In a particular Millikan oil-drop apparatus, the plates are 2.25 cm apart. The o

Question

In a particular Millikan oil-drop apparatus, the plates are 2.25 cm apart. The oil used has a density of 0.825 g/cm3 , and the atomizer that sprays the oil drops produces drops of diameter 1.00×103 mm .

Part A: What strength of electric field is needed to hold such a drop stationary against gravity if the drop contains five excess electrons?

Part B

What should be the potential difference across the plates to produce this electric field?

Part C

If another drop of the same oil requires a plate potential of 74.4 V to hold it stationary, how many excess electrons did it contain?

Express your answer as an integer.

Explanation / Answer

Accoridng to the given problem,

a) Using equlibrium of forces

F = Eq = mg so E = mg/q = 4/3 (5 *10-7)3* 825*9.81 / ( 5 *1.602 *10-19) = 5290.4 V/m

b) E = V/d so V = Ed = 5290.4 (0.0225 ) = 119.03 V

c) excess electrons = 119.03/74.4 = 1.5999 so there must be 1.6 *5 electrons

n = 7.99999 = 8 if the drop (unlikely) is the same size.

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