Question

A short bar magnet is placed with its south pole towards geographical north. The neutral points are situated at a distance of 20 cm from the centre of the magnet. If then the magnetic moment of the magnet is

• A. 9000 ab-amp × cm²
• B. 900 $a b - a m p \times c m ^ { 2 }$
• C. $1200 a b - a m p \times c m ^ { 2 }$
• D. $225 a b - a m p \times c m ^ { 2 }$

Answer 1

Answer: (C)

$1200 a b - a m p \times c m ^ { 2 }$

Answer 2

At neutral point magnetic field due to magnet = Horizontal component of earth's magnetic field

⇒ $\frac { \mu _ { 0 } } { 4 \pi } \cdot \frac { 2 M } { r ^ { 3 } } = B _ { H }$⇒ $\frac { 10 ^ { - 7 } \times 2 \times M \times 1 } { ( 0.2 ) ^ { 3 } } = 0.3 \times 15 ^ { 4 }$

⇒