How do electric and magnetic fields affect the motion of an electron?

[brad sue]

Hi, I have this problem and I need some explanation and help to solve it:

An electron is injected at t=0sec with a velocity of v_o =(2*10^6 m/s) i into a region with parallel electric field E=(1500V/m)j and B=(-.2T)j,respectively.
Calculate the subsequent motion.

What I did is that I know that the motion will be a circular one.
then I computed R =m*v/(q*B)
Can I compute R this way since we have electric and magnetic fields together??

Also How can I find the acceleration? when I do V²/R I don't get the right answer at the back of the textbook.

Thank you
B

 

[gulsen]

Since there's an electric field, the magnitude of it's velocity increases, so the radii will chage. You can plug the Lorentz force into Newton's 2nd law.