Physics - Magnetism Question with Solution | TestHub

Solution:

The particle is negatively charged and falls downwards (velocity, $v$, is downwards). The magnetic field ($B$) is horizontal, pointing North. Using Fleming's Left-Hand Rule, if the thumb points in the direction of force, the forefinger in the direction of the magnetic field, and the middle finger in the direction of the current (opposite to the velocity of a negative charge), the force will be towards the West.

Alternatively, using the Lorentz force formula: $\vec{F}=q(\vec{v}\times \vec{B})$.

Since $q$ is negative, the direction of $\vec{F}$ is opposite to $\vec{v}\times \vec{B}$.

$\vec{v}$ is downwards, $\vec{B}$ is North.

$\vec{v}\times \vec{B}$ points East.

Since $q$ is negative, $\vec{F}$ points West.

The final answer is $\boxed{B}$