Slab Beam of electrons through Neutralising Ions

[Remembrandt]

Homework Statement

A slab beam of electrons of thickness a, width w >> a, charge density
(-ρ_b ), (Take ρ_b to be positive.) and velocity v_be_z passes through a channel of
neutralizing ions with charge density f*ρ_b where f is some number between zero and one
(all parameters given above apply in the "lab" frame).

1. Calculate the electric and magnetic fields produced by this combination of charges and currents in the region y << w / 2 .
2. Find f such that there is no net force on any electron in the beam.

Homework Equations

F[/B] = q(E + v x B)

The Attempt at a Solution

I have an idea as how to begin. But I am not very sure about it.

At any instant of time the new current flowing is given by (ρ_b - f *ρ_b)*w*a*v_b. If this is the current, then the magnetic field is given by Biot Savarts law (μ_o/4π)*I/2R ? I just don't get this .

Can anybody guide me on how to start ?

 

[mfb]

I'm not sure what "channel of neutralizing ions" means, but I don't think those ions move in our lab frame.
How did you apply Biot-Savart to your disk of electrons?