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

Consider aiming a beam of free electrons towards free protons. When they scatter, an electron and a proton cannot combine to produce a H-atom ______.

1. because of energy conservation.
2. without simultaneously releasing energy in the from of radiation.
3. because of momentum conservation.
4. because of angular momentum conservation.

The Bohr model for the spectra of a H-atom ______.

1. will not be applicable to hydrogen in the molecular from.
2. will not be applicable as it is for a He-atom.
3. is valid only at room temperature.
4. predicts continuous as well as discrete spectral lines.

Taking the Bohr radius as a₀ = 53 pm, the radius of Li⁺⁺ ion in its ground state, on the basis of Bohr’s model, will be about ______.

The mass of a H-atom is less than the sum of the masses of a proton and electron. Why is this?

When an electron falls from a higher energy to a lower energy level, the difference in the energies appears in the form of electromagnetic radiation. Why cannot it be emitted as other forms of energy?

A message signal of frequency ω_m is superposed on a carrier wave of frequency ω_c to get an amplitude modulated wave (AM). The frequency of the AM wave will be ______.

Using Bohr model, calculate the electric current created by the electron when the H-atom is in the ground state.

If a proton had a radius R and the charge was uniformly distributed, calculate using Bohr theory, the ground state energy of a H-atom when (i) R = 0.1 Å, and (ii) R = 10 Å.

The inverse square law in electrostatics is |F| = `e^2/((4πε_0).r^2)` for the force between an electron and a proton. The `(1/r)` dependence of |F| can be understood in quantum theory as being due to the fact that the ‘particle’ of light (photon) is massless. If photons had a mass m_p, force would be modified to |F| = `e^2/((4πε_0)r^2) [1/r^2 + λ/r]`, exp (– λr) where λ = m_pc/h and h = `h/(2π)`. Estimate the change in the ground state energy of a H-atom if m_p were 10^-6 times the mass of an electron.

Identify the mathematical expression for amplitude modulated wave ______.

In amplitude modulation, the modulation index m, is kept less than or equal to 1 because ______.

1. m > 1, will result in interference between carrier frequency and message frequency, resulting into distortion.
2. m > 1 will result in overlapping of both side bands resulting into loss of information.
3. m > 1 will result in change in phase between carrier signal and message signal.
4. m > 1 indicates amplitude of message signal greater than amplitude of carrier signal resulting into distortion.

The maximum amplitude of an A.M. wave is found to be 15 V while its minimum amplitude is found to be 3 V. What is the modulation index?

Compute the LC product of a tuned amplifier circuit required to generate a carrier wave of 1 MHz for amplitude modulation.

Why is a AM signal likely to be more noisy than a FM signal upon transmission through a channel?

On radiating (sending out) an AM modulated signal, the total radiated power is due to energy carried by ω_c, ω_c – ωm and ω_c + ω_m. Suggest ways to minimise cost of radiation without compromising on information.

An amplitude modulated wave is as shown in figure. Calculate

1. the percentage modulation
2. peak carrier voltage and
3. peak value of information voltage.

Draw the plot of amplitude versus ‘ω’ for an amplitude modulated wave whose carrier wave (ω_c) is carrying two modulating signals, ω₁ and ω₂ (ω₂ > ω₁). 

1. Is the plot symmetrical about ωc? Comment especially about plot in region ω < ωc.
2. Extrapolate and predict the problems one can expect if more waves are to be modulated.
3. Suggest solutions to the above problem. In the process can one understand another advantage of modulation in terms of bandwidth?

Two concentric and coplanar circular loops P and Q have their radii in the ratio 2:3. Loop Q carries a current 9 A in the anticlockwise direction. For the magnetic field to be zero at the common centre, loop P must carry ______.

The radius of the innermost electron orbit of a hydrogen atom is 5.3 × 10^–11m. The radius of the n = 3 orbit is ______.

The ground state energy of hydrogen atoms is -13.6 eV. The photon emitted during the transition of electron from n = 3 to n = 1 unknown work function. The photoelectrons are emitted from the material with a maximum kinetic energy of 9 eV. Calculate the threshold wavelength of the material used.