Verified Solution • 2025–26 Session moderate

QPages 249-253: Q27: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Due to a Current Pages 249-253: Q27 Solution: Three coplanar parallel wires, each carrying a current of 10 A…

⚠️ Common Exam Traps & Conceptual Pitfalls

When all currents flow in the same direction, the magnetic force between any pair of wires is attractive; for an outer wire the forces from the middle and the opposite outer wire both point toward the centre, so they add rather than cancel. Students often subtract these forces, forgetting that the directions are the same, leading to an underestimated net force per unit length.

Three coplanar parallel wires, each carrying a current of 10 A along the same direction, are placed with a separation 5.0 cm between the consecutive ones. Find the magnitude of the magnetic force per unit length acting on the wires.

Step-by-Step Verified Solution

**Answer:** (a) Middle wire: 0. (b) Outer wires: 4×10−44 \times 10^{-4} N/m. **Step-by-Step Explanation:** Three wires, 10 A each, separated by 5 cm. Middle wire: forces from left and right cancel. Outer wire: force from middle = μ0×10×102π×0.05=4×10−4\frac{\mu_0 \times 10 \times 10}{2\pi \times 0.05} = 4 \times 10^{-4} N/m. Force from other outer wire = μ0×10×102π×0.1=2×10−4\frac{\mu_0 \times 10 \times 10}{2\pi \times 0.1} = 2 \times 10^{-4} N/m. Net = 2×10−42 \times 10^{-4} N/m.

All Questions in this Chapter

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