Millikan’s experiment measures the elementary charge e by
the observation of the motion of small oil droplets in an electric
field. The oil droplets are charged with one or several elementary
charges, and if the (vertical) electric field has the
right magnitude, the electric force on the droplet will balance
its weight, holding the drop suspended in midair. Suppose
that an oil droplet of radius 1.0 104 cm carries a single elementary
charge. What electric field is required to balance the
weight? The density of oil is 0.80 gcm3
.