Science (English Medium) कक्षा १२ - CBSE Question Bank Solutions

When charcoal is prepared from a living tree, it shows a disintegration rate of 15.3 disintegrations of ¹⁴C per gram per minute. A sample from an ancient piece of charcoal shows ¹⁴C activity to be 12.3 disintegrations per gram per minute. How old is this sample? Half-life of ¹⁴C is 5730 y.

A radioactive isotope is being produced at a constant rate dN/dt = R in an experiment. The isotope has a half-life t_1/2. Show that after a time t >> t_1/2 the number of active nuclei will become constant. Find the value of this constant.

Consider the situation of the previous problem. Suppose the production of the radioactive isotope starts at t = 0. Find the number of active nuclei at time t.

The half-life of ⁴⁰K is 1.30 × 10⁹ y. A sample of 1.00 g of pure KCI gives 160 counts s⁻¹. Calculate the relative abundance of ⁴⁰K (fraction of ⁴⁰K present) in natural potassium.

Define the term 'decay constant' of a radioactive sample. The rate of disintegration of a given radioactive nucleus is 10000 disintegrations/s and 5,000 disintegrations/s after 20 hr. and 30 hr. respectively from start. Calculate the half-life and the initial number of nuclei at t= 0. 

Answer the following question.
Describe briefly the functions of the three segments of the n-p-n transistor.

Answer the following question.
Describe briefly the process of transferring the charge between the two plates of a parallel plate capacitor when connected to a battery. Derive an expression for the energy stored in a capacitor.

Solve the following question.
A parallel plate capacitor is charged by a battery to a potential difference V. It is disconnected from the battery and then connected to another uncharged capacitor of the same capacitance. Calculate the ratio of the energy stored in the combination to the initial energy on the single capacitor.   

Identify the nature of the radioactive radiations emitted in each step of the decay process given below.

`""_Z^A X -> _Z^A  _-1^-4 Y ->_Z^A  _-1^-4 W`

Define one Becquerel.

A radioactive substance disintegrates into two types of daughter nuclei, one type with disintegration constant λ₁ and the other type with disintegration constant λ₂ . Determine the half-life of the radioactive substance.

A monoenergetic (18 keV) electron beam initially in the horizontal direction is subjected to a horizontal magnetic field of 0.04 G normal to the initial direction. Estimate the up or down deflection of the beam over a distance of 30 cm (m_e = 9.11 × 10^–31 kg).

The magnetic moment vectors µ_s and µ_l associated with the intrinsic spin angular momentum S and orbital angular momentum l, respectively, of an electron are predicted by quantum theory (and verified experimentally to a high accuracy) to be given by:

µ_s = –(e/m) S,

µ_l = –(e/2m) l

Which of these relations is in accordance with the result expected classically? Outline the derivation of the classical result.

In charging by Induction, a metallic object is charged ______.

For a one dimensional electric field, the correct relation of E and potential V is _________.

A point charge + q is placed at a distance d from an isolated conducting plane. The field at a point P on the other side of the plane is ______.

Two small balls having the same mass and charge and located on the same vertical at heights h₁ and h₂ are thrown in the same direction along the horizontal at the same velocity v. The first ball touches the ground at a distance l from the initial vertical. At what height H₂ will the second ball be at this instant ? The air drag and the effect of the charges induced on the ground should be neglected.