MediumChemistryHigh School (11–12)Structure of the Atom › Subatomic Particles and Atomic ModelsNumericalConceptual

Five oil drops in Millikan's experiment carried the charges 4.806, 12.816, 17.622, 20.826 and…

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Five oil drops in Millikan's experiment carried the charges $4.806$ , $12.816$ , $17.622$ , $20.826$ and $16.020$ , all in units of $10^{-19}\ \text{C}$ . What is the smallest charge that can explain all five, and how many such charges does each drop carry?

Need a reminder? The key ideaShow

Cathode rays are electrons: negative, the same in every gas, with $e/m_e = 1.759 \times 10^{11}\ \text{C kg}^{-1}$ . Millikan found $e = 1.602 \times 10^{-19}\ \text{C}$ , so $m_e = 9.109 \times 10^{-31}\ \text{kg}$ . Canal rays are positive, with $e/m$ depending on the gas. The neutron has no charge. In a field $E$ , a charge $q$ feels $F = qE$ .

The worked solution

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← In Thomson's tube, the charge to mass ratio of the rays was the same for hydrogen, air and carbon…The mass number of sodium is 23, and the radius of a sodium atom is about 1.86 × 10⁻¹⁰ m. →

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