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The effective length of a magnet is 31.4 cm and its pole strength is 0.8 A m. The magnetic moment, if it is bent in the form of a semicircle, is ______ A m².

Asked in GUJCET 2015 · Magnetic moment of magnets and loops

• (1) 1.2
• (2) 1.6
• (3) 0.16
• (4) 0.12

Step-by-step solution

Why the other options are wrong

• (1) No single slip produces 1.2. Sanity check: bent into a semicircle of arc 31.4 cm, the magnet has its poles a diameter apart, 2r=(2×0.314)/π=0.2 m, so m=0.8×0.2=0.16 A m².
• (2) Comes from reading the length as 3.14 m instead of 31.4 cm: then r=1 m, the separation is 2 m and m=0.8×2=1.6 - the right method with the decimal point ten times out.
• (4) No single slip produces 0.12: using the radius 0.1 m as the new separation gives 0.08, and using the arc length itself gives 0.25. Sanity check: the poles end up a diameter apart, 0.2 m, so m=0.16 A m².

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