One oil droplet observed by students carrying out the Millikan experiment has a

Question

One oil droplet observed by students carrying out the Millikan experiment has a radius of 5.0 times 10^-7 m and carries a charge of -4.8 times 10^-19 C. The density of the oil is 886 kg/m^3. How many excess electrons are on this oil droplet? How does this compare with the number of electrons in the atoms that make up the droplet? Mineral oil is a "long-chain hydrocarbon," so assume that there are roughly equal numbers of protons and neutrons in the droplet. In that case, about half the mass of the droplet is assignable to protons, and there are equal numbers of protons and electrons.

Explanation / Answer

Here ,

a) charge , q = -4.8 *10^-19 C

number of excess electron = q/e

number of excess electron = 4.8 *10^-19/(1.6 *10^-19)

number of excess electron = 3

b)

radius , r = 5 *10^-7 m

mass of molecule , m = 4pi/3 * (5 *10^-7)^3 * 886

m = 4.638 *10^-16 m^3

number of nucleons , n = (4.638 *10^-16)/(1.67 *10^-27)

n = 2.77 *10^11

number of electrons = 2.77 *10^11/2 = 1.385 *10^11

ratio of excess electrons to number of electrons = 3/(1.385 *10^11)

ratio of excess electrons to number of electrons = 2.17 *10^-11

the ratio of excess electrons to number of electrons is 2.17 *10^-11

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