CBSE-XI-Physics
46: The Nucleus
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- #Section : i
- Qstn #1If neutrons exert only attractive force, why don’t we have a nucleus containing neutrons alone?Ans : Nuclear forces are short range strong attractive forces that act between two proton-proton, neutron-proton and neutron-neutron pairs. Two protons have strong nuclear forces between them and also exert electrostatic repulsion on each other. However, electrostatic forces are long ranged and have very less effect as compared to the strong nuclear forces.
So, in a nucleus (that is very small in dimension), there's no such significance of repulsive force as compared to the strong attractive nuclear force. On the other hand, an atom contains electrons revolving around its nucleus. These electrons are kept in their orbit by the strong electrostatic force that is exerted on them by the protons present inside the nucleus. Hence, a nucleus contains protons as well as neutrons.
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- Qstn #2Consider two pairs of neutrons. In each pair, the separation between the neutrons is the same. Can the force between the neutrons have different magnitudes for the two pairs?Ans : Neutrons are chargeless particles and they exert only short range nuclear forces on each other. If we have two pairs of neutrons and the separation between them is same in both the pairs. The force between the neutrons will be of same magnitude for the two pairs until there is some other influence on any of them.
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- Qstn #3A molecule of hydrogen contains two protons and two electrons. The nuclear force between these two protons is always neglected while discussing the behaviour of a hydrogen molecule. Why?Ans : Inside the nucleus, two protons exert nuclear force on each other. These forces are short-ranged (a few fm), strong and attractive forces. They also exert electrostatic repulsive force (long-ranged). While discussing the behaviour of a hydrogen molecule, the nuclear force between the two protons is always neglected. This is because the separation between the two protons in the molecule is `` ~``70 pm which is much greater than the range of the nuclear force.
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- Qstn #4Is it easier to take out a nucleonAns : Binding energy per nucleon of a nucleus is defined as the energy required to break-off a nucleon from it.
- #4-afrom carbon or from ironAns : As the binding energy per nucleon of iron is more than that of carbon, it is easier to take out a nucleon from carbon than iron.
- #4-bfrom iron or from lead?Ans : As the binding energy per nucleon of iron is more than that of lead. Therefore, it is easier to take out a nucleon from lead as compared to iron.
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- Qstn #5Suppose we have 12 protons and 12 neutrons. We can assemble them to form either a 24Mg nucleus or two 12C nuclei. In which of the two cases more energy will be liberated?Ans : If we assemble 6 protons and 6 neutrons to form 12C nucleus, 92.15 MeV (product of mass number and binding energy per nucleon of carbon-12) of energy is released. Therefore, the energy released in the formation of two carbon nuclei is 184.3 MeV. On the other hand, when 12 protons and 12 neutrons are combined to form a 24Mg atom, 198.25 MeV of energy (binding energy) is released. Hence, in case of ​24Mg nucleus, more energy is liberated.
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- Qstn #6What is the difference between cathode rays and beta rays? When the two are travelling in space, can you make out which is the cathode ray and which is the beta ray?Ans : Cathode rays consist of electrons that are accelerated using electrodes. They do not carry high energy and do not harm human body. On the other hand, beta rays consist of highly energetic electrons that can even penetrate and damage human cells. Beta rays are produced by the decay of radioactive nuclei. If the two are travelling in space, they can be distinguished by the phenomenon named production of Bremsstrahlung radiation, which is produced by the deceleration of a high energy particle when deflected by another charged particle, leading to the emission of blue light. Only beta rays are capable of producing it.
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- Qstn #7If the nucleons of a nucleus are separated from each other, the total mass is increased. Where does this mass come from?Ans : When the nucleons of a nucleus are separated, a certain amount of energy is to be given to the nucleus, which is known as the binding energy.
Binding energy = [(Number of nucleons) `` \times `` (Mass of a nucleon) - (Mass of the nucleus)]
When the nucleons of a nucleus are separated, the increase in the total mass comes from the binding energy, which is given to the nucleus to break-off its constituent nucleons as energy is related to mass by the relation given below.
E = Δmc2
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- Qstn #8In beta decay, an electron (or a positron) is emitted by a nucleus. Does the remaining atom get oppositely charged?Ans : In beta decay, a neutron from the nucleus is converted to a proton releasing an electron and an antineutrino or a proton is converted to a neutron releasing a positron and a neutrino.
`` \,\mathrm{\,i.e.\,}{\beta }^{-}\,\mathrm{\,decay\,}:n\to p+e+\stackrel{¯}{\nu }``
`` {\beta }^{+}\,\mathrm{\,decay\,}:p\to n+{e}^{+}+\nu ``
`` ``
Since the number of valence electrons present in the parent atom do not change, the remaining atom does not get oppositely charged. Instead, due to a change in the atomic number, there's a formation of a new element.
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- Qstn #9When a boron nucleus
(B510)is bombarded by a neutron, a α-particle is emitted. Which nucleus will be formed as a result?Ans : It is given that when a boron nucleus `` \left({}_{5}{}^{10}\,\mathrm{\,B\,}\right)`` is bombarded by a neutron, an α-particle is emitted.
Let X nucleus be formed as a result of the bombardment.
According to the charge and mass conservation,
`` {}_{5}{}^{10}B+{}_{0}{}^{1}n\to X+{}_{2}{}^{4}He``
`` \,\mathrm{\,Charge\,}:50\overline{)3}2``
`` \,\mathrm{\,Mass\,}:101\overline{)7}4``
`` ``
The mass number of X should be 7 and its atomic number should be 3.
`` \therefore X={}_{3}{}^{7}Li``
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- Qstn #10Does a nucleus lose mass when it suffers gamma decay?Ans : Gamma rays consist of photons that are produced when a nucleus from its excited state comes to its ground state releasing energy. Since gamma rays are chargeless and massless particles, the nucleus does not suffer any loss in mass during the gamma decay.
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- Qstn #11In a typical fission reaction, the nucleus is split into two middle-weight nuclei of unequal masses. Which of the two (heavier or lighter) has greater kinetic energy? Which one has greater liner momentum?Ans : Two photons having equal liner momentum have equal wavelengths as here for both the photons the direction and magnitude of linear momentum will be same. For the rest of the options, magnitude will be same but nothing can be said about the direction of the photons.
Hence the correct option is D.
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