# What is the force on a moving charge in combined magnetic and electric fields, in general, when the magnetic field defines the z-axis?

This is a question from a problem sheet. How am I supposed to find the force applied to a moving charge by magnetic and electric fields when the question only gives me the magnetic field $B=B_z \hat k$. I know that in electromagnetic waves that the fields are perpendicular but even if that was the case here, would it help?

$\mathbf{F_B} = q(\mathbf{v}\times \mathbf{B})$