A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field

Q. No 3:  A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is

(a) towards south  (b) towards east

(c) downward        (d) upward

Ans:

     (d) The direction of the magnetic field can be determined by the Fleming’s left hand rule. According to Fleming’s left hand rule if we stretch the thumb, the centre finger and the middle finger of our left hand such that they are mutually perpendicular to each other. If the centre finger gives the direction of current and middle finger points in the direction of magnetic field then the thumb points towards the direction of the force or motion of the conductor.
Here the direction of positively charged alpha particle is towards west this implies the direction of current is also towards west and the direction of magnetic force is towards north. Hence, according to Fleming’s left hand rule, the direction of magnetic field will be upwards.