Quantum Physics and Millikan's Experiment

Quantum physics or Quantum Mechanics or Quantum Theory

  • It is the study of the microscopic world.
  • When we talk at microscopic level, many quantities are found only in certain minimum (elementary) amounts, or integer multiples of those elementary amounts; these quantities are then said to be quantized.
  • The elementary amount that is associated with such a quantity is called the quantum of that quantity.

Millikan’s Experiment

Millikan determined the value of charge on an electron using oil drop method.
Principle: It is based upon the measurement of
(i) The terminal velocity of an oil drop under the influence of gravity alone,
(ii) The terminal velocity under the joint action of gravity and an electric field opposing the gravity.
Experimental arrangement: The experimental arrangement used by Millikan to determine the charge on an electron is shown in figure 1.
millikan-oil-drop-apparatus1figure 1
Importance of Millikan’s experiment
(i) It proved the quantum nature of charge.
(ii) Millikan’s method also found the mass of electron.
From thomson method, e/m = 1.7598 × 1011 C Kg-1
From millikan’s method, e = 1.6 x 10-19 C
Therefore, mass of electron,
m = e/(e/m) = ( 1.6 x 10-19 / 1.7598 × 1011  ) = 9.1 × 10-31 Kg
(iii) Millikan’s method is also used to determine the radius of electron (1.87 x 10-15 m)

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