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Two magnetic dipoles $X$ and $Y$ are placed at a separation $d$, with their axes perpendicular to each other. The dipole moment of $Y$ is twice that of $X$. A particle of charge $q$ is passing through their mid-point $P$, at angle $\theta = 45^o$ with the horizontal line as shown in the figure. What would be the magnitude of force on the particle at that instant ? ($d$ is much larger than the dimensions of the dipole)
$\left( {\frac{{{\mu _0}}}{{4\pi }}} \right)\frac{M}{{{{\left( {d/2} \right)}^3}}} \times {q^\upsilon }$
$0$
$\left( {\frac{{{\mu _0}}}{{4\pi }}} \right)\frac{2M}{{{{\left( {d/2} \right)}^3}}} \times {q^\upsilon }$
$\sqrt 2 \left( {\frac{{{\mu _0}}}{{4\pi }}} \right)\frac{M}{{{{\left( {d/2} \right)}^3}}} \times {q^\upsilon }$
Solution
$B_{1}=2\left(\frac{\mu_{0}}{4 \pi}\right) \frac{M}{(d / 2)^{3}} ; B_{2}=\left(\frac{\mu_{0}}{4 \pi}\right) \frac{2 M}{(d / 2)^{3}}$
$B_{1}=B_{2}$
$\Rightarrow $ $B_{net}$ is at $45^{\circ}\left(\theta=45^{\circ}\right)$
Velocity of charge and $B_{net}$ are parallel so by
$\overrightarrow{\mathrm{F}}=q(\overrightarrow{\mathrm{b}} \times \overrightarrow{\mathrm{B}})$ force on charge particle is Zero.
