An alpha particle (positively charged particle) enters a magnetic field at a right angle to it,as shown in the figure. Explain the direction of the force acting on the alpha particle with the help of a relevant rule.

(N/A) According to Fleming's left-hand rule,if the forefinger points in the direction of the magnetic field and the central finger points in the direction of the current (the direction of motion of the positively charged alpha particle),then the thumb points in the direction of the force acting on the particle.
In this case,the magnetic field is directed upwards and the alpha particle is moving towards the right. Applying Fleming's left-hand rule,the force acting on the alpha particle will be directed out of the plane of the paper (perpendicular to the plane of the paper and towards the observer).