Millikan oil drop experiment class 11: history, apparatus, procedure, observation

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The time was the year 1909, a curious physicist Robert A. Millikan conducted the “Millikan oil drop experiment” named after the physicist Robert A. Millikan. It was groundbreaking research that helped scientists to determine the fundamental unit of electric charge, known as the elementary charge. This experiment played an important role in advancing our understanding of the nature of the electric charge and revolutionized the field of atomic physics.

When he measured the motion of charged oil droplets in an electric field, he made precise calculations and provided experimental evidence for the existence and quantization of electric charge.

In this article, we will discuss the Millikan oil drop experiment, its historical background, experimental setup, experimental procedure, observations, results, and many more. So what are you waiting for, let’s join us on the journey of understanding the “Millikan oil drop experiment class 11“. So let’s get started…

Historical background of the Millikan oil drop experiment

There is no doubt that the “Millikan oil drop experiment, conducted by American physicist Robert A. Millikan, was a significant milestone in the field of physics. If we want to understand the historical background of this experiment, then it is very important for us to know the scientific situation at that time.

Millikan oil drop experiment class 11: history, apparatus, procedure, observation
Robert. A Millikan in 1891, source: Alchetron

During the late 19th and early 20th centuries, physicists were actively investigating the nature of electricity and also investing that how it is connected to matter. Particularly, their intense field of scientific research is the properties of electric charge and its quantization. Many scientists had made unforgettable contributions to the understanding of electric charge, including Charles-Augustin de Coulomb, who formulated Coulomb’s law describing the force between charged particles, and Michael Faraday, who proposed the concept of electric fields.

However, the precise nature of the electric charge and its fundamental unit, the elementary charge, is still a mystery. This made Robert A. Millikan very curious, and he wanted to reveal the true nature of the electric charge, so this was his curiosity that pushes him to design his now-famous oil drop experiment and revealed the mystery of the true nature of the electric charge.

Millikan’s oil drop experiment was inspired by the work of J.J. Thomson, who had discovered the existence of negatively charged particles called electrons. Upon Thomson’s findings, Millikan aimed to measure the charge of individual electrons and determine whether it was quantized, as suggested by earlier theories.

In the oil drop experiment, held at Ryerson Physical Laboratory at the University of Chicago, tiny oil droplets are suspended in a chamber that is filled with air. An electric field is applied to the chamber. During the electric field application, Millikan observed the motion of the droplets, which became either positively or negatively charged due to ionization by X-rays. He carefully adjusted the electric field strength until the droplets were suspended in mid-air, counteracting the force of gravity.

Millikan then measured the droplets’ terminal velocities and used this information to calculate their charges. By studying numerous droplets and their corresponding charges, he noticed that the charges were always multiples of a certain value, which he identified as the elementary charge.

The success of the Millikan oil drop experiment provided experimental confirmation of the quantized nature of the electric charge and provided a precise measurement of the elementary charge. This groundbreaking achievement not only advanced our understanding of electricity but also laid the foundation for further developments in atomic physics and the study of subatomic particles. Millikan received the Nobel Prize in Physics in 1923 for the precise measurement of the elementary charge.

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Apparatus of the Millikan’s Oil Drop Experiment

If we want to understand the working procedure of the oil drop experiment efficiently, then it is necessary to understand every component or apparatus that is involved in the experiment. The oil drop experiment was conducted by Robert. A Millikan involved various apparatus. A brief description of each apparatus is given below:

Millikan oil drop experiment class 11: history, apparatus, procedure, observation
Apparatus of the Millikan’s Oil Drop Experiment, source: ResearchGate
  1. Chamber: It is typically a glass container with transparent windows, where the experiment took place. The chamber was filled with air, and adjustments could be made to control the pressure and temperature within the chamber.
  2. Atomizer: An atomizer was used to introduce a fine mist of oil droplets into the chamber. The atomizer created tiny oil droplets by forcing oil through a nozzle and breaking it up into smaller particles. The droplets needed to be of a consistent size for accurate measurements.
  3. Electric Field: An electric field was generated within the chamber by applying a potential difference between two electrodes. The electrodes were typically metal plates or grids placed at opposite ends of the chamber. The electric field played an important role in affecting the motion of the oil droplets.
  4. Microscope: A microscope was positioned to view the oil droplets suspended in the chamber. The microscope allowed for magnified observation and precise measurements of the droplets’ position and motion.
  5. Light Source: A light source, such as a lamp or laser, was used to illuminate the oil droplets. This allowed for better visibility and tracking of the droplets’ movement.
  6. Viewing System: To direct the magnified image of the oil droplets to the microscope, a viewing system, normally comprised of lenses and mirrors, had been created. This viewing system had been created to measure and spot the oil droplets precisely.
  7. Measurement Tools: Millikan measured the size and position of the oil droplets using a variety of equipment, including calibrated scales and micrometers. These observations were important for estimating the charges of the droplets.
  8. Adjustments: Fine modifications were possible due to the experimental arrangement. The strength of the electric field might be adjusted to regulate the mobility of the droplets. Additionally, the pressure and temperature within the chamber could be changed to keep the experiment running well.

Oil drop experiment procedure explanation

In the previous section, we discussed the various apparatuses used in the experiment. In this section, we will see how these apparatuses are being used. What are the procedures for doing experiments? What are the step-by-step flows of their experiment? The oil drop experiment, conducted by Robert A. Millikan, involved a step-by-step procedure to observe and measure the charges of oil droplets in an electric field. Below is a detailed explanation of each procedural step:

  1. Chamber Preparation: The experiment starts with the making of the chamber, which is normally a glass container with transparent windows. The chamber is filled with air, and changes are made to control the pressure and temperature within the chamber.
  2. Atomization: To inject a fine mist of oil droplets into the chamber, an atomizer is used. Oil is forced into a nozzle by the atomizer, which breaks it up into smaller particles. The purpose is to create oil droplets of uniform size for precise measurements.
  3. Observation Setup: The oil droplets suspended in the chamber are viewed through a microscope. The droplets are illuminated by a light source, and a viewing system comprised of lenses and mirrors provides an enlarged picture of the droplets to the microscope.
  4. Electric Field Application: Two metal plates or grids serve as electrodes and are set at the opposite ends of the chamber. A potential difference is applied between the electrodes, resulting in the formation of an electric field within the chamber. The mobility of the oil droplets is influenced by this electric field.
  5. Droplet Selection: An appropriate oil droplet for observation is chosen using the microscope. The droplet should be freely suspended in the chamber and moving in a stable manner. A droplet is chosen based on its size, position, and behavior in the electric field.
  6. Motion Observation: Under the microscope, the selected droplet is examined. Its movement is carefully observed and recorded, with attributes like speed, direction, and deviations from a straight path.
  7. Electric Field Adjustment: The strength of the electric field can be controlled by adjusting the potential difference or the distance between the electrodes. The aim is to discover an electric field strength that counteracts the droplet’s gravitational attraction, resulting in the droplet being suspended in mid-air.
  8. Terminal Velocity Measurement: Once the droplet is stable, its terminal velocity is measured. This is done by carefully examining the droplet’s motion and recording the rate at which it falls or rises under the effects of gravity and the electric field.
  9. Multiple Measurements: Steps 5-8 are repeated for several droplets, ensuring a range of measurements for accurate analysis and calculation of charges.
  10. Data Analysis: Using the measurements of droplet size, motion, and terminal velocity, calculations are performed to determine the charge on each droplet. These calculations involve considering the gravitational force, the electric field strength, and the droplet’s properties.
  11. Charge Determination: Patterns and regularities are observed by analyzing the charges of many droplets. It is discovered that the charges of the droplets are always multiples of a specified value known as the elementary charge.

Following this step-by-step approach, Millikan was able to observe and measure the charges of oil droplets in an electric field, providing experimental proof for the quantization of electric charge and figuring out the value of the elementary charge.

Observations of Millikan’s oil drop experiment

Robert A. Millikan’s oil drop experiment included numerous major observations that were important in establishing the charges of the oil droplets and providing evidence for the quantization of electric charge. The following are the key findings from the experiment:

  1. The motion of Oil Droplets: Millikan observed the motion of the suspended oil droplets in the chamber. He noticed that under the influence of gravity and the electric field created by the electrodes, the droplets could either rise or fall. The droplets exhibited various types of motion, such as steady upward motion, steady downward motion, or oscillatory motion.
  2. Terminal Velocity: Millikan carefully measured the terminal velocities of the oil droplets. Terminal velocity is a constant speed at which a droplet falls or rises when the gravitational force is balanced by the drag force due to air resistance and the electric force due to the electric field. These measurements help him to accurately calculate the properties of the oil droplets.
  3. Size of Oil Droplets: Microscopic observation and measurement techniques were used to determine the size of the oil droplets. Millikan ensured that the droplets used in the experiment were all of the same sizes in order to maintain uniformity and accuracy in the measurements.
  4. Charges on Oil Droplets: Millikan calculated the charges on the oil droplets using the motion and terminal velocity observations. Observing several droplets, he observed that the charges were always multiples of a specific value. This provided solid evidence for the quantization of electric charge and resulted in the discovery of the elementary charge.
  5. Quantization of Electric Charge: The consistent quantization of electric charge was one of the experiment’s primary findings. Millikan saw that the charges on the droplets were always integer multiples of a fixed number, which he called the elementary charge. This observation supported the idea that electric charge is distinct and exists in separate packets or units.

These findings, especially the consistent quantization of electric charge, offered strong experimental evidence for the existence and quantization of electric charge. They were essential in confirming the theoretical knowledge of electric charge and establishing the importance of the Millikan oil drop experiment in the realm of atomic physics.

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Derive the formula for electric charge in the Millikan oil drop experiment

By analyzing the observations and measurements from the oil drop experiment, Robert A. Millikan calculated the value of the electric charge. In this section, we will look into the mathematical derivation of the formula for electric charge. In order to derive the formula of electric charge on the oil droplets, there are various terms involved such as:

  • Gravitational force: A downward force acting on the oil drop due to the gravitational pull.
  • Electrostatic force: A upward force is applied to the oil drop due to an electric field.
  • Buoyant force: The buoyancy due to the surrounding air between the plates introduces an upward thrust
  • Drag force: When the drop falls down through the viscous air, it experiences an upward drag force given by Stokes’s law.
Millikan oil drop experiment class 11: history, apparatus, procedure, observation
Derivation of the formula for the electric charge on the oil droplets, source: Doubtnut

Now, let’s derive the formula of electric charge ($q$) on the oil droplets.

When oil droplets are sprayed from the atomizer into the glass chamber. It starts falling down under the influence of gravity, we can slow down the falling of droplets by giving an upward force (electrostatic force) by adjusting the electric field.

The downward force acting on the oil drop is due to the gravitational pull is:
$$F_G=m g$$
The upward force applied to the oil drop due to an electric field is:
$$F_E=E q$$
The electric field can be expressed as a function of the voltage $V$ across the plates and the spacing $d$ between them:
$$E=\frac{V}{d}$$

In the correct conditions, the electrostatic force and the force of gravity can be balanced, such that the oil drop is brought to rest. In this case, we can express the charge on the oil drop as follows:
$$
\begin{gathered}
F_E=F_G, \\
q=\frac{m g V}{d}
\end{gathered}
$$
Because measuring the mass of individual oil drops is difficult, it is useful to re-express the mass of the oil drop in terms of the density of the oil as follows:
$$
\rho_{\text {oil }}=m / V_{\text {sphere }}
$$
Assuming that the oil drop is approximately spherical, its volume is given by the following formula:
$$
V_{\text {sphere }}=\frac{4}{3} \pi r^3
$$
Where $r$ is the radius of the oil drop. Thus, the gravitational pull on the oil drop can be expressed as:
$$
F_G=\frac{4}{3} g \pi r^3 \rho_{o i l}
$$

There is a third force acting on the oil drops. The force of buoyancy due to the surrounding air between the plates introduces an upward thrust:
$$
F_b=\frac{4}{3} g \pi r^3 \rho_{a i r}
$$

In a balanced condition, when the sum of electrostatic force and buoyant force is equal to the gravitational force, oil droplets remain at rest. $$F_E + F_b = F_G$$

Therefore the equation for the charge of the oil drop can be re-expressed as
$$
q=\left(\frac{d}{V}\right)\left(\frac{4}{3}\right) \pi r^3\left(\rho_{\text {oil }}-\rho_{a i r}\right)
$$
Among the various parameters in this equation, the radius $r$ is difficult to measure. Let’s see how $r$ can be determined.

The radius of an oil drop is very small and difficult to measure. However, Millikan found that $r$ could be measured indirectly by switching off the voltage and letting the drop fall.

As the drop falls down through the viscous air, it experiences an upward drag force given by Stokes’s law as follows:
$$
F_{drag}=6 \pi \eta r v
$$
In this equation, $\eta$ is the viscosity of air and $v$ is the speed of the oil drop.

When the upward buoyant and viscous forces balance the downward gravitational force, then the drop no longer accelerates and moves at its terminal velocity $v_t$. The drops reach terminal velocity very quickly, and this can be measured by timing how long it takes a droplet to fall a measured distance.

By equating the forces acting on an oil drop traveling at its terminal velocity, we can derive an expression for the radius of the oil drop.

Equating these forces:
$$
F_G=F_b+F_{d r a g}
$$
The forces acting on the drop are:
$$
\begin{aligned}
& F_{d r a g}=6 \pi \eta r v, \\
& F_G=\frac{4}{3} g \pi r^3 \rho_{o i l}, \\
& F_b=\frac{4}{3} g \pi r^3 \rho_{a i r}
\end{aligned}
$$
Therefore,
$$
\begin{gathered}
6 \pi \eta r v_t=\left(\frac{4}{3}\right) \pi r^3\left(\rho_{o i l}-\rho_{a i r}\right) \\
r=3 \sqrt{\frac{\eta v_t}{2 g\left(\rho_{o i l}-\rho_{a i r}\right)}}
\end{gathered}
$$
Substituting this back into the expression for $q$ we get:
$$
q=\left(\frac{9 \pi d}{V}\right) \sqrt{\frac{2 \eta^3 v_t^3}{g\left(\rho_{o i l}-\rho_{a i r}\right)}}
$$

Significance of oil drop experiment

The Millikan oil drop experiment is significant in the realm of physics because it helped us understand electricity and the basic nature of the electric charge. Here are some essential points that emphasize the significance of the oil drop experiment:

  1. Determination of Elementary Charge: One of the most important achievements of the oil drop experiment was the accurate measurement of the elementary charge (the fundamental unit of electric charge). Millikan discovered the value of the elementary charge with incredible precision by examining the charges on several oil droplets and noting that they were always found to be multiples of a certain value. This measurement gave definitive proof for the discrete nature of the electric charge and set the path for later advancements in atomic and particle physics.
  2. Confirmation of Quantization of Electric Charge: The oil drop experiment provided experimental support for the quantization of electric charge, which had previously been proposed by J.J. Thomson and others. Millikan strengthened the concept that electric charge exists in discrete packets or units by observing that the charges on the oil droplets were always quantized in specified multiples of the elementary charge.
  3. Verification of Thomson’s Model of the Atom: The oil drop experiment authenticated J.J. Thomson’s atomic model, sometimes known as the “plum pudding” or “raisin bun” model. The atom, according to this theory, is a positively charged spherical with negatively charged electrons contained within it. Millikan gave experimental proof for the existence and distinct character of electrons within the atom by measuring the charges on the oil droplets.
  4. Fundamental Understanding of Electric Forces: The oil drop experiment allowed researchers to gain a better knowledge of electric forces and their effects on charged particles. Millikan gained insights into the interaction between electric charges and the forces exerted on them by carefully analyzing the motion and behavior of oil droplets under the influence of electric fields. This discovery broadened our understanding of electrostatics and opened the way for future advancements in the discipline.
  5. Educational Impact: The oil drop experiment is a popular educational tool for teaching students about electric charge, forces, and the scientific method. It gives a physical and visual demonstration of essential physics principles and allows students to engage in hands-on experimentation and data analysis.

The Millikan oil drop experiment continues to be recognized as an important milestone in physics, contributing significantly to our knowledge of electric charge, quantization, and atomic structure. Its significance extends throughout various fields of science and continues to drive fresh studies and discoveries in the realms of fundamental particles and electricity.

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Criticisms and Controversies of oil drop experiment

The oil drop experiment conducted by Robert Millikan was not without criticisms and controversies. Below are some points of criticism of the oil drop experiment at that time, which had to suffer Robert. A Millikan:

  1. Assumption of Spherical Droplets: The experiment was criticized for assuming that the oil droplets were completely spherical. In actuality, the droplets could have had irregular forms, which might have affected measurement accuracy and brought uncertainty into the calculations.
  2. Neglecting Air Turbulence: The experiment assumed that there was no air turbulence impacting the mobility of the oil droplets. However, air currents and turbulence could have impacted the droplets’ mobility, which led to errors in the measurement of terminal velocities and subsequent mathematical calculations.
  3. Influence of Cosmic Rays: Cosmic rays, which are high-energy particles from space, have the ability to ionize the air and affect the charges on the oil droplets. It was claimed that the presence of cosmic rays could have interfered with the experiment, causing errors in the observed charges.
  4. Data Selection and Bias: There were worries about potential bias in the selection of data points that supported the desired outcomes. It was hypothesized that Millikan may have chosen data that fit with his assumptions, thus resulting in a biased measurement of the elementary charge.
  5. Lack of Reproducibility: Some researchers had trouble recreating Millikan’s findings and obtaining reliable measurements. This lack of reproducibility caused concerns regarding the trustworthiness and consistency of the experimental approach and calculations.

Applications of Millikan oil drop experiment

The oil drop experiment conducted by Robert Millikan has found various applications in different fields of science and technology. Here are some simplified applications of the oil drop experiment:

  1. Understanding Electrical Charges: The oil drop experiment helps scientists understand the basic nature of electrical charges. By measuring the charges on tiny oil droplets, researchers can determine how electric charges are quantized and how they interact with each other.
  2. Developing Particle Detectors: The principles of the oil drop experiment have been used to create detectors that can track and analyze particles. These detectors are important tools in fields like particle physics, where scientists study the properties and behavior of subatomic particles.
  3. Studying Nanoparticles: The oil drop experiment has been modified to investigate very small particles known as nanoparticles. Scientists can analyze the electrical properties of suspended nanoparticles and gain knowledge about their size, charge, and behavior by applying electric fields to them. This understanding is useful in fields such as nanotechnology and materials science.
  4. Teaching Physics Concepts: The oil drop experiment is commonly used as an educational tool to teach students about electric charges and forces. It provides a hands-on approach, allowing students to conduct experiments and analyze data, helping them grasp fundamental physics concepts.
  5. Contributing to Scientific Knowledge: The oil drop experiment has played a significant role in advancing our understanding of electricity and charged particles. It has provided valuable data and information that contribute to the broader scientific knowledge base, enabling further research and discoveries in related fields.

Simply, the oil drop experiment helps us understand how electric charges work, assists in creating detectors for studying particles, allows us to explore the properties of nanoparticles, helps teach physics concepts, contributes to scientific knowledge, and many more.

Conclusion

Summing up all discussion, we can conclude that the Millikan oil drop experiment is regarded as an important milestone in physics, having made significant advancements to our knowledge of electric charge and its fundamental nature. The experiment confirmed the existence of the elementary charge by demonstrating the discrete and quantized characteristics of electric charge through precise measurements and observations.

The accurate determination of the elementary charge value by the experiment has served as a foundational constant in various fields of physics. Despite significant criticism and controversy, the oil drop experiment remains a critical milestone in scientific discovery, influencing our understanding of electricity, particle physics, and the structure of matter. Its significance extends to applications like particle detection, nanoparticle characterization, and science education, where its ideas continue to encourage interest and research in science.

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Frequently Asked Questions – FAQs

What did the Millikan oil drop experiment prove?

The Millikan oil drop experiment proved the discrete nature of the electric charge, provided a precise measurement of the elementary charge, and supported the concept of quantization of electric charge.

What was the conclusion of the oil-drop experiment?

The conclusion of the oil drop experiment was that electric charge is quantized and exists in discrete units, confirming the existence of the elementary charge and providing precise measurements of its value.

Which oil is used in the Millikan oil drop method?

In the Millikan oil drop experiment, typically a fine mist of oil is used, such as mineral oil (commonly referred to as “light oil”) or sometimes a specific type of oil called “Dioctyl sebacate.” The choice of oil depends on factors such as its stability, uniformity, and suitability for the experiment’s requirements.

Why oil is used in the oil drop experiment?

The oil was used in vacuum apparatus and was chosen for its extremely low vapor pressure. Ordinary oils would evaporate under the heat of the light source, causing the mass of the oil drops to vary during the course of the experiment.

What is the working principle of the Millikan oil drop experiment?

The working principle of the Millikan oil drop experiment is based on the balance between gravitational force, buoyant force, and electric force acting on charged oil droplets. By measuring the motion of these droplets in an electric field, the experiment allows for the determination of the charge on individual droplets and provides insights into the discrete nature of the electric charge.

Stay tuned with Laws Of Nature for more useful and interesting content.

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