Question

A negative point charge moves with a velocity in the negative z direction.As the point charge moves through the origin,in what direction is its magnetic field at a point on the positive y-axis?

A) +x

B) -x

C) +y

D) -y

E) -z

(I picked +x,but that was wrong,so a little explanation would be nice since I am a little confused on way I got this wrong)

Answer 1

v = - k

q = -ve

r = j

Now, for the magnetic field direction

B = q * v X r

B = - (-k) X j

B = - i

hence , the direction of magnetic field is - x

(you must have not taken the sign of charge in account)