Millikans Oil Drop

TITLE:

        Millikan’s Oil Drop

OBJECTIVES:

[TAKEN FROM PHYS1224: ‘INTRODUCTION TO THERMODYNAMICS AND

MODERN PHYSICS’ LABORATORY MANUAL; PG 11]

• To determine the elementary charge of an electron and to calculate the charge of a drop of oil.
• To understand the concepts and investigate the quantization of charge.

ABSTRACT:

        This experiment served the basis of exploring the discoveries Robert Millikan found in 1910. The experimenter goal in this experiment was to determine the radius, mass and charge of a charged droplet of oil utilizing the PASCO Scientific Model AP-8210. The experiment was carried out by ionizing a single oil droplet five times and recorded the rise and fall time between an allocated distance (0.0005m)

After five trails the radius of the oil drop was determined to be between the ranges of 4.01 x 10-7m to 4.34 x 10-7m. The mass of the oil drop for the five trials was determined to be between 2.39 x 10-16kg and 3.03 x 10-16kg. The charge of the oil droplet was found to be between the ranges of 3.28 x 10-20C and 1.15 x 10-19 C. The average elementary charge was not identical to the expected theoretical value however this discrepancy can be accounted for within the sources of errors.

INTRODUCTION:

        In 1833, Michael Faraday along with his assistant, Davy, developed the laws of electrolysis in 1833. This new breakthrough, suggested that in a solution, each molecular fragment or atom has a fixed electric charge or a multiple of that fixed charged. In 1897, Sir J. J. Thompson discovered the electron. He studied the flow of electricity in rarefied gases under high voltage. From this, it became evident that currents were carried by positive and negative particles in the gas. Sir Thompson however isolated one type of particle and studied its characteristics. This particle was known as the ‘electron.’

         This supported the basis of which Robert Millikan began his research in 1910.  At the California Institute of Technology, Millikan successfully sprayed tiny droplets of oil and measured the charge of the electron. He sprayed the oil droplets from an atomizer into an electric field; there existed a force between the parallel plates.  The droplets observed became electrically charged and started to fall slowly in the field. When Millikan adjusted the voltage, he was able to stop the moving oil drops i.e. the electric force was equivalent to the gravitational force acting upon the drop.

He was able to measure both electric force and magnitude of the electric field on the charge of the oil drop. From the data obtained, he was able to determine the magnitude of the charge itself. In 1913, Robert Millikan’s report stated that he discovered the charge of an electron was 1.602 x 10-19C.

From past experiments, it was established that all free charges are integral multiples of a basic unit of charge denoted by e. As such the charge on a body, q, is given by:

Q = n x e

Here, n is an integer, whether positive or negative. The basic unit of charge is that of either an electron or proton. By convention, the electron charge is negative as such the charge is denoted as ‘-e.’ The fact that the electric charge is always an integral multiple od e, it is termed the ‘quantization of charge.’