Verified Solution • 2025–26 Session moderate

QPages 249-253: Q55: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Due to a Current Pages 249-253: Q55 Solution: A long solenoid is fabricated by closely winding a wire of…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often take the wire radius as the spacing between turns, computing $n=1/r$ instead of the correct $n=1/(2r)$ (the wire diameter) and also forget to convert mm to metres before inverting. Always use the wire's diameter as the pitch for a tightly wound solenoid and convert units first.

A long solenoid is fabricated by closely winding a wire of radius 0.5 mm over a cylindrical nonmagnetic frame so that the successive turns nearly touch each other. What would be the magnetic field BB at the centre of the solenoid if it carries a current of 5 A?

Step-by-Step Verified Solution

**Answer:** B=6.28×10−3B = 6.28 \times 10^{-3} T. **Step-by-Step Explanation:** Wire radius = 0.5 mm, so turns per metre n=1/(2×0.5×10−3)=1000n = 1/(2 \times 0.5 \times 10^{-3}) = 1000 turns/m. B=μ0ni=4π×10−7×1000×5=6.28×10−3B = \mu_0 n i = 4\pi \times 10^{-7} \times 1000 \times 5 = 6.28 \times 10^{-3} T.

All Questions in this Chapter

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