Team Quanta gladly presents all possible short questions of Modern Physics & Electronics’ Chapter#05: Nuclear Physics.
Q.8.1 what is atomic mass unit (a. m. u.)? Show that $1a.\ m.\ u.=1.66\times{10}^{-27}kg$
Answer: Atomic mass unit is unit of mass used in nuclear physics as adopted by international union of pure and applied physics (IUPAP).
$$1a.\ m.\ u.=\frac{1}{12}\left(mass\ of\ carbon\ atom\right)$$
$$\Rightarrow1a.\ m.\ u.=\frac{1}{12}\times\frac{12\times{10}^{-3}kg}{6.023\times{10}^{23}}=\ \frac{1}{12}\times\ 1.99267\times{10}^{-27}kg$$
$$\Rightarrow1a.\ m.\ u.=1.66\times{10}^{-27}kg$$
Q.8.2 Prove that 1a. m. u. =931MeV.
Answer: We know that,
$$1a.\ m.\ u.\ =1.66\times{10}^{-27}kg$$
Using $E={mc}^2,$, we have
$$1a.\ m.\ u.=1.66\times{10}^{-27}\times3\times{10}^8J=1.4925\times{10}^{-10}J$$
$$1a.\ m.\ u.=\frac{1.4925\times{10}^{-10}}{1.6\times{10}^{-19}}eV=931.5\times{10}^6eV\cong931MeV\$$
Q.8.3 how it is possible that a large number of protons excite in a very small space inside nucleus?
Answer: It is due to strongest nuclear force, which is attractive and overcome the electrostatic force of repulsion among protons.
Q.8.4 what are basic difference between alpha, beta, and gamma rays?
Answer: Alpha particles are helium nuclei and very massive. Alpha particles have low penetration power. A beta particle is a negatively charged electron and has very low mass. Penetration power of beta particles is somewhat greater than that of alpha particles. A gamma ray is a high energy photon or high frequency electromagnetic wave and has high penetrating power.
Q.8.5 A student sys in class that a heavy form of hydrogen decays by alpha emission. What will you say about this statement?
Answer: I will tell to student, that heavy form of hydrogen emit an alpha particle because an alpha particle contains two protons and two neutrons whereas hydrogen nucleus contains only one proton.
Q.8.6 If film is kept in a box, alpha particles from a radioactive source outside the box cannot expose the film, but beta particles can. Why?
Answer: Since beta particles have greater penetration power as compared to that of alpha particles, so beta particles can expose the film.
Q.8.7 why would a fusion reactor produce less radioactive waste in comparison with a fission reactor?
Answer: A fusion reactor produces less radioactive waste as compared to fission reactor because fusion reactor uses helium which is not radioactive and is inert.
Q.8.8 An alpha particle has twice the charge of a beta particle. Why does the former deflect less than the latter when passing between electrically charged plates, assuming they both have the same speed?
Answer: Since mass of alpha particle is almost 7000 times greater than that of beta particle, so it is to deflect alpha particle in comparison with beta particle.
Q.8.9 The number of neutrons is more than number of protons in heavy nuclei? Explain why this happen.
Answer: When number of protons becomes large, long range electrostatic force of repulsion will be grater. For nuclear stability, short range nuclear force which is attractive must dominate. Due to this reason, number of neutrons must be greater than number of protons.
Q.8.10 The atomic masses are not whole numbers for most of atom. Why?
Answer: The atomic mass is average mass of different stable element weighted over their natural abundance, which is not a whole number.
Q.8.11 if a heavy nucleus is initially at rest and undergoes alpha decay. Which has more kinetic energy after the decay, the alpha particle or the daughter nucleus?
Answer: Kinetic energy is related to momentum by relation,
$$K=\frac{p^2}{2m}$$
This equation shows that under given conditions, after decay, alpha particle must have much more of the decay energy than daughter nucleus.
Q.8.12 what is half-life of free neutron? Name at least two particles having infinite life time.
Answer: The life time of neutron is 13 minutes. Electron and proton have infinite life time.
Q.8.13 what factors make a fusion reaction difficult to achieve?
Answer: There are two factors in fusion reactor which make difficult to achieve is:
- High plasma density
- A high plasma temperature
Q.8.14 Will a single nucleus emit alpha, beta and gamma rays together?
Answer: No, a single nucleus can emit either alpha particle or beta particle at one time.
Q.8.15 what fraction of a radioactive sample has decayed after two half-lives has elapsed?
Answer: After two half-life, fraction of radioactive sample decayed is,
$$\frac{1}{2}+\frac{1}{2}\left(\frac{1}{2}\right)=\frac{1}{4}$$
Q.8.16 Consider two heavy nuclei X and Y having similar mass numbers. If X has the higher binding energy, which nucleus tends to be more unstable?
Answer: We know that a nucleus with the higher binding energy requires more energy for breaking into its constituents and has less energy to release in decay, so nucleus Y is more unstable.
Q.8.17 Is there any difference between beta particles and electrons emitted by photoelectric emission and thermionic emission?
Answer: Yes, in photoelectric and thermionic emissions, orbital electrons are emitted and kinetic energy is low.
In case of radio activity, beta particles are emitted from nucleus and have high kinetic energy.
Q.8.18 what are thermal neutrons? Why are neutrons considered as ideal particles for nuclear fission?
Answer: The neutrons which are in thermal equilibrium with medium through which they pass are called thermal neutrons. Neutrons are considered as ideal particles because these are neutral and do not experience any Coulomb force of attraction or repulsion and easily enters in the nucleus for fusion.
Q.8.19 Can we separate isotopes of an element by chemical method?
Answer: No, the isotopes of an element cannot be separated by chemical methods because chemical properties of all isotopes of an element are alike. Due to this reason, physical methods are used to separate isotopes.
Q.8.20 why do isotopes of an element have different physical properties, such as mass, but the same chemical properties?
Answer: Isotopes of an element have different numbers of neutrons and same number of electrons. So the physical properties of nuclei, such as mass are different. The chemical properties are governed by electrons, so isotopes of a given element have same chemical behavior.
Review Questions
R.8.1 In beta decay, the energy of the electron or position emitted from the nucleus lies somewhere in a relatively large range of possibilities. In alpha decay however, the alpha particle energy can only have discrete values. Explain this difference.
Answer: In alpha decay, there are only two product particles, alpha particle and the daughter nucleus. There are conservation laws of energy and of momentum to be satisfied. Whereas, in beta decay, there are particles, beta particle, the neutrino (or antineutron), and the daughter nucleus. The energy and momentum may be shared in a variety of ways among the three particles satisfying the conservation laws of energy and of momentum. Due to this reason, energy spectrum of beta particles is continuous.
R.8.2 Do the laws of conservation of energy, momentum and charge remain applicable during a nuclear reaction?
Answer: Yes, laws of conservation of energy, momentum and charge remain applicable during a nuclear reaction.
R.8.3 what is difference between natural and artificial radioactivity?
Answer: The process of emitting radiations randomly and spontaneously is called natural radioactivity.
Radioactivity induced by bombarding stable nuclei with high energy particles is called artificial radioactivity.
R.8.4 Can carbon-14 dating be used to measure the age of a rock?
Answer: Generally carbon dating cannot be used to estimate the age of a rock, because the rock was not alive to receive carbon.
R.8.5 what is chain reaction? Explain.
Answer: We know that when absorbs a neutron; it breaks into two nuclei almost of equal masses along with two or three neutrons and release of energy. This fission reaction can be maintained continuously by proper use of neutrons emitted during fission reaction of . Such a process is called fission chain reaction.
R.8.6 what is role of moderator in the nuclear reactor?
Answer: The role of moderator in nuclear reactor is to slow down the speed of fast neutrons, produced during fission process.
R.8.7 The radioactive element . Since earth is about 4 billion years old, how can you explain why we still can find this element in nature?
Answer: Half-life of ${_{88}^{226}}Ra\ is\ 1.6\times{10}^3y$ but its total life time is infinity. It is due to fact that radioactive decay is a random process that lasts up to infinity until last atom of element has also decayed. Number of half- lives in 4 billion years is,
$$\frac{4\times{10}^8}{1.6\times{10}^3}=2.5\times{10}^6$$
So we can find still radium in nature.
