CBSE Class 12 Physics 2026: question 19

Hints

Open one hint at a time and try again before the next.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

1. The torque is maximum when the field lies in the plane of the coil: ${\tau }_{max}=NIAB$. Since $N$, $I$ and $B$ are the same for both coils, ${\tau }_{max}\propto A$.

   ½ mark

2. Square coil: perimeter of one turn $=\frac{L}{N}$, so side $=\frac{L}{4N}$ and $A_s=\frac{L^2}{16N^2}$.

   ½ mark

3. Circular coil: $2\pi r=\frac{L}{N}\Rightarrow r=\frac{L}{2\pi N}$, so $A_c=\pi r^2=\frac{L^2}{4\pi N^2}$.

   ½ mark

4. $\frac{{\tau }_s}{{\tau }_c}=\frac{A_s}{A_c}=\frac{L^2/16N^2}{L^2/4\pi N^2}=\frac{\pi }{4}$

   ½ mark

Answer: ${\tau }_{square}:{\tau }_{circle}=\pi :4$

Revise the chapter

• Chapter: Moving Charges and Magnetism (notes, formulas, where marks are lost and every past question)
• Topic: Torque on a Current Loop and Magnetic Dipole