A positively charged particle (alpha-particle) is projected towards west in a horizontal plane. It is deflected towards north in the same plane by a magnetic field. Which one among the following is the direction of the magnetic field ?

The direction of the magnetic field can be determined using Fleming's Left-Hand Rule. According to this rule, if we stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to each other:

• The Middle finger represents the direction of the current (which is the same as the direction of motion for a positively charged particle like an α-particle). In this case, it points West.
• The Thumb represents the direction of the force or deflection. In this case, it points North.
• The Forefinger represents the direction of the Magnetic Field.

By aligning the middle finger to the West and the thumb to the North, the forefinger points vertically upwards. Therefore, the magnetic field is directed vertically upwards.