Millikan's oil drop experiment provided strong experimental evidence that electric charge is quantized. It also measured the magnitude of the elementary charge, now accepted as approximately C.
How the Experiment Worked
This is part of IB Physics D.2 Electric and magnetic fields and is required at both SL and HL.
Millikan sprayed tiny, electrically charged oil droplets between two horizontal metal plates. A potential difference across the plates produced a nearly uniform electric field of magnitude
,
where is the potential difference and is the plate separation.
Each droplet experienced forces including its weight, an electric force, and effects due to air resistance and buoyancy. Millikan first observed a droplet moving at terminal speed, allowing its radius and effective weight to be determined.
He then adjusted the electric field until the droplet was suspended. At equilibrium, the resultant force was zero, so the upward electric force balanced the effective downward weight:
.
Therefore, the droplet's charge could be calculated from
.
Repeating the procedure for many droplets produced different charge values. However, every measured charge was consistent with an integer multiple of one smallest value:
,
where is an integer and is the elementary charge. This showed that charge does not normally occur in arbitrary amounts; it is transferred in discrete units.
A common misconception is that Millikan discovered the electron. He did not. The electron had already been identified through J. J. Thomson's experiments. Millikan measured the magnitude of its charge and provided evidence for charge quantization.
Exam Technique
For an IB Explain question, state both conclusions: Millikan determined and showed that measured charges were integer multiples of . Do not merely write that he “measured charge”; connect the repeated measurements explicitly to quantization.