# NCERT Exemplar Solutions for CBSE Class 12 Physics ‒ Chapter 5: Magnetism and Matter (SA)

Mayank UttamNCERT Exemplar Solutions for CBSE Class 12 Physics are available here. With this article, you will get solutions from question number 5.16 to question number 5.20. Solutions of multiple choice questions with single correct answer **(MCQ I)**, multiple choice questions with multiple correct answer **(MCQ II) **and very short answer type questions **(VSA)** are already available. Solutions of long answer type questions of this chapter will be available in other part.

*NCERT Exemplar Solutions for CBSE Class 12 Physics, Chapter 5 (from question number 5.16 to 5.20) **are given below:*

**Question 16:** Verify the Gauss's Law for magnetic field of a point dipole of dipole moment **m** at the origin for the surface which is a sphere of radius *R.*

**Solution 16:**

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**Question 17: **Three identical bar magnets are rivetted together at centre in the same plane as shown in Fig. 5.1. This system is placed at rest in a slowly varying magnetic field. It is found that the system of magnets does not show any motion. The north-south poles of one magnet is shown in the figure. Determine the poles of the remaining two.

**Solution 17:**

When the net force on the system is zero and net torque on the system is also zero then the system is in stable equilibrium. This is possible only when the poles of the remaining two magnets are as shown in the figure given below\

**Question 18: **Suppose we want to verify the analogy between electrostatic and magnetostatic by an explicit experiment. Consider the motion of (i) electric dipole **p** in an electrostatic field **E** and (ii) magnetic dipole **M** in a magnetic field **B**. Write down a set of conditions on **E**, **B**, **p**, **M** so that the two motions are verified to be identical. (Assume identical initial conditions).

**Solution 18:**

Suppose that the angle between **M** and *B* is q.

Torque on magnetic dipole moment **M** in magnetic field **B**,

τ*' = MB sin *q

Two motions will be identical, if** ***p**E* sin q = MB sin q

⇒ *pE = MB *,..(i)

But, *E = cB*

Putting this value in Eq, (i).

*pcB= MB*

⇒ *p* = *M*/*c*.

**Question 19: **A bar magnet of magnetic moment *M *and moment of inertia *I *(about centre, perpendicular to length) is cut into two equal pieces, perpendicular to length. Let *T *be the period of oscillations of the original magnet about an axis through the mid-point, perpendicular to length, in a magnetic field *B. *What would be the similar period *T’* for each piece?

**Solution 19:**

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**Question 20:** Use (i) the Ampere's law for H and (ii) continuity of lines of **B**, to conclude that inside a bar magnet, (a) lines of **H** run from the *N*-pole to *S- *pole, while (b) lines of **B** must run from the *S*-pole to *N*-pole.

**Solution 20:**

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**NCERT Solutions for CBSE Class 12 Physics: All Chapters**