NCERT Exemplar Solutions for Class 12 Physics Chapter 13 – Nuclei are available here. This is one of the most important chapters of CBSE Class 12th Physics and questions from this chapter are frequently asked in board exams and competitive exams (like JEE Main, NEET, WBJEE, UPSEE etc).
NCERT Exemplar books are completely different from NCERT textbooks. In each chapter of Class 12 Physics NCERT textbooks you will get theory, derivations, solved examples and exercises. But, in Class 12 Physics NCERT Exemplar books, students will only find exercises. But the level of questions given in NCERT Exemplar books is quite high. A student can’t solve questions of NCERT Exemplar until and unless he or she doesn’t have a strong hold on basic concepts given in textbooks.
Students who don’t have a clear understanding of the topics (on which problems are based) will neither understand the problems nor the solutions. So, it is suggested that students preparing for Class 12 board exams or competitive exams must go through NCERT textbooks along with NCERT Exemplar book.
Important topics of the Chapter 13 – Nuclei:
• Composition and size of nucleus
• Atomic masses
• Isotopes, isobars, isotones
• Alpha, beta and gamma particles/rays and their properties
• Radioactive decay law
• Mass-energy relation, mass defect
• Binding energy per nucleon and its variation with mass number
• Nuclear fission and nuclear fusion
Each chapter of Class 12 Physics NCERT Exemplar book has variety of questions from multiple choice to subjective questions. There are 5 categories of problems and we have provided the solutions (PDF format) accordingly.
Types and number of questions in this chapter:
In this chapter there are - 7 MCQ I (Multiple choice questions with single correct option), 3 MCQ II (Multiple choice questions with multiple correct options), 5 VSA (Very short answer type questions), 5 SA (Short answer type questions), 6 LA (Long answer type questions).
Few problems and their solutions from this chapter:
Suppose we consider a large number of containers each containing initially 10000 atoms of a radioactive material with a half life of 1 year. After 1 year,
(a) all the containers will have 5000 atoms of the material.
(b) all the containers will contain the same number of atoms of the material but that number will only be approximately 5000.
(c) the containers will in general have different numbers of the atoms of the material but their average will be close to 5000.
(d) none of the containers can have more than 5000 atoms.
Solution 13.1: (c)
Radioactivity is a process due to which a radioactive material spontaneously decays. In half-life (t = 1yr) of the material on the average half the number of atoms will decay. Therefore, the containers will in general have different number of atoms of the material, but their average will be approx 5000.
The gravitational force between a H-atom and another particle of mass m will be given by Newton’s law:
F = G [(M.m)/r2], where r is in km and
(a) M = mproton + melectron
(b) M = mproton + melectron ‒ (B/c2) [B = 13.6 eV].
(c) M is not related to the mass of the hydrogen atom.
(d) M = mproton + melectron ‒ (|V|/c2) [|V| = 13.6 magnitude of the potential energy of electron in the H-atom)].
Solution 13.2 (b)
When a nucleus in an atom undergoes a radioactive decay, the electronic energy levels of the atom
(a) do not change for any type of radioactivity .
(b) change for α and β radioactivity but not for γ-radioactivity.
(c) change for α-radioactivity but not for others.
(d) change for β-radioactivity but not for others.
Solution 13.3 (b)
α particle – 2 unit of positive charge
β particles – 1 unit positive charge
γ particles – No charge
Due to this reason, energy levels of the atom change for α and β but not for others.
Mx and My denote the atomic masses of the parent and the daughter nuclei respectively in a radioactive decay. The Q-value for a β – decay is Q1 and that for a β+ decay is Q2. If me denotes the mass of an electron, then which of the following statements is correct?
(a) Q1 = (Mx – My) c2 and Q2 = (Mx – My – 2me) c2
(b) Q1 = (Mx – My) c2 and Q2 = (Mx – My ) c2
(c) Q1 = (Mx – My – 2me) c2 and Q2 = (Mx – My +2 me) c2
(d) Q1 = (Mx – My + 2me) c2 and Q2 = (Mx – My +2 me) c2
Solution 13.4: (a)
Let the nucleus is ZXA.
Now, ß+ decay is represented as
zXA → Z+1YA + +1e + v + Q2
∴ Q2 = [mN (zXA) - mN (Z-1YA)-me]c2
= [mN (ZXA) + Z me - mn (Z-1YA) - (z-1) me-2me] c2
= [mN (ZXA) - m (Z-1YA) -2me]c2
= (Mx -My -2me) c2
ß- decay is represented as
Q1 = [mN (zXA) -mN (z + 1YA) -me] c2 = [mN (zXA) -m (z + 1YA)] c2 = (Mx –My) c2.
Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into p + ē + ν. If one of the neutrons in Triton decays, it would transform into He3 nucleus. This does not happen. This is because
(a) Triton energy is less than that of a He3 nucleus.
(b) the electron created in the beta decay process cannot remain in the nucleus.
(c) both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a He3 nucleus.
(d) because free neutrons decay due to external perturbations which is absent in a triton nucleus.
Solution 13.5: (a)
Heavy stable nucle have more neutrons than protons. This is because of the fact that
(a) neutrons are heavier than protons.
(b) electrostatic force between protons are repulsive.
(c) neutrons decay into protons through beta decay.
(d) nuclear forces between neutrons are weaker than that between protons.
Solution 13.6: (b)
This is because electrostatic force between protons is repulsive, it may reduce stability of the nucleus.
In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose because
(a) they will break up.
(b) elastic collision of neutrons with heavy nuclei will not slow them down.
(c) the net weight of the reactor would be unbearably high.
(d) substances with heavy nuclei do not occur in liquid or gaseous state at room temperature.
Solution 13.7: (b)
The moderator used have light nuclei like proton. When protons undergo perfectly elastic collision with the neutron emitted their velocities are exchanged, it means, neutrons come to rest and protons move with the velocity of neutrons.
To access the problems and solutions of the complete chapter please download the PDFs’ from the able given below
Link to Download NCERT Exemplar Solutions for Class 12 Physics: Chapter 13 – Nuclei
Types of Question
Link to Download PDF
7 MCQ I (Multiple choice questions with single correct option)
3 MCQ II (Multiple choice questions with multiple correct options)
5 VSA (Very short answer type questions)
5 SA (Short answer type questions)
6 LA (Long answer type questions)
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