# NCERT Exemplar Solutions for CBSE Class 12 Physics ‒ Chapter 4: Moving Charges & Magnetism (Part IV)

NCERT Exemplar Solutions for Class12 Physics – Chapter 4 (Moving Charges & Magnetism) are available here. In this article, you will get solutions to short answer type questions i.e., from question number 4.17 to 4.22. Solutions of question number 4.1 to 4.16 are already available in **Part I**, **Part II** and **Part III**. Solutions of very long answer type questions will be available in next part. These questions can be asked in CBSE Class 12 board exams.

*NCERT Exemplar Solutions for CBSE Class 12 Physics, Chapter 4 (from question number 4.17 to 4.22) are given below:*

**Question 4.17:** A current carrying loop consists of 3 identical quarter circles of radius R, lying in the positive quadrants of the x-y, y-z and z-x planes with their centres at the origin, joined together. Find the direction and magnitude of **B** at the origin.

**Solution 4.17:**

**
**

**CBSE Class 12 Video & Toppers' Interview **

**Question 4.18:** A charged particle of charge e and mass m is moving in an electric field **E** and magnetic field **B**. Construct dimensionless quantities and quantities of dimension [T ]^{–1}.

**Solution 4.18:**

No dimensionless quantity can be constructed using given quantities.

**Question 4.19:** An electron enters with a velocity **v** = *v*_{0}** î** into a cubical region (faces parallel to coordinate planes) in which there are uniform electric and magnetic fields. The orbit of the electron is found to spiral down inside the cube in plane parallel to the

*x*-

*y*plane. Suggest a configuration of fields

**E**and

**B**that can lead to it.

**Solution 4.19:**

In the given question, orbit of electron is spiral down inside the cube in a plane parallel to *x*-*y* plane, so, direction of magnetic field is along +*z* direction. Therefore, ** B** = B

_{o}ˆ

**Also, electric field will be along positive**

*k.**x*-axis, i.e.,

**= E**

*E*_{o}

**, where**

*î**E*

_{o}> 0.

**Question 4.20:** Do magnetic forces obey Newton’s third law. Verify for two current elements *dl*_{1} = *dl* ** î** located at the origin and

*dl*_{2}=

*dl*

**located at (0,**

*ĵ**R*, 0). Both carry current

*I*.

**Solution 4.20:**

**
**

**Question 4.21:** A multirange voltmeter can be constructed by using a galvanometer circuit as shown in Fig. 4.2. We want to construct a voltmeter that can measure 2V, 20V and 200V using a galvanometer of resistance 10Ω and that produces maximum deflection for current of 1 mA. Find *R*_{1}, *R*_{2} and *R*_{3} that have to be used.

**Solution 4.21:**

In the given question,

*G* = 10 Ω, *i*_{G} = 1 mA = 10^{‒3} A

For 2 V range,

*i*_{G }(*G* + *R*_{1}) = 2

⇒ *i*_{G} = 2 k Ω

For 20V range,

*i*_{G} (*G* + *R*_{1}+ *R*_{2}) = 20

⇒ *R*_{2} = 18 k Ω

For 200V range,

*i*_{G} (*G* + *R*_{1}+ *R*_{2} + *R*_{3}) = 200

⇒ *R*_{3} = 180 k Ω.

**Question 4.22:** A long straight wire carrying current of 25A rests on a table as shown in Fig. 4.3. Another wire PQ of length 1m, mass 2.5 g carries the same current but in the opposite direction. The wire PQ is free to slide up and down. To what height will PQ rise?

**Solution 4.22:**

**
**

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