**Nuclei Important Questions for CBSE Class 12 Physics Mass Defect and Binding Energy**

**3.**Binding Energy The binding energy of nucleus is defined as the minimum energy required to separate its nucleons and place them at rest and infinite distance apart.

**Previous Year Examinations Questions**

**2 Marks Questions**

**1.Using the curve for the binding energy per nucleon as a function of mass number A, state clearly how the release in energy in the processes of nuclear fission and nuclear fusion can be explained. [All India 2011]**

**Ans.**

Explanation of Release of Energy in Nuclear Fission and Fusion The curve reveals that binding energy per nucleon is smaller for heavier nuclei than the middle level nuclei. This shows that heavier nuclei are less stable than middle level nuclei. In nuclear fission, binding energy per nucleon of reactants (heavier nuclei) changes from nearly 7.6 MeV to 8.4 MeV (for nuclei of middle level mass).Higher value of the binding energy of the nuclear product results in the liberation of energy during the phenomena of nuclear fission

**2.Draw a plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions which you can draw regarding the nature of nuclear forces. [All India 2010]**

**Ans.**

**3.Draw a plot of the binding energy per nucleon as a function of mass number for a large number of nuclei 20 > A > 240 . How do you explain the constancy of binding energy per nucleon in the range of 30<A<170 using the property that nuclear force is short-ranged? [All India 2010]**

**Ans.**

Explanation of Release of Energy in Nuclear Fission and Fusion The curve reveals that binding energy per nucleon is smaller for heavier nuclei than the middle level nuclei. This shows that heavier nuclei are less stable than middle level nuclei. In nuclear fission, binding energy per nucleon of reactants (heavier nuclei) changes from nearly 7.6 MeV to 8.4 MeV (for nuclei of middle level mass).Higher value of the binding energy of the nuclear product results in the liberation of energy during the phenomena of nuclear fission

The binding energy per nucleon in the range of 30 < A < 170 has average binding energy per nucleon = 8.5 MeV. The higher value of binding energy per nucleon is due to stability of these nucleons. Neutron-proton ratio is higher in this range of mass number which leads to stability of the nuclei. Also, the nuclear force is strongly attractive enough to overcome the coulombian repulsive force acting between positively charged protons,

**4.A heavy nucleus X of mass number 240 and binding energy per nucleon 7.6 MeV is splitted into two fragments Y and Z of mass numbers 110 and 130. The binding energy of nucleons in Y and Z is 8.5 MeV per nucleon. Calculate the energy released per fission in MeV. [hots; Delhi 2010]**

**Ans.**

**Ans.**

**6.(i) The mass of a nucleus in its ground state is always less than the total mass of its constituents neutrons and protons. Explain, (ii) Plot a graph showing the variation of potential energy of a pair of nucleons as a function of their separation. [All India 2009]**

**Ans.**

(ii)

**Ans.**

**8.Sketch a graph showing the variation of binding energy per nucleon as a function of mass number Afor large number of nuclei. State briefly from which region of the graph can release of energy in the process of nuclear fusion be explained.[Foreign 2008]**

**Ans.**

Explanation of Release of Energy in Nuclear Fission and Fusion The curve reveals that binding energy per nucleon is smaller for heavier nuclei than the middle level nuclei. This shows that heavier nuclei are less stable than middle level nuclei. In nuclear fission, binding energy per nucleon of reactants (heavier nuclei) changes from nearly 7.6 MeV to 8.4 MeV (for nuclei of middle level mass).Higher value of the binding energy of the nuclear product results in the liberation of energy during the phenomena of nuclear fission

**Ans.**

the fast neutrons using appropriate substance namely moderator into slow thermal neutrons.Nuclei which have comparable mass to that of neutrons should be preferable be used to slow down fast neutrons. It is due to the fact that the elastic collision between fast neutrons and slow moving protons lead to interchange the velocities

**Ans.**

**11.Draw a plot of potential energy between a pair of nucleons as a function of their separation. Mark the regions where potential energy is**

** (i)positive and**

** (ii)negative.[Delhi 2013]**

**Ans.**

**12.Answer the following.**

** (i)Why is the binding energy per nucleon found to be constant for nuclei in the range of mass number (A) lying between 30 and 170?**

** (ii)When a heavy nucleus with mass number A = 240 breaks into two nuclei, A = 120, energy is released in the process. [Foreign 2012]**

**Ans.**

**Ans.**

**14.(i)What characteristic property of nuclear force explains the constancy of binding energy per nucleon (BE/A) in the range of mass number A lying 30 < A < 170?**

** (ii) Show that the density of nucleus over a wide range of nuclei is constant and independent of mass number A.[Delhi 2012]**

**Ans.**(i)The saturation effect of nuclear force explains the constancy of BE/A over wide range of mass number, 70 > A > 30. Saturation effect imply that nucleon interacts only with its neighbouring nucleons and does not interact with nucleons which are not in direct contact with it.

**15.Draw a plot of potential energy of a pair of nucleons as a function of their separations. Mark the regions where the nuclear force is (i) attractive and (ii) repulsive. Write any two characteristic features of nuclear forces. [All India 2012]**

**Ans.**

**16.Explain giving necessary reactions, how energy is released during**

** (i)fission (ii) fusion [All India 2011 c]**

**Ans.**

**5 Marks Questions**

**17.(i) Draw the plot of binding energy per nucleon (BE/A) as a function of mass number A. Write two important conclusions that Can be drawn regarding the nature of nuclear force.**

** (ii)Use this graph to explain the release of energy in both the processes of nuclear fusion and fission.**

** **

**Ans.**

nucleon, nucleons are tightly bound. This implies that energy will be released in the process which justifies the energy released in fission reaction

**Nuclear Fusion** When two light nuclei (A < 10) are combined to form a heavier nuclei, the binding energy of the fused heavier nuclei is more that the binding energy per nucleon of the lighter nuclei. Thus, the final system is more tightly bound than the initial system. Again the energy will be released in fusion reaction.

**18.Define the O-value of a nuclear process. When can a nuclear process not proceed spontaneously? If both the number of protons and the number of neutrons are conserved in a nuclear reaction in what way is mass converted into energy (or vice-versa) in the nuclear reaction? [All India 2010 c ]**

**Ans.**

**19.Draw a plot of binding energy per nucleon (BE/A) versus mass number (A) for a large number of nuclei lying between 2 < A < 240. Using this graph, explain clearly how the energy is released in both the process of nuclear fission and fusion? [All India 2009 C]**

**Ans.**

nucleon, nucleons are tightly bound. This implies that energy will be released in the process which justifies the energy released in fission reaction

**Nuclear Fusion** When two light nuclei (A < 10) are combined to form a heavier nuclei, the binding energy of the fused heavier nuclei is more that the binding energy per nucleon of the lighter nuclei. Thus, the final system is more tightly bound than the initial system. Again the energy will be released in fusion reaction.

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