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The NCERT Exemplar class 12 physics solutions for Chapter 13 Nuclei are meticulously prepared by the highly experienced faculty at Infinity Learn. These solutions are designed to assist students in their board exam preparation, aligning with the CBSE syllabus and exam patterns. Solving these exemplar questions helps students clear their doubts and gain a thorough understanding of the chapter on Nuclei.
This exemplar solution page includes answers to the questions from the NCERT book, supplemented with extra questions curated by our subject experts. It also incorporates questions from previous years’ question papers, sample papers, and additional resources. Utilizing these solutions enables students to grasp the core concepts effectively and achieve high scores in their board exams.
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Q. When a β-particle is emitted from a nucleus then its neutron-proton ratio:
(a) Increases
(b) Decreases
(c) Remains unchanged
(d) May increase or decrease depending upon the nucleus
Answer: (b) Decreases
Q. In nuclear reactors, the control rods are made of:
(a) Cadmium
(b) Graphite
(c) Krypton
(d) Plutonium
Answer: (a) Cadmium
Q. Light energy emitted by a star is due to:
(a) Breaking of nuclei
(b) Joining of nuclei
(c) Burning of nuclei
(d) Reflection of solar light
Answer: (b) Joining of nuclei
Q. In nuclear reaction, there is conservation of:
(a) Mass only
(b) Energy only
(c) Momentum only
(d) Mass, energy, and momentum
Answer: (d) Mass, energy, and momentum
Also Read: NCERT Syllabus
Q. Tritium is an isotope of hydrogen whose nucleus contains 2 neutrons and 1 proton. Free neutrons decay into p + e + v. If one of the neutrons in Triton decays, it would transform into He-3 nucleus. This does not happen because:
(a) Triton energy is less than that of a He-3 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 He-3 nucleus
(d) Free neutrons decay due to external perturbations which is absent in a triton nucleus
Answer: (a) Triton energy is less than that of a He-3 nucleus
Q. Heavy stable nuclei have more neutrons than protons. This is because:
(a) Neutrons are heavier than protons
(b) Electrostatic force between protons is repulsive
(c) Neutrons decay into protons through beta decay
(d) Nuclear forces between neutrons are weaker than that between protons
Answer: (b) Electrostatic force between protons is repulsive
Q. Fusion processes, like combining two deuterons to form a He nucleus, are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact:
(a) Nuclear forces have short range
(b) Nuclei are positively charged
(c) The original nuclei must be completely ionized before fusion can take place
(d) The original nuclei must first break up before combining with each other
Answer: (a) Nuclear forces have short range, (b) Nuclei are positively charged
Q. For thorium (A = 232 and Z = 90), at the end of some radioactive disintegrations, we obtain an isotope of lead with (A = 208, Z = 82). The number of emitted α and β particles are:
(a) α = 4, β = 6
(b) α = 5, β = 5
(c) α = 6, β = 4
(d) α = 6, β = 6
Answer: (c) α = 6, β = 4
Q. If 10% of a radioactive material decays in 5 days, then the amount of the original material left after 20 days is nearly:
(a) 60%
(b) 70%
(c) 75%
(d) 66%
Answer: (d) 66%
Q. Which of the following is the best nuclear fuel?
(a) Thorium-236
(b) Plutonium-239
(c) Neptunium-239
(d) Uranium-236
Answer: (b) Plutonium-239
Q. Heavy water is used as a moderator in a nuclear reactor. The function of the moderator is to:
(a) Absorb neutrons and stop chain reaction
(b) Cool the reactor
(c) Slow down the neutrons to thermal energies
(d) Control the energy released
Answer: (c) Slow down the neutrons to thermal energies
Read More: NCERT Syllabus Class 12
Q. The volume of a nucleus is smaller than that of an atom by a factor of:
(a) 10
(b) 105
(c) 1015
(d) 1010
Answer: (c) 1015
Q. Fusion reactions take place at high temperature because:
(a) Kinetic energy is high enough to overcome repulsion between nuclei
(b) Nuclei break up at high temperature
(c) Atoms are ionized at high temperature
(d) Molecules break up at high temperature
Answer: (a) Kinetic energy is high enough to overcome repulsion between nuclei
Short Answer Types
Q. Why do stable nuclei never have more protons than neutrons?
Ans. Stable nuclei have more neutrons than protons to counterbalance the repulsive electrostatic forces between protons through the strong nuclear force.
Q. Consider a radioactive nucleus A which decays to a stable nucleus C through the following sequence: A → B → C. Plot the graph showing the variation of the number of atoms of A and B versus time.
The number of atoms of A decreases exponentially over time, while B increases initially and then decreases as it decays into C, forming a peak on the graph.
Q. Are nucleons fundamental particles, or do they consist of smaller parts?
Ans. Nucleons are not fundamental particles; they consist of quarks held together by the strong nuclear force mediated by gluons.
Q. Why do heavy stable nuclei have more neutrons than protons?
Ans. Heavy stable nuclei have more neutrons than protons to mitigate the repulsive electrostatic forces between protons and to provide additional binding through the strong nuclear force.
Q. Explain why fusion processes like combining two deuterons to form a helium nucleus are impossible at ordinary temperatures and pressures.
Ans. Fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between positively charged nuclei.
Q. Describe the role of a moderator in a nuclear reactor.
Ans. A moderator slows down the neutrons produced during fission to increase the probability of further fission reactions by maintaining the chain reaction.
Q. What is pair annihilation, and how is momentum conserved in this process?
Ans. Pair annihilation occurs when an electron and a positron destroy each other to produce gamma radiation. Momentum is conserved as the total momentum of the electron-positron pair is transferred to the photons produced.
Q. In radioactive decay, what is meant by the term ‘half-life’?
Ans. Half-life is the time required for half of the radioactive nuclei in a sample to decay.
Q. What are the products of alpha decay?
Ans. Alpha decay produces a new nucleus with a mass number reduced by 4 and an atomic number reduced by 2, along with an alpha particle( 24He).
Q. Explain beta-minus decay.
Ans. Beta-minus decay occurs when a neutron in an unstable nucleus is converted into a proton, an electron (beta particle), and an antineutrino.
Q. What is the significance of binding energy per nucleon in nuclear stability?
Ans. The binding energy per nucleon indicates the stability of a nucleus; higher binding energy per nucleon implies a more stable nucleus.
Q. How does the mass defect of a nucleus relate to its binding energy?
Ans. The mass defect is the difference between the mass of a nucleus and the sum of its constituent nucleons. This mass defect, converted to energy, corresponds to the binding energy holding the nucleus together.
Q. What are isotopes and give an example?
Ans. Isotopes are atoms of the same element with different numbers of neutrons. Example: Carbon-12 and Carbon-14 are isotopes of Carbon.
More Resources for Class 12
- NCERT Exemplar Solutions for Class 12 Chemistry
- NCERT Exemplar Solutions for Class 12 Biology
- NCERT Exemplar Solutions for Class 12 Maths
Topics Discussed in Class 12 Chapter 13 Nuclei
- Introduction
- Atomic Masses and Composition of Nucleus
- Size of the Nucleus
- Mass-energy and Nuclear Binding Energy
- Mass-energy
- Nuclear binding energy
- Nuclear Force
- Radioactivity
- Law of radioactive decay
- Alpha decay
- Beta-decay
- Gamma decay
- Nuclear Energy
- Fission
- Nuclear reactor
- Nuclear fusion – Energy generation in stars
- Controlled thermonuclear fusion
