12th Physics - Book Back Answers - Unit 3 Magnetism and Magnetic Effects of Electric Current - English Medium Guides

  

 

Plus Two / 12th Physics - Book Back Answers - Unit 3 - English Medium

Tamil Nadu Board 12th Standard Physics - Unit 3: Book Back Answers and Solutions

    This post covers the book back answers and solutions for Unit 3 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 3 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. The magnetic field at the centre O of the following current loop is

(a) $\frac{{\mu }_{o}I}{4r}\otimes$ 

(b) $\frac{{\mu }_{o}I}{4r}\odot$

(c) $\frac{{\mu }_{o}I}{2r}\otimes$

(d) $\frac{{\mu }_{o}I}{r}\odot$

Answer Key: 

(a) $\frac{{\mu }_{o}I}{4r}\otimes$ 

2. An electron moves in a straight line inside a charged parallel plate capacitor of uniform charge density σ. The time taken by the electron to cross the parallel plate capacitor undeflected when the plates of the capacitor are kept under constant magnetic field of induction ( )

(a) ${ϵ}_{o }\frac{elB}{\sigma }$  

(b) ${ϵ}_{o }\frac{lB}{\sigma l}$

(c) ${ϵ}_{o }\frac{lB}{e\sigma }$

(d) ${ϵ}_o \frac{lB}{\sigma }$

Answer Key: 

(d) ${ϵ}_o \frac{lB}{\sigma }$ 

 

3. A particle having mass m and charge q accelerated through a potential difference V. Find the force experienced when it is kept under perpendicular magnetic field $\overrightarrow{B}$

(a) $\frac{\sqrt{2{\mathrm{q}}^3\mathrm{BV}}}{\mathrm{m}}$ 

(b) $\frac{\sqrt{{\mathrm{q}}^3{\mathrm{B}}^2\mathrm{V}}}{2\mathrm{m}}$

(c) $\frac{\sqrt{2{\mathrm{q}}^3{\mathrm{B}}^2\mathrm{V}}}{2\mathrm{m}}$

(d) $\frac{\sqrt{2{\mathrm{q}}^3\mathrm{BV}}}{{\mathrm{m}}^3}$

Answer Key: 

(c) $\frac{\sqrt{2{\mathrm{q}}^3{\mathrm{B}}^2\mathrm{V}}}{2\mathrm{m}}$ 

 

4. A circular coil of radius 5 cm and 50 turns carries a current of 3 ampere. The magnetic dipole moment of the coil is nearly
(a) 1.0 A $m^2$ 

(b) 1.2 A $m^2$
(c) 0.5 A $m^2$

(d) 0.8 A $m^2$

Answer Key: 

(b) 1.2 A $m^2$ 

 
5. A thin insulated wire forms a plane spiral of N = 100 tight turns carrying a current I = 8 m A (milli ampere). The radii of inside and outside turns are a = 50 mm and b = 100 mm respectively.
The magnetic induction at the centre of the spiral is
(a) 5 μT 

(b) 7 μT
(c) 8 μT 

(d) 10 μT

Answer Key: 

(b) 7 μT 

 
6. Three wires of equal lengths are bent in the form of loops. One of the loops is circle, another is a semi-circle and the third one is a square. They are placed in a uniform magnetic field and same electric current is passed through them. Which of the following loop configuration will experience greater torque ?
(a) Circle 

(b) Semi-circle
(c) Square 

(d) All of them

Answer Key: 

(a) Circle  

  
7. Two identical coils, each with N turns and radius R are placed coaxially at a distance R as shown in the figure. If I is the current passing through the loops in the same direction, then the 

(a) $\frac{8\mathrm{N}{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\sqrt{5 }\mathrm{R}}$

(b) $\frac{8\mathrm{N}{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{5^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\mathrm{R}}$

(c) $\frac{8\mathrm{N}{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{5\mathrm{R}}$

(d) $\frac{4\mathrm{N}{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\sqrt{5}\mathrm{R}}$

Answer Key: 

(b) $\frac{8\mathrm{N}{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{5^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\mathrm{R}}$ 

 
8. A wire of length l carrying a current I along the Y direction is kept in a magnetic field given by $\overrightarrow{\mathit{B}}\mathit{=}\frac{\mathit{\beta }}{\sqrt{\mathit{3}}}\mathit{(}\widehat{\mathit{i}}\mathit{+}\widehat{\mathit{j}}\mathit{+}\widehat{\mathit{k}}\mathit{)}\mathit{T}$. The magnitude of Lorentz force acting on the wire is
(a) $\sqrt{\frac{2}{3}}\mathrm{\beta }\mathrm{I}l$

(b) $\sqrt{\frac{1}{3}}\mathrm{\beta }\mathrm{I}l$

(c) $\sqrt{2}\mathrm{\beta }\mathrm{I}l$

(d) $\sqrt{\frac{1}{2}}\mathrm{\beta }\mathrm{I}l$ 

Answer Key: 

(a) $\sqrt{\frac{2}{3}}\mathrm{\beta }\mathrm{I}l$ 

 
9. A bar magnet of length l and magnetic moment ${\mathit{P}}_{\mathit{m}}$ is bent in the form of an arc as shown in figure. The new magnetic dipole moment will be

(a) ${\mathit{p}}_{\mathit{m}}$

(b) $\frac{\mathbf{3}}{\mathit{\pi }}{\mathit{p}}_{\mathit{m}}$
(c) $\frac{\mathbf{2}}{\mathit{\pi }}{\mathit{p}}_{\mathit{m}}$
(d) $\frac{\mathbf{1}}{\mathbf{2}}{\mathit{p}}_{\mathit{m}}$

Answer Key: 
(b) $\frac{\mathbf{3}}{\mathit{\pi }}{\mathit{p}}_{\mathit{m}}$

${\mathit{ }}_{}$
10. A non-conducting charged ring carrying a charge of q, mass m and radius r is rotated about its axis with constant angular speed ω. Find the ratio of its magnetic moment with angular momentum is
(a) $\frac{\mathit{q}}{\mathit{m}}$
(b) $\frac{\mathbf{2}\mathit{q}}{\mathit{m}}$
(c) $\frac{\mathit{q}}{\mathbf{2}\mathit{m}}$
(d) $\frac{\mathit{q}}{\mathbf{4}\mathit{m}}$
Answer Key: 

(c) $\frac{\mathit{q}}{\mathbf{2}\mathit{m}}$ 

 
11. The BH curve for a ferromagnetic material is shown in the figure. The material is placed inside a long solenoid which contains 1000 turns/cm. The current that should be passed in the solenonid to demagnetize the ferromagnet completely is 

(a) 1.00 m A  

(b) 1.25 mA

(c) 1.50 mA

(d) 1.75 mA

Answer Key: 

(c) 1.50 mA

  

12. Two short bar magnets have magnetic moments 1.20 A ${\mathit{m}}^{\mathbf{2}}$ and 1.00 A ${\mathit{m}}^{\mathbf{2}}$ respectively. They are kept on a horizontal table parallel to each other with their north poles pointing towards south. They have a common magnetic equator and are separated by a distance of 20.0 cm. The value of the resultant horizontal magnetic induction at the mid-point O of the line joining their centres is (Horizontal components of Earth’s magnetic induction is 3.6 × ${\mathbf{10}}^{\mathbf{-}\mathbf{5}}$ W${\mathit{m}}^{\mathbf{-}\mathbf{2}}$ (NSEP 2000-2001)
(a) 3.60 × 10${\mathbf{10}}^{\mathbf{-}\mathbf{5}}$ Wb ${\mathbf{m}}^{\mathbf{-}\mathbf{2}}$
(b) 3.5 × 10 Wb ${\mathbf{m}}^{\mathbf{-}\mathbf{2}}$

(c) 2.56 × 10${\mathbf{10}}^{\mathbf{-}\mathbf{4}}$ Wb ${\mathbf{m}}^{\mathbf{-}\mathbf{2}}$

(d) 2.2 × 10 Wb ${\mathbf{m}}^{\mathbf{-}\mathbf{2}}$

Answer Key: 

(c) 2.56 × 10${\mathbf{10}}^{\mathbf{-}\mathbf{4}}$ Wb ${\mathbf{m}}^{\mathbf{-}\mathbf{2}}$

 
13. The vertical component of Earth’s magnetic field at a place is equal to the horizontal component. What is the value of angle of dip at this place?
(a) ${\mathbf{30}}^{\mathbf{°}}$ 

(b) $\mathbf{45}\mathbf{°}$
(c) $\mathbf{60}\mathbf{°}$ 

(d) $\mathbf{90}\mathbf{°}$

Answer Key: 

(b) $\mathbf{45}\mathbf{°}$ 

 
14. A flat dielectric disc of radius R carries an excess charge on its surface. The surface charge density is σ. The disc rotates about an axis perpendicular to its plane passing through the centre
with angular velocity ω. Find the magnitude of the torque on the disc if it is placed in a uniform magnetic field whose strength is B which is directed perpendicular to the axis of rotation
(a) $\frac{\mathbf{1}}{\mathbf{4}}\mathbf{\sigma }\mathbf{\omega }\mathbf{\pi }\mathbf{B}\mathbf{R}$

(b) $\frac{\mathbf{1}}{\mathbf{4}}\mathbf{\sigma }\mathbf{\omega }\mathbf{\pi }{\mathbf{BR}}^{\mathbf{2}}$
(c) $\frac{\mathbf{1}}{\mathbf{4}}\mathbf{\sigma }\mathbf{\omega }\mathbf{\pi }{\mathbf{BR}}^{\mathbf{3}}$

(d) $\frac{\mathbf{1}}{\mathbf{4}}\mathbf{\sigma }\mathbf{\omega }\mathbf{\pi }{\mathbf{BR}}^{\mathbf{4}}$
Answer Key: 

(d) $\frac{\mathbf{1}}{\mathbf{4}}\mathbf{\sigma }\mathbf{\omega }\mathbf{\pi }{\mathbf{BR}}^{\mathbf{4}}$

${\mathbf{ }}^{}$ 

15. The potential energy of magnetic dipole whose dipole moment is
${\overrightarrow{\mathbf{p}}}_{\mathbf{m}}\mathbf{=}\mathbf{(}\mathbf{-}\mathbf{0.5}\widehat{\mathbf{I}}\mathbf{+}\mathbf{0.4}\widehat{\mathbf{J}\mathbf{)}}\mathbf{ }\mathbf{A}{\mathbf{m}}^{\mathbf{2}}$ kept in uniform magnetic field $\overrightarrow{\mathbf{B}}\mathbf{=}\mathbf{0.2}\widehat{\mathbf{i}}\mathbf{T}$   

(a) –0.1 J 

(b) –0.8 J
(c) 0.1 J 

(d) 0.8 J  

Answer Key:  
(c) 0.1 J  

II. Short answer questions.

12th Physics

III. Long answer questions. 

12th Physics

IV. Exercises.   

12th Physics