Question

If you were building a velocity selector where the electric field pointed along the upward direction and a positively charged particle was traveling left to right;

a.) Would you rather the magnetic field go into or out of the page. Explain.

b.) Show that the veloicty will not be deflected. v=E/B

c.) Would your answer to part A change if the charge was negative. Explain.

Answer 1

a) The electric and magnetic fields should act in the same region (usually, an electromagnetic field created between 2 plates). If Fe is upward, the electromagnetic force (F_L=Lorentz's force) should be downward in order, for the (positive) ion, to move straightforward (condition for the same speed).

So, B should exit the page.

b)

Fe=F_L <---> neE=nevB --=> v=E/B.

ne=ion charge

c) If the electric field has the same direction and sense (upward), Fe will be downward for a negative ion. Thus, the Lorentz's force should be upward. In this case B exits the page as well since the charge is negative.