JEE Main 2014 · 9 Apr · Shift 1 · Physics Q19

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PhysicsSingle correct+4 / -1Question 19 of 90

The mid points of two small magnetic dipoles of length $d$ in end-on positions, are separated by a distance $x$ , ( $x\gg d$ ). The force between them is proportional to $x^{-n}$ where $n$ is :

Two bar magnets on one line, end-on: N-S on the left and N-S on the right (S of the first facing N of the second), centres a distance x apart.

Answer and worked solution

Official answer: D

The field of a short dipole on its axis is $B=\dfrac{\mu_0}{4\pi}\dfrac{2m}{x^3}$ . The force on the second dipole is $m\dfrac{dB}{dx}\propto x^{-4}$ , so $n=4$ .

Topics:Magnetic dipole and bar magnet

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JEE Main 2014 · 9 Apr · Shift 1, Physics question 19