The classic Millikan oil drop experiment was the first to obtain an accurate measurement of the charge on an electron. In it, oil drops were suspended against the gravitational force by a vertical electric field. (See Figure 18.54.) Given the oil drop to be 1.00 μm in radius and have a density of 920 kg/m³:

(a) Find the weight of the drop. 

(b) If the drop has a single excess electron, find the electric field strength needed to balance its weight.

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