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

Five long wires A, B, C, D and E, each carrying current I are arranged to form edges of a pentagonal prism as shown in figure. Each carries current out of the plane of paper.

1. What will be magnetic induction at a point on the axis O? AxisE is at a distance R from each wire.
2. What will be the field if current in one of the wires (say A) is switched off?
3. What if current in one of the wire (say) A is reversed?

What are the dimensions of χ, the magnetic susceptibility? Consider an H-atom. Guess an expression for χ, upto a constant by constructing a quantity of dimensions of χ, out of parameters of the atom: e, m, v, R and µ₀. Here, m is the electronic mass, v is electronic velocity, R is Bohr radius. Estimate the number so obtained and compare with the value of |χ| ~ 10^–5 for many solid materials.

The mutual inductance M₁₂ of coil 1 with respect to coil 2 ______.

1. increases when they are brought nearer.
2. depends on the current passing through the coils.
3. increases when one of them is rotated about an axis.
4. is the same as M₂₁ of coil 2 with respect to coil 1.

There are two coils A and B seperated by some distance. If a current of 2A flows through A, a magnetic flux of 10^-2 Wb passes through B (no current through B). If no current passes through A and a current of 1A passes through B, what is the flux through A?

Consider sunlight incident on a slit of width 10⁴ A. The image seen through the slit shall ______.

Consider sunlight incident on a pinhole of width 10³A. The image of the pinhole seen on a screen shall be ______.

1. a sharp white ring.
2. different from a geometrical image.
3. a diffused central spot, white in colour.
4. diffused coloured region around a sharp central white spot.

The wavelength of a photon needed to remove a proton from a nucleus which is bound to the nucleus with 1 MeV energy is nearly ______.

1. In the explanation of photo electric effect, we assume one photon of frequency ν collides with an electron and transfers its energy. This leads to the equation for the maximum energy E_max of the emitted electron as E_max = hν – φ₀  where φ₀ is the work function of the metal. If an electron absorbs 2 photons (each of frequency ν) what will be the maximum energy for the emitted electron?
2. Why is this fact (two photon absorption) not taken into consideration in our discussion of the stopping potential?

There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength.

A student performs an experiment on photoelectric effect, using two materials A and B. A plot of V_stop vs ν is given in Figure.

1. Which material A or B has a higher work function?
2. Given the electric charge of an electron = 1.6 × 10^–19 C, find the value of h obtained from the experiment for both A and B.

Comment on whether it is consistent with Einstein’s theory:

The Balmer series for the H-atom can be observed ______.

1. if we measure the frequencies of light emitted when an excited atom falls to the ground state.
2. if we measure the frequencies of light emitted due to transitions between excited states and the first excited state.
3. in any transition in a H-atom.
4. as a sequence of frequencies with the higher frequencies getting closely packed.

Let E_n = `(-1)/(8ε_0^2) (me^4)/(n^2h^2)` be the energy of the nth level of H-atom. If all the H-atoms are in the ground state and radiation of frequency (E₂ - E₁)/h falls on it ______.

1. it will not be absorbed at all.
2. some of atoms will move to the first excited state.
3. all atoms will be excited to the n = 2 state.
4. no atoms will make a transition to the n = 3 state.

Positronium is just like a H-atom with the proton replaced by the positively charged anti-particle of the electron (called the positron which is as massive as the electron). What would be the ground state energy of positronium?

In the Auger process an atom makes a transition to a lower state without emitting a photon. The excess energy is transferred to an outer electron which may be ejected by the atom. (This is called an Auger electron). Assuming the nucleus to be massive, calculate the kinetic energy of an n = 4 Auger electron emitted by Chromium by absorbing the energy from a n = 2 to n = 1 transition.

In Figure, Vo is the potential barrier across a p-n junction, when no battery is connected across the junction ______.

In the circuit shown in figure, if the diode forward voltage drop is 0.3 V, the voltage difference between A and B is ______.

When an electric field is applied across a semiconductor ______.

1. electrons move from lower energy level to higher energy level in the conduction band.
2. electrons move from higher energy level to lower energy level in the conduction band.
3. holes in the valence band move from higher energy level to lower energy level.
4. holes in the valence band move from lower energy level to higher energy level.

Consider an npn transistor with its base-emitter junction forward biased and collector base junction reverse biased. Which of the following statements are true?

1. Electrons crossover from emitter to collector.
2. Holes move from base to collector.
3. Electrons move from emitter to base.
4. Electrons from emitter move out of base without going to the collector.

Consider an npn transistor with its base-emitter junction forward biased and collector base junction reverse biased. Which of the following statements are true?

1. Electrons crossover from emitter to collector.
2. Holes move from base to collector.
3. Electrons move from emitter to base.
4. Electrons from emitter move out of base without going to the collector.

Figure shows the transfer characteristics of a base biased CE transistor. Which of the following statements are true?

At V_i = 0.4 V, transistor is in active state.

At V_i = 1 V, it can be used as an amplifier.

At V_i = 0.5 V, it can be used as a switch turned off.

At V_i = 2.5 V, it can be used as a switch turned on.