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QPages 277-278: Q1: HC Verma: Concepts of Physics - Volume 2 Permanent Magnets Pages 277-278: Q1 Solution: A long bar magnet has a pole strength of 10 Am. Find the…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students frequently apply the isolated pole formula $B=\frac{\mu_0}{4\pi}\frac{m}{r^2}$ directly, forgetting that a bar magnet has both north and south poles whose fields superpose on the axis. Always include the contribution of the opposite pole (or use the correct dipole expression) and measure $r$ from the pole, not from the magnet’s centre, to avoid a factor‑of‑two error.

A long bar magnet has a pole strength of 10 Am. Find the magnetic field at a point on the axis of the magnet at a distance of 5 cm from the north pole of the magnet.

Step-by-Step Verified Solution

**Answer:**

B = 4 \times 10^{-4} \text{ T}

. **Step-by-Step Explanation:** Given: pole strength

m = 10 \text{ Am}

, distance

r = 5 \text{ cm} = 0.05 \text{ m}

. The magnetic field due to a magnetic pole is

B = \frac{\mu_0}{4\pi} \frac{m}{r^2}

.

B = 10^{-7} \times \frac{10}{(0.05)^2} = 10^{-7} \times \frac{10}{0.0025} = 4 \times 10^{-4} \text{ T}

[citation:17].

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