Verified Solution • 2025–26 Session moderate

QPages 249-253: Q26: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Due to a Current Pages 249-253: Q26 Solution: A straight, long wire carries a current of 20 A. Another wire…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students frequently fail to convert the wire length from centimeters to meters before substituting it into the force formula, leading to a result that is off by a factor of 100. Always ensure $l$ is in meters to maintain consistency with the SI units of the permeability constant $\mu_0$.

A straight, long wire carries a current of 20 A. Another wire carrying equal current is placed parallel to it. If the force acting on a length of 10 cm of the second wire is 2.0×10−5 N2.0 \times 10^{-5} \text{ N}, what is the separation between them?

Step-by-Step Verified Solution

**Answer:** d=2d = 2 cm. **Step-by-Step Explanation:** i1=i2=20i_1 = i_2 = 20 A, F=2×10−5F = 2 \times 10^{-5} N, l=0.1l = 0.1 m. F=μ0i1i2l2πd⇒d=4π×10−7×20×20×0.12π×2×10−5=0.02F = \frac{\mu_0 i_1 i_2 l}{2\pi d} \Rightarrow d = \frac{4\pi \times 10^{-7} \times 20 \times 20 \times 0.1}{2\pi \times 2 \times 10^{-5}} = 0.02 m = 2 cm.

All Questions in this Chapter

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