An electron experiences the greatest force as it travels 1.7 ✕ 106 m/s in a magnetic field when it is moving southward. The force is upward and of magnitude 2.5 ✕ 10-12 N. What are the magnitude and direction of the magnetic field?
F = qv X B. (Lorentz's law, ref.) The cross product means that when
viewed in the direction of F, the direction of the smaller-angle
rotation of +qv into +B is clockwise. Note for an electron, q =
-1.6022E-19 C. This means qv is negative southward, +qv is northward and
thus +B points westward. At the maximum point, F = qvB ==>
B = F/(qv) T.