Science (English Medium) Class 12 - CBSE Important Questions

Find the angular momentum of an electron revolving in the second orbit in Bohr's hydrogen atom.

The wavelength of the second line of the Balmer series in the hydrogen spectrum is 4861 Å. Calculate the wavelength of the first line of the same series.

When alpha particles are sent through a thin gold foil, most of them go straight through the foil, because ______.

The energy of an electron in the n^th orbit of the hydrogen atom is E_n = -13.6/n²eV. The negative sign of energy indicates that ______.

 In a nuclear reaction

`"_2^3He + _2^3He -> _2^4He +_1^1H +_1^1H + 12.86 Me V` though the number of nucleons is conserved on both sides of the reaction, yet the energy is released. How? Explain.

Using the curve for the binding energy per nucleon as a function of mass number A, state clearly how the release in energy in the processes of nuclear fission and nuclear fusion can be explained.

A heavy nucleus X of mass number 240 and binding energy per nucleon 7.6 MeV is split into two fragments Y and Z of mass numbers 110 and 130. The binding energy of nucleons in Y and Z is 8.5 MeV per nucleon. Calculate the energy Q released per fission in MeV.

Two nuclei have mass numbers in the ratio 1: 2. What is the ratio of their nuclear densities?

Write two distinguishing features of nuclear forces.

James Chadwick, in 1932 studied the emission of neutral radiations when Beryllium nuclei were bombarded with alpha particles. He concluded that emitted radiations were neutrons and not photons. Explain.

Two nuclei may have the same radius, even though they contain different numbers of protons and neutrons. Explain.

The curve of binding energy per nucleon as a function of atomic mass number has a sharp peak for helium nucleus. This implies that helium nucleus is ______.

The radius of \[\ce{^27_13X}\] nucleus is R. The radius of \[\ce{^125_53Y}\] nucleus will be ______.

What happens when a forward bias is applied to a p-n junction?

What causes the setting up of high electric field even for small reverse bias voltage across the diode?

In the following diagram 'S' is a semiconductor. Would you increase or decrease the value of R to keep the reading of the ammeter A constant when S is heated? Give reason for your answer.

Draw its I – V characteristics of photodiode

A zener diode is fabricated by heavily doping both p- and n- sides of the junction. Explain, why?

Explain briefly with the help of necessary diagrams, the forward biasing of a p-n junction diode. Also draw characteristic curves.

Explain briefly with the help of necessary diagrams, the  reverse biasing of a p-n junction diode. Also draw characteristic curves.