# 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?

According to Right-hand rule, the direction of force ($\stackrel{\to }{F}$) on positively charged particle will be perpendicular to the plane of paper upward and on negatively charged particle will be perpendicular to the plane of paper downwards. As,
If , then
$F=1×1×B×1=B$
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 $\stackrel{\to }{F}$ is the direction of cross-product of velocity $\stackrel{\to }{v}$ and magnetic field $\stackrel{\to }{B}$, which is perpendicular to the plane containing  $\stackrel{\to }{v}$ and  $\stackrel{\to }{B}$.
Thus, from the cross product rule and Right-hand rule, the direction of magnetic field is along + Z-axis.

• 17
What are you looking for?