Oil is used in oil drop experiments primarily because it has a very low vapor pressure, minimizing evaporation which could affect the experiment's accuracy.
Detailed Explanation
Millikan's oil drop experiment aimed to determine the elementary electric charge (the charge of a single electron). To do this accurately, he needed to observe the behavior of tiny, charged droplets. Here's why oil was the optimal choice:
Minimizing Evaporation
The most crucial reason for using oil was to prevent rapid evaporation.
- Low Vapor Pressure: Oil, especially a type with low vapor pressure, evaporates very slowly. This is critical because any change in the size of the droplet due to evaporation would alter its mass and, consequently, its behavior under the influence of gravity and electric fields. Accurate measurements of the droplet's motion are essential for calculating its charge.
- Maintaining Constant Mass: By using oil, Millikan could ensure that the mass of the droplet remained relatively constant throughout the experiment. This allowed for more precise and reliable calculations. He even performed repeated experiments with the same drop, noting any systematic changes in fall time that might indicate evaporation.
Other Important Considerations
While minimizing evaporation was the primary concern, other factors also contributed to the choice of oil:
- Ease of Atomization: Oil can be easily atomized into tiny droplets using a sprayer.
- Viscosity: The viscosity of the oil allowed for relatively slow and measurable movement under the influence of the applied electric field. If the droplets moved too quickly, accurate observations would be difficult.
- Availability: Suitable oils with the desired properties were readily available to Millikan at the time.
In summary, oil was selected for its low vapor pressure, which minimized evaporation and allowed for accurate measurements of the charged droplets, enabling the determination of the elementary electric charge.
