A beam of alpha particles projected along +x axis, experience a force due to magnitic field along the +y axis. What is the direction of the magnetic field?

If $v=1,q=1and\mathrm{sin}\theta =1or\theta ={90}^{o}$, then

$F=1\times 1\times B\times 1=B$

Thus the magnetic field induction at a point in the field is equal to the force experienced by a unit charge moving with a unit velocity perpendicular to the direction of magnetic field at that point.

The direction of $\overrightarrow{F}$ is the direction of cross-product of velocity $\overrightarrow{v}$ and magnetic field $\overrightarrow{B}$, which is perpendicular to the plane containing $\overrightarrow{v}$ and $\overrightarrow{B}$.

Thus, from the cross product rule and Right-hand rule, the direction of magnetic field is along + Z-axis.

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