CBSE Class 12 Physics sample paper 2025-26: question 20(I)

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Step-by-step solution

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1. Force per unit length between two long parallel wires carrying currents $I_1$ and $I_2$ separated by distance $r$: $\frac{F}{L}=\frac{{\mu }_0{I}_1{I}_2}{2\pi r}$ (attractive for like currents, repulsive for unlike).

   1 mark

2. Putting $I_1=I_2=1$ A, $r=1$ m: $\frac{F}{L}=\frac{4\pi \times 10^{-7}}{2\pi }=2\times 10^{-7}$ N/m. Hence one ampere is that steady current which, flowing in each of two very long, straight, parallel conductors placed 1 m apart in vacuum, produces a force of $2\times 10^{-7}$ N per metre of length on each conductor.

   1 mark

Answer: $F/L={\mu }_0{I}_1{I}_2/(2\pi r)$; 1 A gives $2\times 10^{-7}$ N/m between wires 1 m apart in vacuum

Revise the chapter

• Chapter: Moving Charges and Magnetism (notes, formulas, where marks are lost and every past question)