Cracking the SAT Physics Subject Test

electric field, E, crossed with a perpendicular magnetic field, B. If

E = 2 × 10^4 N/C and B = 0.5 T, what must be the speed of the

particle if it is to cross this region without being deflected?

Here’s How to Crack It

If the particle is to pass through undeflected, the electric force it feels has to be
canceled by the magnetic force. In the previous diagram, the electric force on the
particle is directed upward and the magnetic force is directed downward. So FE

and FB point in opposite directions, and for their magnitudes to balance, qE must

equal qvB, so v must equal E/B, which in this case gives

4. The figure below shows a uniform magnetic field, B, whose
   field lines point up in the plane of the page, and three particles, all
   with the same positive charge, q, and all moving with the same
   speed, v. Which particle—X, Y, or Z—will experience the greatest
   magnetic force?