Answer
$1.1\times10^{-6}m/s$ west
Work Step by Step
Set the magnetic force equal in magnitude to the electron’s weight. Let the electron’s velocity be perpendicular to the magnetic field, so the magnetic force is maximum. The magnitude is given in equation 20–4.
$$mg=qvB$$
$$v=\frac{mg}{qB}$$
$$v=\frac{(9.11\times10^{-31}kg)(9.80m/s^2)}{(1.60\times10^{-19}C)(0.50\times10^{-4}T)}$$
$$=1.1\times10^{-6}m/s$$
To balance the weight, the magnetic force must point upward. The electron must be moving to the west.
Use the right hand-rule, as described on page 566, to calculate the direction of the force on a positively -charged particle. Since this charge is negative, the answer we want is opposite to the result given by the right-hand rule. Some instructors advocate the use of a “left-hand rule” for negative charges, which also works.
