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Magnetism — JEE physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceJEE physicsMagnetism1 Mark
MCQ
Force between two identical bar magnets whose centres are $r$ metre apart is $4.8 \mathrm{~N}$, when their axes are in the same line. If separation is increased to $2 r$, the force between them is reduced to
  • A
    $2.4 \mathrm{~N}$
  • B
    $1.2 \mathrm{~N}$
  • C
    $0.6 \mathrm{~N}$
  • ✓
    $0.3 \mathrm{~N}$
✓

Answer

Correct option: D.
$0.3 \mathrm{~N}$
In magnetic dipole, force $\propto \frac{1}{r^4}$Hence new force $=\frac{4.8}{2^4}=\frac{4.8}{16}=0.3 \mathrm{~N}$

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