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

Suppose there is a circuit consisting of only resistances and batteries and we have to double (or increase it to n-times) all voltages and all resistances. Show that currents are unaltered. Do this for circuit of Example 3.7 in the NCERT Text Book for Class XII.

A square of side L meters lies in the x-y plane in a region, where the magnetic field is given by `B = Bo(2hati + 3hatj + 4hatk)`T, where B₀ is constant. The magnitude of flux passing through the square is ______.

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.

Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.

An electrical device draws 2kW power from AC mains (voltage 223V (rms) = `sqrt(50,000)` V). The current differs (lags) in phase by `phi(tan phi = (-3)/4)` as compared to voltage. Find (i) R, (ii) X_C – X_L, and (iii) I_M. Another device has twice the values for R, X_C and X_L. How are the answers affected?

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")]`

Hole is ______.

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.

An ac voltage V = V₀ sin ωt is applied across a pure inductor of inductance L. Find an expression for the current i, flowing in the circuit and show mathematically that the current flowing through it lags behind the applied voltage by a phase angle of `π/2`. Also draw graphs of V and i versus ωt for the circuit.

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

Why is doping of intrinsic semiconductors necessary?

What is meant by doping of an intrinsic semiconductor?

Name the two types of atoms used for doping of Ge/Si.