Answer :
Given :
An electron moving in the positive x direction experiences a magnetic force in the positive z direction.
To Find :
The direction of the magnetic field.
Solution :
We know, force is given by :
[tex]\vec{F}=q(\vec{v}\times \vec{B)}[/tex]
Here, q = -e.
[tex]\vec{F}=(-e)(\vec{v}\times \vec{B)}\\\\\hat{k}=(-e)(\hat{i}\times \vec{B})[/tex]
Now, for above condition to satisfy :
[tex]\hat{i}\times \vec{B}=-\hat{k}[/tex]
So, [tex]\vec{B}=-\hat{j}[/tex]
Therefore, direction of magnetic field is negative y direction.
Hence, this is the required solution.