A magnet is 10 cm long and its pole strength is 120 CGS units(1 CGS unit of pole strength=0.1 Am). Find the magnitude of magnetic field ‘B’ at a point on its axis at a distance 20 cm from it.

The pole strength is m=120 CGS units= 12 AmMagnetic length (distance between poles) is 2l=10 cm or  l=0.05 mDistance from the center of magnet is d=20 cm=0.2 m.The field B at a point on axial position is B=μ04π.2Mdd2−l22 ⇒B=μ04π.4mldd2−l22 ⇒B=10−7.4×12×0.05×0.2(0.2)2−(0.05)22B=3.4×10−5 T .