Introduction
The oil drop experiment was performed in 1909 by Robert A. Millikan and Harvey Fletcher to measure the elementary electric charge (it means the electron's charge). This experiment took place in the Ryerson Physical Laboratory, which is present at the University of Chicago. Also, this experiment has proved to be very crucial in physics.
Before this experiment, the existence of subatomic particles was not accepted universally. Millikan's apparatus has an electric field created between a parallel pair of metal plates held apart by an insulating material. The oil droplets, which are electrically charged, enter the electrical field and are balanced between two plates by altering the field. When the charged drops fell at a constant rate, the gravitational forces and electric forces on it were equal.
Principles of Millikan's Experiment
The Millikan experiment is complicated and fiddly while performing in school. It is more likely that we will use a simulation or a film clip of the experiment to show its principles to the students. Few of such principles are,
An oil drop can fall under its own weight. If a charge is given to the drop, it can be suspended by using an electric field. At this point, the electrostatic force balances the weight of every drop. Then the size of the electrostatic force depends entirely on the drop. So Millikan should have figured out the charge as soon as he knew the weight.
Millikan allowed the drop to fall through the air to find the weight of the drop. It reaches its terminal velocity quickly. At this point, the weight is balanced by the viscous drag of the air. Drag can be calculated from the Stokes' Law, which allowed Millikan to determine the weight.
Millikan repeated the same experiment thoroughly for over 150 oil drops and selected 58 of Millikan oil drop experiment results and got to find the highest common factor. It means the single unit of charge that could be multiplied up to give the charge he measured on all of his oil drops.
Oil Drop Experiment
Millikan allowed charged small oil droplets to travel through a hole into an electric field in the experiment. With the electric field's varying strength, the charge over an oil droplet is calculated, and it always comes as a fundamental value of 'e.'
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Apparatus
Millikan and Fletcher designed the experiment apparatus. It included two metal plates held at a distance by an insulated rod. There were four holes in the plate, three of which were there to allow light to pass through, and one was there to allow viewing through the microscope.
They did not use ordinary oil for this experiment, as it would evaporate by the heat of the light, and could cause an error in the Millikan Oil Drop Experiment. The oil, which is usually used in a vacuum apparatus with low vapour pressure, was also used.
Procedure
Oil passes through the atomizer, from where it came in tiny droplets form. The same droplets pass through the holes in the upper plate of the apparatus.
The droplet's downward movements are observed through the microscope and the mass of the oil droplets, and then their terminal velocity is measured.
The air present inside the chamber is ionised by passing through the X-ray beam. Collisions obtain the electrical charge on these oil droplets with gaseous ions produced by the ionisation of air.
Then, the electric field is set up between the two plates so that the motion of the charged oil droplets can be affected by the same electric field.
Now, gravity attracts the oil in a downward direction, and the electric field pushes the charge upwards. Also, the electric field strength is regulated so that all the oil droplets reach an equilibrium position with gravity.
The charge on the droplet is calculated at equilibrium, which depends on the mass of the droplet and strength of the electric field.
Millikan Oil Drop Experiment Calculations
The experiment initially allows the oil drops to fall between the plates in the absence of the electric field. They accelerate first due to gravity, but gradually the oil droplets slow down because of air resistance.
The Millikan oil drop experiment formula can be given as below.
Fup = Q ⋅ E Fdown = m
Where Q is an electron’s charge, m is the droplet’s mass, E is the electric field, and g is gravity.
Q ⋅ E = m ⋅ g
Q = m.gE
By this, one can identify how an electron charge is measured by Millikan. Millikan also found that all the drops had charges, which were 1.6x 10-19 C multiples.
Importance of Millikan's Oil Drop Experiment
Millikan's experiment is quite essential because it establishes the charge on an electron.
Millikan used a simple apparatus in which he balanced the actions of electric, gravitational, and air drag forces.
Using the apparatus, he was able to calculate the charge on an electron as 1.60 × 10-19 C.
Conclusion
The charge for any oil droplet is always an integral value of e (1.6 x 10-19). Thus, Millikan's Oil Drop Experiment concludes that the charge is said to be quantized, which means that the charge on any particle will be an integral multiple of e always.
Millikan discovered the charge on a single electron using a uniform electric field between the oil drops and two parallel charged plates.
FAQs on Millikan's Oil Drop Experiment
1. What is the primary purpose of Millikan's Oil Drop experiment?
The primary purpose of Millikan's Oil Drop experiment, conducted in 1909, was to determine the precise value of the elementary electric charge, which is the charge of a single electron. This experiment provided the first direct and compelling evidence that electric charge is quantized, meaning it exists in discrete, fundamental units.
2. What was the main conclusion drawn from Millikan's Oil Drop experiment?
The most significant conclusion from the experiment was the quantization of charge. Millikan observed that the electric charge on any oil droplet was always an integer multiple of a fundamental value. He calculated this fundamental unit of charge 'e' to be approximately 1.602 × 10⁻¹⁹ Coulombs, confirming it as the charge of one electron.
3. Can you explain the basic step-by-step procedure of the experiment?
The experiment follows a systematic procedure to measure the charge on tiny oil droplets:
- Atomization: A fine mist of oil droplets is sprayed into the upper section of the apparatus.
- Gravitational Fall: The droplets fall under gravity through a small pinhole into a region between two parallel metal plates.
- Ionization: The air in the chamber is ionized using X-rays, which causes the neutral oil droplets to acquire an electric charge (typically negative) by capturing electrons.
- Application of Electric Field: A uniform electric field is applied between the plates. By adjusting the voltage, the upward electric force on a charged droplet can be controlled.
- Balancing Forces: The voltage is adjusted until the upward electric force on a single droplet exactly balances the downward gravitational force, causing the droplet to be suspended motionlessly.
- Calculation: By measuring the electric field strength required to suspend the droplet and determining the droplet's mass, the charge on the droplet is calculated.
4. What forces act on an oil droplet during this experiment?
Three primary forces act on an oil droplet:
- Gravitational Force (F_g): This is the weight of the droplet (mass × acceleration due to gravity), always acting in a downward direction.
- Electric Force (F_e): This force acts on the charged droplet when the electric field is on. Its direction (upward or downward) depends on the charge of the drop and the polarity of the plates. To suspend a negatively charged drop, this force must act upwards.
- Viscous Drag Force: This is the force of air resistance, which always opposes the droplet's motion. It is significant when the droplet is moving but becomes zero when the droplet is held stationary.
5. Why was oil specifically used in the experiment and not a more common liquid like water?
Oil was chosen primarily because of its very low rate of evaporation. The experiment required the mass of the oil droplet to remain constant throughout the observation period. If a liquid like water were used, it would evaporate under the heat of the illuminating lamp, causing the droplet's mass to decrease and leading to highly inaccurate charge measurements.
6. How is the charge on an oil drop calculated from the experimental setup?
The charge is calculated when a droplet is suspended motionless in the electric field. At this point, the upward electric force (qE) is equal to the downward gravitational force (mg). The equation is qE = mg. The mass (m) is determined by observing the droplet's terminal velocity as it falls without the electric field. With the mass (m), the electric field strength (E), and the acceleration due to gravity (g) all known, the charge (q) can be calculated using the formula: q = mg / E.
7. What is the importance of 'quantization of charge' as a concept?
The concept of charge quantization is a cornerstone of modern physics. It establishes that electric charge is not a continuous fluid but exists in fundamental, indivisible packets. Every observable charge in the universe is an integer multiple of the elementary charge 'e'. Millikan's experiment provided the first concrete proof of this idea, which was essential for the development of the atomic model and quantum mechanics.
8. What was the purpose of using X-rays to ionize the air in the chamber?
The X-rays served a critical function: to impart an electric charge onto the otherwise neutral oil droplets. The X-ray beam would knock electrons off air molecules, creating free electrons and positive ions. The oil droplets would then acquire a net negative charge by capturing one or more of these free electrons. Without this step, the droplets would be uncharged and would not be affected by the electric field, making the experiment impossible.
