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Tamil Nadu Board 12th Standard Physics - Unit 5: Book Back Answers and Solutions

    This post covers the book back answers and solutions for Unit 5 from the Tamil Nadu State Board 12th Standard Physics textbook. These detailed answers have been carefully prepared by our expert teachers at KalviTips.com.

    We have explained each answer in a simple, easy-to-understand format, highlighting important points step by step under the relevant subtopics. Students are advised to read and memorize these subtopics thoroughly. Once you understand the main concepts, you’ll be able to connect other related points with real-life examples and confidently present them in your tests and exams.

    By going through this material, you’ll gain a strong understanding of Unit 5 along with the corresponding book back questions and answers (PDF format).

Question Types Covered:

• 1 Mark Questions: Choose the correct answer, Fill in the blanks, Identify the correct statement, Match the following 
• 2 Mark Questions: Answer briefly 
• 3, 4, and 5 Mark Questions: Answer in detail

All answers are presented in a clear and student-friendly manner, focusing on key points to help you score full marks.

All the best, Class 12 students! Prepare well and aim for top scores. Thank you!

I. Multiple choice questions.

1. Suppose an alpha particle accelerated by a potential of V volt is allowed to collide with a nucleus of atomic number Z, then the distance of closest approach of alpha particle to the nucleus is

(a) $\mathbf{14.4}\frac{\mathbf{Z}}{\mathbf{V}}\mathbf{\AA }$ 

(b) $\mathbf{14.4}\frac{\mathbf{V}}{\mathbf{Z}}\mathbf{\AA }$
(c) $\mathbf{1.44}\frac{\mathbf{Z}}{\mathbf{V}}\mathbf{\AA }$

(d) $\mathbf{1.44}\frac{\mathbf{V}}{\mathbf{Z}}\mathbf{\AA }$

$\mathbf{<b>Answer\; Key:</b>}$

(b) $\mathbf{14.4}\frac{\mathbf{V}}{\mathbf{Z}}\mathbf{\AA }$$\mathbf{ }$

$\mathbf{ }$
2. In a hydrogen atom, the electron revolving in the fourth orbit, has angular momentum equal to
(a) h

(b) $\frac{\mathbf{h}}{\mathbf{\pi }}$
(c) $\frac{\mathbf{4}\mathbf{h}}{\mathbf{\pi }}$
(d) $\frac{\mathbf{2}\mathbf{h}}{\mathbf{\pi }}$

$\mathbf{<b>Answer\; Key:</b>}$

(d) $\frac{\mathbf{2}\mathbf{h}}{\mathbf{\pi }}$

$\frac{\mathbf{ }}{}$$\mathbf{<b> </b>}$$\frac{\mathbf{ }}{}$
3. Atomic number of H-like atom with ionization potential 122.4 V for n = 1 is

(a) 1 

(b) 2 

(c) 3 

(d) 4

$\mathbf{<b>Answer\; Key:</b>}$

(c) 3 $\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$ 

4. The ratio between the radius of first three orbits of hydrogen atom is
(a) 1:2:3 

(b) 2:4:6
(c)1:4:9 

(d)1:3:5

$\mathbf{<b>Answer\; Key:</b>}$

(c)1:4:9$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
5. The charge of cathode rays particle is
(a) positive 

(b) negative
(c) neutral 

(d) not defined

$\mathbf{<b>Answer\; Key:</b>}$

(b) negative$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
6. In J.J. Thomson e/m experiment, electrons are accelerated through 2.6 kV enter the region of crossed electric field and magnetic field of strength 3.0 × ${\mathbf{10}}^{\mathbf{4}}$ Vm–1 and 1.0 × ${\mathbf{10}}^{\mathbf{-}\mathbf{3}}$ T, respectively, and pass through it and undeflected, then the specific charge is

(a) 1.6 × ${\mathbf{10}}^{\mathbf{10} \mathbf{C}\mathbf{ }\mathbf{k}{\mathbf{g}}^{\mathbf{-}\mathbf{1}}}$
(b) 1.7 × ${\mathbf{10}}^{\mathbf{11} \mathbf{C}\mathbf{ }\mathbf{k}{\mathbf{g}}^{\mathbf{-}\mathbf{1}}}$

(c) 1.5 × ${\mathbf{10}}^{\mathbf{11} \mathbf{C}\mathbf{ }\mathbf{k}{\mathbf{g}}^{\mathbf{-}\mathbf{1}}}$
(d) 1.8 × ${\mathbf{10}}^{\mathbf{11} \mathbf{C}\mathbf{ }\mathbf{k}{\mathbf{g}}^{\mathbf{-}\mathbf{1}}}$

$\mathbf{<b>Answer\; Key:</b>}$

(b) 1.7 × ${\mathbf{10}}^{\mathbf{11} \mathbf{C}\mathbf{ }\mathbf{k}{\mathbf{g}}^{\mathbf{-}\mathbf{1}}}$$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$${{\mathbf{ }}^{}}^{}$
7. The ratio of the wavelengths radiation emitted for the transition from n =2 to n = 1 in ${\mathrm{Li}}^{++}$, ${\mathrm{H}}^{+}$ and H is
(a)1: 2: 3 

(b) 1: 4: 9
(c) 3:2:1 

(d) 4: 9: 36

$\mathbf{<b>Answer\; Key:</b>}$

(d) 4: 9: 36$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
8. The electric potential of an electron is given by $\mathbf{V}\mathbf{=}{\mathbf{V}}_{\mathbf{0}}\mathbf{ln}\left(\frac{\mathbf{r}}{{\mathbf{r}}_{\mathbf{0}}}\right)$, where ${\mathbf{r}}_{\mathbf{0}}$ is a constant. If Bohr atom model is valid, then variation of radius of nth orbit ${\mathbf{r}}_{\mathbf{n}}$ with the principal quantum number n is
(a) ${\mathbf{r}}_{\mathbf{n}}\mathbf{\propto }\frac{\mathbf{1}}{\mathit{n}}$ 

(b) ${\mathbf{r}}_{\mathbf{n}}\mathbf{\propto }\mathit{n}$
(c) ${\mathbf{r}}_{\mathbf{n}}\mathbf{\propto }\frac{\mathbf{1}}{{\mathit{n}}^{\mathit{2}}}$ 

(d)

$\mathbf{<b>Answer\; Key:</b>}$

(b) ${\mathbf{r}}_{\mathbf{n}}\mathbf{\propto }\mathit{n}$$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$${\mathbf{<math\; xmlns="http://www.w3.org/1998/Math/MathML"><msup><mn\; mathvariant="bold"> </mn></msup></math>}}_{}$
9. If the nuclear radius of ${\mathbf{27}}_{\mathbf{A}\mathbf{l}}$ is 3.6 fermi, the approximate nuclear radius of ${64}_{\mathbf{C}\mathbf{u}}$ in fermi is
(a) 2.4 

(b) 1.2
(c) 4.8 

(d) 3.6

$\mathbf{<b>Answer\; Key:</b>}$

(c) 4.8 $\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
10. The nucleus is approximately spherical in shape. Then the surface area of nucleus having mass number A varies as
(a) ${\mathbf{A}}^{\raisebox{1ex}{$\mathbf{2}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{3}$}\right.}$ 

(b) ${\mathbf{A}}^{\raisebox{1ex}{$\mathbf{4}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{3}$}\right.}$
(c) ${\mathbf{A}}^{\raisebox{1ex}{$\mathbf{1}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{3}$}\right.}$ 

(d) ${\mathbf{A}}^{\raisebox{1ex}{$\mathbf{5}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{3}$}\right.}$

$\mathbf{<b>Answer\; Key:</b>}$

(a) ${\mathbf{A}}^{\raisebox{1ex}{$\mathbf{2}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{3}$}\right.}$ $\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
11. The mass of a $\mathrm{Li}$ nucleus is 0.042 u less than the sum of the masses of all its nucleons. The average binding energy per nucleon of $\mathrm{Li}$ nucleus is nearly
(a) 46 MeV 

(b) 5.6 MeV
(c) 3.9MeV 

(d)23 MeV

$\mathbf{<b>Answer\; Key:</b>}$

(b) 5.6 MeV$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$ 

12. ${\mathbf{M}}_{\mathbf{p}}$ denotes the mass of the proton and ${\mathbf{M}}_{\mathbf{n}}$ denotes mass of a neutron. A given nucleus of binding energy B, contains Z protons and N neutrons. The mass M(N,Z) of the nucleus is given by(where c is the speed of light)
(a) $\mathbf{M}\mathbf{(}\mathbf{N}\mathbf{,}\mathbf{Z}\mathbf{)}\mathbf{=}\mathbf{N}{\mathbf{M}}_{\mathbf{n}}\mathbf{+}\mathit{Z}{\mathbf{M}}_{\mathbf{p}}\mathbf{-}\mathit{B}{\mathit{c}}^{\mathbf{2}}$
(b) $\mathbf{M}\left(\mathbf{N}\mathbf{,}\mathbf{Z}\right)\mathbf{=}{\mathbf{NM}}_{\mathbf{n}}\mathbf{+}\mathit{Z}{\mathbf{M}}_{\mathbf{p}}\mathbf{+}\mathit{B}{\mathit{c}}^{\mathbf{2}}$
(c) $\mathbf{M}\mathbf{(}\mathbf{N}\mathbf{,}\mathbf{Z}\mathbf{)}\mathbf{=}\mathbf{N}{\mathbf{M}}_{\mathbf{n}}\mathbf{+}\mathbf{Z}{\mathbf{M}}_{\mathbf{p}}\mathbf{-}\raisebox{1ex}{$\mathbf{B}$}\!\left/ \!\raisebox{-1ex}{${\mathbf{c}}^{\mathbf{2}}$}\right.$
(d) $\mathbf{M}\left(\mathbf{N}\mathbf{,}\mathbf{Z}\right)\mathbf{=}{\mathbf{NM}}_{\mathbf{n}}\mathbf{+}{\mathbf{ZM}}_{\mathbf{p}}\mathbf{+}\raisebox{1ex}{$\mathbf{B}$}\!\left/ \!\raisebox{-1ex}{${\mathbf{c}}^{\mathbf{2}}$}\right.$

$\mathbf{<b>Answer\; Key:</b>}$

(c) $\mathbf{M}\mathbf{(}\mathbf{N}\mathbf{,}\mathbf{Z}\mathbf{)}\mathbf{=}\mathbf{N}{\mathbf{M}}_{\mathbf{n}}\mathbf{+}\mathbf{Z}{\mathbf{M}}_{\mathbf{p}}\mathbf{-}\raisebox{1ex}{$\mathbf{B}$}\!\left/ \!\raisebox{-1ex}{${\mathbf{c}}^{\mathbf{2}}$}\right.$$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
13. A radioactive nucleus (initial mass number A and atomic number Z) emits two α-particles and 2 positrons. The ratio of number of neutrons to that of proton in the final nucleus will be
(a) $\frac{\mathrm{A}-\mathrm{Z}-4}{\mathrm{Z}-2}$
(b) $\frac{\mathrm{A}-\mathrm{Z}-2}{\mathrm{Z}-6}$
(c) $\frac{\mathrm{A}-\mathrm{Z}-4}{\mathrm{Z}-6}$
(d) $\frac{\mathrm{A}-\mathrm{Z}-12}{\mathrm{Z}-4}$
$\mathbf{<b>Answer\; Key:</b>}$

(b) $\frac{\mathrm{A}-\mathrm{Z}-2}{\mathrm{Z}-6}$$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
14. The half-life period of a radioactive element A is same as the mean life time of another radioactive element B. Initially both have the same number of atoms. Then
(a) A and B have the same decay rate initially
(b) A and B decay at the same rate always
(c) B will decay at faster rate than A
(d) A will decay at faster rate than B.

$\mathbf{<b>Answer\; Key:</b>}$

(c) B will decay at faster rate than A$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$
15. A radiative element has N0 number of nuclei at t=0. The number of nuclei remaining after half of a half-life (that is, at time $\mathbf{t}\mathbf{=}\frac{\mathbf{1}}{\mathbf{2}}{\mathbf{T}}_{\frac{\mathbf{1}}{\mathbf{2}}}$
(a) $\frac{{\mathbf{N}}_{\mathbf{0}}}{\mathbf{2}}$
(b) $\frac{{\mathbf{N}}_{\mathbf{0}}}{\sqrt{\mathbf{2}}}$
(c) $\frac{{\mathbf{N}}_{\mathbf{0}}}{\mathbf{4}}$
(d) $\frac{{\mathbf{N}}_{\mathbf{0}}}{\mathbf{8}}$

$\mathbf{<b>Answer\; Key:</b>}$

(b) $\frac{{\mathbf{N}}_{\mathbf{0}}}{\sqrt{\mathbf{2}}}$$\mathbf{<b> </b>}$

$\mathbf{<b> </b>}$ 

II. Short answer questions.

12th Physics

III. Long answer questions. 

12th Physics

IV. Exercises.   

12th Physics