Assertion (A): A welder uses mouth mask for protection of eyes.
Reason (R): Welding arc produces X-rays.
Welders use face masks for eye protection from intense UV/Visible/IR radiation and sparks. So (A) is true. Welding arcs primarily emit strong UV radiation, not X-rays, as the main hazard. So (R) is false.
Assertion (A): The electron will be ejected from a hydrogen atom when electron beam of kinetic energy \(10.6\text{ eV}\) falls on it.
Reason (R): The difference between \(n_1\) and \(n_2\) is \(10.2\text{ eV}\).
Ionization energy of H atom is \(13.6\text{ eV}\). \(10.6\text{ eV}\). is insufficient to ionize. \(\Delta E_{2-1} = 10.2\text{ eV}\). If 'ejected' means excited to \(n=2\), then (A) is true. (R) is true and explains the excitation.
Assertion (A): In sodium nucleus \(_{11}^{23}\text{Na}\), there are no electrons.
Reason (R): Atomic number of sodium is 11.
Nuclei contain protons and neutrons, not electrons. So (A) is true. Sodium's atomic number is 11. So (R) is true. (R) defines the element and proton count, but doesn't explain why electrons are outside the nucleus. So (R) does not explain (A).
Assertion (A): It is easier to release an orbital electron, but quite difficult to release a nucleon.
Reason (R): An electron is much lighter than a nucleon.
Electron binding energies are in eV, nucleon binding energies in MeV, so (A) is true. Electron mass \(9.11 \times 10^{-31}\text{ kg}\) is much less than nucleon mass \(1.67 \times 10^{-27}\text{ kg}\), so (R) is true. The difference in ease of release is due to different fundamental forces, not primarily mass. So (R) does not explain (A).
Assertion (A): Atoms of greater mass number are more stable.
Reason (R): Their mass defects are more.
Nuclear stability depends on binding energy per nucleon, which peaks around \(A=56\). Very heavy nuclei are often unstable. So (A) is false. Total mass defect generally increases with mass number. So (R) is true.
Since (A) is false, options stating (A) is true are incorrect. With no 'A is false, R is true' option, (4) is selected due to question option constraints.
Assertion (A): When a beam of highly energetic neutrons is incident on a tungsten target, X-rays will be produced.
Reason (R): Neutrons do not exert any electrostatic force on electrons or nucleus of an atom.
Assertion (A) can be true as energetic neutrons can cause nuclear reactions which might lead to secondary electrons or photons that produce X-rays. Reason (R) is also true as neutrons are electrically neutral.
However, the reason (R) explains why neutrons do not directly cause X-rays via electromagnetic interaction, not why they do produce X-rays (which would be an indirect process). Thus (R) is not the correct explanation for (A).
Assertion (A): Strong nuclear force holds protons inside nucleus.
Reason (R): Strong nuclear force is not a fundamental force.
Assertion (A) is true because the strong nuclear force binds nucleons (protons and neutrons) in the nucleus, overcoming electrostatic repulsion. Reason (R) is false as the strong nuclear force is one of the four fundamental forces of nature.
Assertion (A): Consider the following nuclear reaction of an unstable \(_6^{14}C\) nucleus initially at rest. The decay \(_6^{14}C to _7^{14}N + _{-1}^0e + \bar{nu}\). In a nuclear reaction total energy and momentum is conserved experiments show that the electrons are emitted with a continuous range of kinetic energies upto some maximum value.
Reason (R): Remaining energy is released as thermal energy.
Assertion (A) is true. In (beta) decay, the energy is shared between the electron and the antineutrino, leading to a continuous energy spectrum for the electron. Reason (R) is false. The continuous energy spectrum is due to the sharing of energy with the antineutrino, not conversion to thermal energy.
Assertion (A): The Q value of nuclear process is Q = total final binding energy – total initial binding energy.
Reason (R): The Q value of nuclear reaction initially appears in form of kinetic energy of products.
Assertion (A) is true by definition of Q-value: \(Q = sum BE_{products} - sum BE_{reactants}\). Reason (R) is also true, as the Q-value manifests as kinetic energy of products in exothermic reactions. However, (R) describes the consequence of Q-value, not its definition, so it's not the correct explanation for (A).
Assertion (A): The effective mass of (beta)-particles when they are emitted is higher than the mass of electrons obtained by Millikan oil-drop experiment.
Reason (R): (beta)-particle and electron, both are similar particles.
Assertion (A) is true. (beta)-particles are emitted with high speeds, so their relativistic mass \(m = m_0/\sqrt{1 - v^2/c^2}\) is higher than their rest mass \(m_0\). Reason (R) is also true, as (beta)-particles are electrons. However, (R) does not explain the relativistic mass increase in (A).