oil drop experiment explanation

Dalton's Model of the Atom / J.J. Thomson / Millikan's Oil Drop Experiment / Rutherford / Niels Bohr / DeBroglie / Heisenberg / Planck / Schrödinger / Chadwick

from- http://en.wikipedia.org/wiki/Oil_drop_experiment

With this data and J.J. Thompsons charge to mass  ratio the mass of the electron could be calculated.

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The Millikan oil drop experiment

In this experiment I calculated the electric charge of an oil drop by measuring the force it experienced from a known electric field. I also calculated how the force, on a charged particle, varied when the electric field changed. Since the force exerted by an electric field on an object with very little charge is also very small, I had to observe tiny oil drops falling and rising really small distances. The main result of this experiment was that focusing on tiny dots for more than 15 minutes made my vision blurry.

The experiment consisted of a two plate capacitator that had a plastic spacer between them. The capacitator was placed within a chamber and had a small hole in the middle where I could introduce the oil drops . There was also a viewing scope mounted so that I could see the hole in the spacer and observe the falling oil drops. I was able to see the drops thanks to a halogen lamp that illuminated the drops. The viewing scope had a grid, whose major lines were separated by 0.5 mm, that allowed me to measure the distance the drops fell.

The oild drops I saw through the scope were kind of like this, just smaller and bright gold.

The capacitators were given voltage by a high voltage DC power supply. There was also a thermistor mounted on the bottom capacitator plate. A thermistor is a resistor whose resistance varies a lot with temperature, and I measured its resistance with a multimeter. With the given resistance I was able to calculate the temperature within the chamber, which was important for later calculations. Lastly, there was an ionization source within the chamber that could be turned on and off. This would create excess electrons that attached themselves to some of the oil drops, and increased their electric charge.

The viewing scope is right in front of the cylindrical chamber, and is placed at eye level.

The high voltage DC power supply is on the bottom. Above it, there are two multimeters.

I observed the fall and rise of oil drops in the chamber because the force they experienced also affected their velocities. The force from the electric field would speed up the drops fall and rise. On the other hand, gravity slowed the rise of the particles, so the oil drops velocity was different if it was falling or rising. It seems a little crazy that oil drops can rise, but that was possible because I could reverse the polarization of the electric field. When I did this, I made the electric field and the force point upwards.

It took me forever, but I was able to choose a single oil drop and make it fall and rise in different electric fields, caused by different voltages. I measured the time it took to fall and rise a distance of 0.5 mm, with a voltage of 306.5v, 397v, 483v, 206.2v and 103.1v. I observed that the greater the voltage was, the smaller the time interval was.

The velocity of falling and rising drops in different voltages increases linearly with voltage. Note that the graph is not centered at the origin.

I also measured the fall and rise of a drop that was exposed to the ionization source. I did not change the electric field and kept the voltage at 245.7 volts. First, I measured the velocity of the drop without turning on the ionization source. Then, I turned on the ionization source for 5 s, and measured the velocity of the drop. I turned on the source one last time and measure the velocity of the drop again. When I turned on the source, it emitted excess electrons that attached themselves to the drops. This increased the charge of the drop and as a result increased the force and the velocity of the drop. This occurred in the first time I turned on the source, but the second time the drop slowed down considerably. Something happened so that the drop didn’t gain electrons, but lost them.

The drop was exposed to an ionization source for 5 second intervals. Curiously, the second exposure resulted in the drop losing electrons instead of gaining them.

I was able to calculate the electric charge of each drop whose velocity I measured, as well as the drop whose charge changed. I did this with the following equation:

I found the charge of the electron with this equation

I calculated the charge of each drop, divided it with the charge of the electron and rounded up the result. The drop I whose velocity I measured as function of voltage had six electrons. The drop that wasn’t exposed to the source in the field created by a voltage of 2457 v had 14 electrons. The drop then gained 4 electrons, and had a charge of q=18 e. The drop was exposed a second time to the source and it lost 10 electrons, so it had a charge of q=8e. The loss of electrons cannot be explained as the result of the ionization source, but it is possible another drop stole the electrons. There might be another, better explanation.

In conclusion, the Millikan oil drop experiment is a very pretty experiment and the fact that you can measure the charge of the electron so precisely is kind of amazing. On the other hand, it was very difficult to NOT to lose a drop, and it took hours to find a drop that was a “perfect match” (Wenqi’s words). So, I’m not sure whether I like this experiment or not.

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• Millikan's Oil Drop Experiment

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.'

(Image will be uploaded soon)

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.

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.

F up = Q ⋅ E   F down = 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

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.

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 Millikan’s Oil Drop experiment?

In 1909, Robert Millikan and Harvey Fletcher conducted the canvas drop trial to determine the charge of an electron. They suspended bitsy charged driblets of canvas between two essence electrodes by balancing downcast gravitational force with upward drag and electric forces. The viscosity of the canvas was known, so Millikan and Fletcher could determine the driblets’ millions from their observed diameters (since from the diameters they could calculate the volume and therefore, the mass). Using the known field and therefore the values of graveness and mass, Millikan and Fletcher determined the charge on canvas driblets in mechanical equilibrium. By repeating the trial, they verified that the charges were all multiples of some abecedarian value. They calculated this value to be1.5924 × 10 −19 Coulombs (C), which is within 1 of the presently accepted value of1.602176487 × 10 −19 C. They proposed that this was the charge of one electron.

2. How did the process work?

The outfit incorporated a brace of essence plates and a specific type of canvas. Millikan and Fletcher discovered it had been stylish to use a canvas with a particularly low vapor pressure, similar together designed to be used during a vacuum outfit. Ordinary canvas would dematerialize under the heat of the light source, causing the mass of the canvas to drop to change over the course of the trial.

By applying an implicit difference across a resemblant brace of vertical essence plates, an invariant electric field was created in the space between them. A ring of separating material was used to hold the plates piecemeal. Four holes were dug into the ring — three for illumination by a bright light and another to permit viewing through a microscope. A fine mist of canvas driblets was scattered into a chamber above the plates. The canvas drops came electrically charged through disunion with the snoot as they were scattered. Alternatively, the charge could be convinced by including an ionizing radiation source ( similar to an X-ray tube).

3. Describe the Millikan’s Oil Drop experiment procedure?

Canvas is passed through the atomizer from where it came in the form of bitsy driblets. They pass the driblets through the holes present in the upper plate of the outfit.

The downcast movements of driblets are observed through a microscope and the mass of canvas driblets also measure their terminal haste.

The air inside the chamber is ionised by passing a ray of X-rays through it. The electrical charge on these canvas driblets is acquired by collisions with gassy ions produced by the ionisation of air.

The electric field is set up between the two plates and so the stir of charged canvas driblets can be affected by the electric field.

Graveness attracts the canvas in a downcast direction and the electric field pushes the charge overhead. The strength of the electric field is regulated so that the canvas drop reaches an equilibrium position with graveness.

The charge over the drop is calculated at equilibrium, which depends on the strength of the electrical field and the mass of the drop.

4. Explain Millikan’s Oil Drop experiment in detail?

Millikan’s original trial or any modified interpretation, similar to the following, is called the canvas-drop trial. An unrestricted chamber with transparent sides is fitted with two resemblant essence plates, which acquire a positive or negative charge when an electric current is applied. At the launch of the trial, an atomizer sprays a fine mist of canvas driblets into the upper portion of the chamber. Under the influence of gravity and air resistance, some of the canvas driblets fall through a small hole cut in the top essence plate. When the space between the essence plates is ionized by radiation (e.g., X-rays), electrons from the air attach themselves to the falling canvas driblets, causing them to acquire a negative charge. 

A light source, set at right angles to a viewing microscope, illuminates the canvas driblets and makes them appear as bright stars while they fall. The mass of a single charged drop can be calculated by observing how presto it falls. By confirming the implicit difference, or voltage, between the essence plates, the speed of the drop’s stir can be increased or dropped; when the quantum of upward electric force equals the given downcast gravitational force, the charged drop remains stationary. The quantum of voltage demanded to suspend a drop is used along with its mass to determine the overall electric charge on the drop.

Through the repeated operation of this system, the values of the electric charge on individual canvas drops are always whole- number multiples of the smallest value — that value being the abecedarian electric charge itself (about1.602 × 10 −19 coulomb). From the time of Millikan’s original trial, this system offered satisfying evidence that electric charge exists in introductory natural units. All posterior distinct styles of measuring the introductory unit of electric charge point to its having the same abecedarian value. 

5. How does Millikan’s Oil Drop experiment work?

Simplified scheme of Millikan’s canvas-drop trial This outfit has a resemblant brace of vertical essence plates. An invariant electric field is created between them. The ring has three holes for illumination and one for viewing through a microscope. A specific type of canvas is scattered into the chamber, where drops come electrically charged. The driblets enter the space between the plates and can be controlled by changing the voltage across the plates. 

The driblets entered the space between the plates and, because they were charged, they could be controlled by changing the voltage across the plates. Originally, the canvas drops were allowed to fall between the plates with the electric field turned off. The snappily reached terminal haste due to disunion with the air in the chamber. The field was turned on and, if it was large enough, some of the drops (the charged bones) would start to rise. This is because the overhead electric force, FE, is lesser for them than the down gravitational force,g. (A charged rubber rod can pick up bits of paper in the same way.) A likely-looking drop was named and kept in the middle of the field of view by alternatively switching off the voltage until all the other drops fell. The trial was continued with this single drop. Millikan’s canvas drop trial measured the charge of an electron. Before this trial, the actuality of subatomic patches wasn't widely accepted. 

Millikan’s outfit contained an electric field created between a resemblant brace of essence plates, which were held piecemeal by separating material. Electrically charged canvas driblets entered the electric field and were balanced between two plates by altering the field. 

6. Why was the Negative Plate Earthed in Millikan's Oil Drop Experiment?

There are three possible reasonable ways to clear it.

The first reason is safety. Grounding ("earthing," in this context), the equipment is so important, particularly the time when you are working with high voltages. The same would be applied to protecting the equipment and for personal safety as well.

The second reason would be to establish a good stable reference point for the voltage measurement. A massive and solidly connected grounding cable would perform that job in a better way.

Finally, from an electrical standpoint, the two plates used in Millikan's experiment form a capacitor. On the other side, this capacitor is being charged to a very high voltage. In such cases, it is suggested to have a discharge path on one of the terminals or plates in order to avoid damage to either humans or equipment as well. Therefore, the negative plate is earthed.

7. Why do we Use Oil Instead of Other Liquids in the Millikan Oil-drop Experiment?

Oil is one of the best liquids for Millikan's oil drop experiment. It retains its mass over a while and exposes to higher temperatures. Also, we employ an atomizer for ultra-fine droplets. So less dense liquids like water and oils are preferred over water because water cannot survive at such higher temperatures.

The atomizer employment is also an important reason behind using oil for this experiment. Moreover, it should be noted that oil would retain the exact volume/mass/weight. This would enable an exact measurement of the charge. Other liquids would separate or dissipate or even evaporate.

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The Millikan Oil Drop Experiment

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Description This simulation is a simplified version of an experiment done by Robert Milliken in the early 1900s. Hoping to learn more about charge, Milliken sprayed slightly ionized oil droplets into an electric field and made observations of the droplets. When the voltage is zero and the run button is pressed, the drop will fall due to the force of gravity. It will reach a terminal velocity (v t ) as it falls. Pause the simulation while you record the terminal velocity. This terminal velocity can be used to determine the mass of the drop. Use the equation: mass = kv t 2 to determine the mass of the particle. The value of k in this simulation is 4.086 x 10 -17 kg s 2 /m 2 . Once the terminal velocity is recorded and the mass calculated, with the simulation still paused increase the voltage between the plates until the two force vectors are approximately equal length. This will produce an upward field and an upward force on the positive droplets. If the upward force of the electric field is equal to the downward force of gravity, and the drag force is zero, the particle will not accelerate. To be sure that the lack of acceleration is not related to drag forces, the velocity must also be zero as well as the acceleration in order to be sure that the two forces are balanced. Increase and decrease the voltage (use the left/right arrow keys) until both the acceleration and velocity are at zero. The velocity may not stay at exactly zero, but find the voltage that has the velocity changing most slowly as it passes v = 0. Use the methods discussed above to ultimately determine the charge on ten (or more) different oil-drops. Use V = Ed to calculate the field strength (d = 5 cm = 0.05 m). Use Eq = mg when the velocity is zero to determine the charge q on the droplet. Record all your data in a table or spreadsheet. After you get each q, create a new particle and start again. When you have the table filled in, look at the various values for q. Is there any pattern to them, or are they seemingly random? Can you draw any conclusions from the Q measurements?

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Understanding Millikan's oil-drop experiment

This is quoted from A.P. French's Newtonian Mechanics about Millikan's oil-drop experiment :

The droplets randomly produced in a mist of oil vapor are of various sizes. The ones that Millikan found most suitable were the smallest. But these droplets were so tiny that even through a medium-power microscope, they appeared against a dark background merely as points of light; no direct measurement of size could be made. However, he used a clever trick of exploiting the law of viscous resistance by applying it to the fall of the droplet under the gravitational force alone. Under these conditions: $$F = \dfrac{4\pi\rho r^3 g}{3}.$$ The terminal velocity of the droplet under gravitational field is then given by: $$v_g = \dfrac{4\pi}{3} \cdot \dfrac{\rho g}{c_1} r^2.$$ Putting in the approximate numerical values, we find, $$v_g \approx 10^8 r^2.$$ Putting $r \approx 1 \mu = 10^{-6}$ , we have $$v \approx 10^{-4} ~\text{m/s}.$$ Such a droplet would take over 1 min to fall 1 cm in air under its own weight, thus allowing precision measurements of its speed. It is worth noting the dynamical stability of this system, and indeed of any situation involving a constant driving force that increases monotically with speed. If by chance the droplet should slow down a little, there is a net force that will speed it up. Conversely, if it should speed up, it is subjected to a net retarding force. [. . .] Millikan was able to follow the motion of a given droplet for many hours on end, using its electric charge as a handle by which to pull it up or down at will. In the course of such protracted observations, the charge on the drop would often change spontaneously, and several different values of terminal velocities would be obtained. The crucial observation was that in such experiment, with a given value of the voltage, the terminal speed was limited to a set of sharp and distinct values, implying that the electric charge comes in discrete units.

If he already knew the radius, what advantage did he get by measuring the terminal velocity in the absence of electric field?

What thing does speed up the drop when it slows down?

Why should the charge on a given droplet change as mentioned in the last para?

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• $\begingroup$ How is $r=1\mu$ found? and is it important to know $r$ to calculate the charge $q$? $\endgroup$ –  innisfree Commented Apr 17, 2015 at 11:51
• $\begingroup$ @innisfree: Sir, I'm not understanding it. It is written by the author. And that's what I want to know what was the purpose behind all that if he knew everything $\endgroup$ –  user36790 Commented Apr 17, 2015 at 12:10
• $\begingroup$ The quote is clear on the reason: "no direct measurement of size could be made. However, he used a clever trick of exploiting the law of viscous resistance by applying it to the fall of the droplet under the gravitational force alone." - this is done to get the size of the droplet. $\endgroup$ –  ACuriousMind ♦ Commented Apr 17, 2015 at 12:17
• 1 $\begingroup$ $r=1\mu m$ is a result found from the freefall experiment rather than prior information - he didn't know it before he began his experiment. @ACuriousMind to be fair, i don't think the text is particularly clear on that point. $\endgroup$ –  innisfree Commented Apr 17, 2015 at 12:23

Answering your three questions:

• He knows the relationship between radius and terminal velocity, but the drops are too small to measure their radius with any accuracy (1 µm is tiny - looking at such a drop from a distance makes it no more than a speck of light, even with a "micro telescope"). Meauring the terminal velocity, he can deduce the size accurately.
• If you slow the drop down, it no longer is at terminal velocity - gravity is unchanged, but the drag becomes less. The difference is a net force that accelerates the drop. This is why they stay at the terminal velocity (seems a convoluted way to explain why "at terminal velocity" is a stable state... but that is basically what they are saying)
• The charge on the droplet is a net charge. Interactions with ions in the air, with ionizing (background) radiation etc can cause an electron to jump on or off (net charge on drop is almost never exactly zero... and if it is non zero, you can expect it not to be absolutely constant). These changes in charge turned out to occur in discrete steps - calculated changes in the electrical force on the droplet corresponded to charge adding or subtracting in discrete units.

It was a very clever experiment - so much to learn from the methods used as well as from the results obtained.

• $\begingroup$ Again thanking you for this answer, I want to ask you again question no:1. Why did Mr. Millikan performed the experiment in absence of electric field? He wanted to know the radius, right? But as French wrote " Putting in the approximate numerical values, we find, $v_g \approx 10^8 r^2$. Putting $r \approx 1μ = 10^{−6}$...", it seems that he knew before experiment that the radius is one micron. What is it all about then? $\endgroup$ –  user36790 Commented May 26, 2015 at 9:38
• $\begingroup$ He knew the radius is approximately one micron - but could only determine it with sufficient accuracy by doing the free fall (no field) observation. $\endgroup$ –  Floris Commented May 26, 2015 at 10:24
• $\begingroup$ "Such a droplet would take over 1 min to fall 1 cm in air under its own weight, thus allowing precision measurements of its speed." So, he measured the velocity first & when it matched, he concluded that the radius was one micron, right? $\endgroup$ –  user36790 Commented May 26, 2015 at 18:13
• 1 $\begingroup$ Yes and no. "One micron" is the order of magnitude, not the exact size. Bigger drops fell too fast and literally "dropped out" of the experiment. The experiment selected for very small but observable drops, whose exact size was measured from highly precise terminal velocity measurement. $\endgroup$ –  Floris Commented May 26, 2015 at 18:15

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