetic field `B = B_o(hati + hatk)`T is present in the region. The flux passing through the loop ABCDEFA (in that order) is ______.

A cylindrical bar magnet is rotated about its axis (Figure). A wire is connected from the axis and is made to touch the cylindrical surface through a contact. Then

Consider a closed loop C in a magnetic field (Figure). The flux passing through the loop is defined by choosing a surface whose edge coincides with the loop and using the formula φ = B₁.dA₁ + B₂.dA₂ +... Now if we chose two different surfaces S₁ and S₂ having C as their edge, would we get the same answer for flux. Jusity your answer.

The optical properties of a medium are governed by the relative permittivity (ε_r) and relative permeability (µ_r). The refractive index is defined as `sqrt(µ_r ε_r) = n`. For ordinary material ε_r > 0 and µ_r > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with ε_r < 0 and µ_r < 0. Since then such ‘metamaterials’ have been produced in the laboratories and their optical properties studied. For such materials `n = - sqrt(µ_r ε_r)`. As light enters a medium of such refractive index the phases travel away from the direction of propagation.

1. According to the description above show that if rays of light enter such a medium from air (refractive index = 1) at an angle θ in 2^nd quadrant, them the refracted beam is in the 3^rd quadrant.
2. Prove that Snell’s law holds for such a medium.

Consider an electron in front of metallic surface at a distance d (treated as an infinite plane surface). Assume the force of attraction by the plate is given as `1/4 q^2/(4πε_0d^2)`. Calculate work in taking the charge to an infinite distance from the plate. Taking d = 0.1 nm, find the work done in electron volts. [Such a force law is not valid for d < 0.1nm].

A 1 KW signal is transmitted using a communication channel which provides attenuation at the rate of – 2dB per km. If the communication channel has a total length of 5 km, the power of the signal received is `["gain in dB" = 10 log ("P"_0/"P"_"i")]`

Show that the first few frequencies of light that is emitted when electrons fall to the n^th level from levels higher than n, are approximate harmonics (i.e. in the ratio 1 : 2 : 3...) when n >> 1.

What is the minimum energy that must be given to a H atom in ground state so that it can emit an H_γ line in Balmer series. If the angular momentum of the system is conserved, what would be the angular momentum of such H_γ photon?

The first four spectral lines in the Lyman series of a H-atom are λ = 1218 Å, 1028Å, 974.3 Å and 951.4 Å. If instead of Hydrogen, we consider Deuterium, calculate the shift in the wavelength of these lines.

Deutrium was discovered in 1932 by Harold Urey by measuring the small change in wavelength for a particular transition in ¹H and ²H. This is because, the wavelength of transition depend to a certain extent on the nuclear mass. If nuclear motion is taken into account then the electrons and nucleus revolve around their common centre of mass. Such a system is equivalent to a single particle with a reduced mass µ, revolving around the nucleus at a distance equal to the electron-nucleus separation. Here µ = m_eM/(m_e + M) where M is the nuclear mass and m e is the electronic mass. Estimate the percentage difference in wavelength for the 1st line of the Lyman series in ¹H and ²H. (Mass of ¹H nucleus is 1.6725 × 10^–27 kg, Mass of ²H nucleus is 3.3374 × 10^–27 kg, Mass of electron = 9.109 × 10^–31 kg.)

A basic communication system consists of ______.

1. transmitter.
2. information source.
3. user of information.
4. channel.
5. receiver.

Choose the correct sequence in which these are arranged in a basic communication system:

Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. Which of the following statements are true?

1. The sideband frequencies are 1506 kHz and 1494 kHz.
2. The bandwidth required for amplitude modulation is 6 kHz.
3. The bandwidth required for amplitude modulation is 3 MHz.
4. The side band frequencies are 1503 kHz and 1497 kHz. 

Which of the following would produce analog signals and which would produce digital signals?

1. A vibrating tuning fork.
2. Musical sound due to a vibrating sitar string.
3. Light pulse.
4. Output of NAND gate.

Figure shows a communication system. What is the output power when input signal is of 1.01 mW? (gain in dB = 10 log10 (P_o/P_i).

An audio signal is modulated by a carrier wave of 20 MHz such that the bandwidth required for modulation is 3 kHz. Could this wave be demodulated by a diode detector which has the values of R and C as

1. R = 1 kΩ, C = 0.01 µF.
2. R = 10 kΩ, C = 0.01 µF.
3. R = 10 kΩ, C = 0.1 µF.

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 ______.

A current of 0.8 A flows in a conductor of 40 Ω for 1 minute. The heat produced in the conductor will be ______.

In a coil of resistance 100 Ω a current is induced by changing the magnetic flux through it. The variation of current with time is shown in the figure. The magnitude of change in flux through the coil is ______.

Using Huygens’s construction of secondary wavelets draw a diagram showing the passage of a plane wavefront from a denser to a rarer medium. Using it verifies Snell’s law.

(i) State Bohr's quantization condition for defining stationary orbits. How does the de Broglie hypothesis explain the stationary orbits?

(ii) Find the relation between three wavelengths λ₁, λ₂ and λ₃ from the energy-level diagram shown below.