Find the instantaneous direction of fore in following case (magnetic field and velocity are indicated).

Explanation

As charge in positive, the direction of force is same as direction of `(vecvxxvecB)`. So by right hand thumb rule, curling the fingers of right hand from `vecv` to `vecB` thumb extends in direction of -x. Thus magnetic force at the instant shown in along `-hati` direction.
We can also do the problem by writing v and B in vectror form.
`barv=vhatj`
`vecB=B(-hatk)`
`vecF=q(vecvxxvecB)=q(vhatjxxB(-hatk))=-qvB[hatjxxhatk]=qvBhati`
Thus magnetic force in in `(-hati)` direction.