Modern Physics Questions and Answers – Page 3

Binding energy is a crucial concept in nuclear physics. Option B is correct: Binding energy of a nucleus is the energy released when a nucleus is formed by bringing together its constituent protons and neutrons (nucleons) from infinity. Equivalently, it is the energy required to completely disassemble a nucleus into its individual nucleons. A more stable nucleus has higher binding energy. Iron-56 has the highest binding energy per nucleon (~8.8 MeV) and is therefore the most stable nucleus. Option A is incorrect: Energy required to remove an electron from an atom is called ionization energy, which is an atomic property, not a nuclear property. It is measured in electron volts and is much smaller. Option C is incorrect: Energy stored in chemical bonds is chemical bond energy, which involves electron interactions between atoms. It is millions of times smaller than nuclear binding energy. Option D is incorrect: Kinetic energy of electrons relates to their movement in atomic orbitals, which is described by quantum mechanics and is not binding energy of the nucleus.

Uranium-235 is the most widely used nuclear fuel in reactors worldwide. Option C is correct: U-235 is a fissile material, meaning it can sustain a nuclear chain reaction when bombarded with slow neutrons. Natural uranium contains only 0.7% U-235 and 99.3% U-238. Enriched uranium with 3-5% U-235 is used in most commercial reactors. Option A is incorrect: Plutonium-239 is produced in reactors from U-238 and is used in some fast breeder reactors and nuclear weapons, but it is not the primary fuel in most commercial reactors. It is fissile but not the main fuel used worldwide. Option B is incorrect: Thorium-232 is fertile, not fissile. India has large thorium reserves and is developing thorium-based reactors. Option D is incorrect: Radium-226 is a naturally occurring radioactive element that emits alpha particles. It is not used as nuclear reactor fuel.

Radioactivity is a natural nuclear phenomenon discovered by Henri Becquerel in 1896. Option B is correct: Radioactivity is the spontaneous emission of radiation (alpha, beta, or gamma) from the unstable nuclei of certain elements. This process occurs without any external stimulus and cannot be controlled by physical or chemical means. Option A is incorrect: Emission of visible light by atoms describes phosphorescence or fluorescence, which are optical phenomena, not radioactivity. Option C is incorrect: Absorption of X-rays by matter describes X-ray attenuation, which is completely different from radioactivity. X-rays are not emitted during radioactive decay. Option D is incorrect: Chemical decomposition of compounds is a chemical reaction involving electron bonds, not nuclear processes. Radioactivity is purely a nuclear phenomenon and does not depend on the chemical state or temperature of the substance. Marie Curie and Pierre Curie were the first to study radioactivity systematically.

A chain reaction is a self-sustaining sequence of nuclear fission events. Option B is correct: In nuclear fission, when U-235 absorbs a neutron and splits, it releases 2-3 new neutrons. These new neutrons can each cause fission in other U-235 nuclei, releasing more neutrons, which cause more fissions, and so on. This self-propagating sequence is a chain reaction. If each fission releases exactly 1 neutron that causes another fission, it is a controlled chain reaction (used in reactors). If more than 1 neutron causes further fission, it becomes an uncontrolled chain reaction (atomic bomb). Option A is incorrect: Chain reactions are self-sustaining; they do not require external energy once started. The initial fission provides energy for subsequent reactions. Option C is incorrect: While a controlled chain reaction can be slow, the definition does not restrict it to being slow. An uncontrolled chain reaction is extremely fast and violent. Option D is incorrect: Chain reactions occur in nuclear reactors and bombs on Earth, not only in stars. Nuclear fusion occurs in stars.

Among the three main types of radioactive radiation, gamma rays have the highest penetrating power. Option C is correct: Gamma rays are electromagnetic radiation with very short wavelength and high energy. Option A is incorrect: Alpha particles are helium nuclei (2 protons + 2 neutrons) and have the lowest penetrating power. They have the highest ionizing power but lowest penetration. Option B is incorrect: Beta particles are fast-moving electrons or positrons. They have moderate penetrating power and can be stopped by a few millimetres of aluminium. They are more penetrating than alpha but much less than gamma. Option D is partially correct but X-rays are not radioactive decay products; they are produced by electron transitions. Gamma rays are more energetic than typical X-rays. The order is: Alpha < Beta < Gamma.

Half-life is a fundamental concept in radioactive decay. Option B is correct: The half-life of a radioactive element is the time required for exactly half of the radioactive atoms in a sample to undergo decay. For example, if Carbon-14 has a half-life of 5730 years, half of a C-14 sample will decay in 5730 years, and half of the remaining half will decay in the next 5730 years, and so on. Option A is incorrect: No radioactive substance fully decays to zero in a finite time because the decay is exponential; theoretically, some atoms always remain. Option C is incorrect: Radioactive decay is a disintegration process; atoms do not double. Option D is incorrect: Radioactive decay of individual atoms is a random process governed by probability; it is impossible to predict when a single atom will decay. Half-life is used in carbon dating, nuclear medicine, and determining the age of rocks.

In a nuclear reactor, slow neutrons (also called thermal neutrons) are used to initiate and sustain nuclear fission. Option C is correct: Slow neutrons have the right energy level to be captured efficiently by U-235 nuclei, making them unstable and causing fission. Option A is incorrect: Protons carry positive charge and are repelled by the positively charged nucleus due to electrostatic repulsion, making them ineffective for initiating fission. Option B is incorrect: Alpha particles are helium nuclei with +2 charge and are also repelled by the nucleus due to Coulomb repulsion, making them unsuitable. Option D is incorrect: Beta particles are fast-moving electrons emitted during radioactive decay; they do not initiate nuclear fission. A moderator (heavy water or graphite) is used in reactors to slow down fast neutrons to thermal speeds required for sustained fission.

Nuclear fission is the process in which a heavy nucleus splits into two or more lighter nuclei, releasing enormous energy. Option B is correct: When Uranium-235 absorbs a slow neutron, it becomes unstable and splits into Barium-141 and Krypton-92, releasing 3 neutrons and large amounts of energy. Option A is incorrect: Fusion of two light nuclei describes nuclear fusion, the opposite process, which occurs in stars. Option C is incorrect: Emission of electrons from the nucleus describes beta decay, a type of radioactive decay, not fission. Option D is incorrect: Conversion of proton to neutron describes beta-plus decay or electron capture, not fission. Nuclear fission is the basis of nuclear power plants and atomic bombs.

The correct answer is Option C — Molecule. A molecule is the smallest unit of a substance that can exist independently in a free state and retain all the properties of that substance. For example, H₂O (water molecule) is the smallest independent unit of water. Option A (Electron): An electron is a subatomic particle, NOT a unit of substance. It cannot exist independently as a substance. Option B (Atom): An atom CAN exist independently for noble gases (like He, Ne, Ar), but for most substances, the molecule is the basic unit. An atom is a unit of an element, not a compound. Option D (Ion): An ion is a charged atom/molecule. It cannot exist freely in a neutral state.

The lifetime of a charged pi-meson (pion) is approximately 2.6 × 10⁻⁸ seconds. This is an extremely short lifetime, typical of subatomic particles governed by the strong nuclear force. Option A (10⁻⁴ s): Too long - this is the lifetime of a muon, not a pion. This is not the pion lifetime. Option D (10⁻¹⁰ s): Too short - this is shorter than the actual pion lifetime.