$A$ uniform magnetic field is directed vertically upwards. In which direction should an $\alpha$-particle (which is a positively charged particle) be projected in this field so that it is deflected southwards? Name and state the rule used to find the direction in this case.

(EAST) The $\alpha$-particle is a positively charged particle. According to Fleming's Left-Hand Rule,the force $(F)$ on a moving charge is perpendicular to both the magnetic field $(B)$ and the velocity $(v)$ of the charge.
Given: Magnetic field $(B)$ is vertically upwards. Force $(F)$ is towards the south.
Using Fleming's Left-Hand Rule: If the forefinger points upwards (magnetic field) and the thumb points south (force),the middle finger will point towards the east.
Therefore,the $\alpha$-particle should be projected towards the east.
Fleming's Left-Hand Rule states: Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current (or motion of positive charge),then the thumb points in the direction of the force acting on the particle.