Millikan's oil drop experiment

[Arnav]

In Millikan's oil drop experiment, when the oil droplets fell through the hole in the top plate, and passed through the ionised air, did only electrons get attached to the droplets? Or both the electrons and the cations? 

In my book, only electrons are shown as attached to the droplet. Why only electrons got stuck? Why not the positive ions as well? Or did they?

[exchemist]

1 hour ago, Arnav said:

In Millikan's oil drop experiment, when the oil droplets fell through the hole in the top plate, and passed through the ionised air, did only electrons get attached to the droplets? Or both the electrons and the cations? 

In my book, only electrons are shown as attached to the droplet. Why only electrons got stuck? Why not the positive ions as well? Or did they?

It's a good question. I suspect it is to do with speed of motion. The electrons ejected from the oxygen and nitrogen molecules are light and move fast compared to the molecular cations left behind, so they will tend to encounter the oil droplets more quickly and more often, I imagine.  

[studiot]

1 hour ago, Arnav said:

In Millikan's oil drop experiment, when the oil droplets fell through the hole in the top plate, and passed through the ionised air, did only electrons get attached to the droplets? Or both the electrons and the cations? 

In my book, only electrons are shown as attached to the droplet. Why only electrons got stuck? Why not the positive ions as well? Or did they?

It's a very good and perceptive question. +1

It should be noted that the original experiment as conducted by Millikan was very different from the simplified one studied in schools and actually carried out in some of them.

In particular neither Millikan nor anybody else originally knew what the charge was.

The droplets from the atomiser were ionized by means of X rays.

Since then it has been discovered that friction within the atomiser is sufficient to ionise the droplets and that they then carry a negative charge.

Also it was not originally known that there was a unit charge, e.

This actually came out in the original experiment.

Millikan found that charges by balancing gravitational, frictional (viscous) and electrostatic forces on specific drops.

He discovered that the charge was negative and could  only be changed in integer multiples of a base unit of charge, e and the change could not be made smaller than this.

[MigL]

Ah, memories ...

I remember doing a simplistic Millikan's oil drop experiment in high school, and a much more elaborate one in 1st year Uni.
Or maybe, it was being in my late teens that I remember fondly.

[Arnav]

10 hours ago, studiot said:

In particular neither Millikan nor anybody else originally knew what the charge was.

 

Could you please elaborate what do u mean by "charge"? I mean, are u refering to "kind of charge(+ or -) on the droplets"?

One of my ideas is this: 

The oil droplets caught both - and + charges,(some caught only -, some only +, while some both).

Millikan knew how he had arranged the electrodes in the apparatus and how positive and negative charges would interact with them.

If the charge on a droplet was net +, it would accelerate down when Electric field strength was increased while it would decelerate if it had net - charge. If it was neutral, there would be no effect.

So, Millikan could determine the kind of charge on the droplet just by observing the effect of changing electric field on its vertical speed.

Could the situation have been like this?

11 hours ago, studiot said:

He discovered that the charge was negative and could  only be changed in integer multiples of a base unit of charge, e and the change could not be made smaller than this.

Do you mean that he discovered that the charge was negative on every droplet? If so, why no + charge was found?

[exchemist]

1 hour ago, Arnav said:

Could you please elaborate what do u mean by "charge"? I mean, are u refering to "kind of charge(+ or -) on the droplets"?

One of my ideas is this: 

The oil droplets caught both - and + charges,(some caught only -, some only +, while some both).

Millikan knew how he had arranged the electrodes in the apparatus and how positive and negative charges would interact with them.

If the charge on a droplet was net +, it would accelerate down when Electric field strength was increased while it would decelerate if it had net - charge. If it was neutral, there would be no effect.

So, Millikan could determine the kind of charge on the droplet just by observing the effect of changing electric field on its vertical speed.

Could the situation have been like this?

Do you mean that he discovered that the charge was negative on every droplet? If so, why no + charge was found?

You may be onto something there. If there were any +ve charged oil drops, they would not be held suspended by the electric field but would be accelerated downwards. So the design of the experiment would have effectively filtered out any +ve charged droplets, allowing the experimenter to concentrate on those with -ve charge. But as I said in my first reply, my suspicion is there would have been relatively few +ve charged droplets anyway.

Edited June 8, 2021 by exchemist

[swansont]

1 hour ago, exchemist said:

You may be onto something there. If there were any +ve charged oil drops, they would not be held suspended by the electric field but would be accelerated downwards. So the design of the experiment would have effectively filtered out any +ve charged droplets, allowing the experimenter to concentrate on those with -ve charge. But as I said in my first reply, my suspicion is there would have been relatively few +ve charged droplets anyway.

It’s likely Millikan tried both polarities, and pursued the configuration that was giving more interesting results. There had to be the ability to form more than one charge state, and there may be reasons that attaching multiple ions may not work (i.e. a chemical bond would be involved), but works for the electrostatic attraction with electrons

[John Cuthber]

By the time Millikan was doing his experiment, the nature of the electron was sort of know.
Thomson had characterised cathode rays as a stream of some sort of particle.
It's plausible that Millikan was trying to characterise the particles better.

[studiot]

Sorry I wasn't completely clear.

The thing is that 'Millikan's Experiment' was not one single experiment at all.
It was a determined effort by Millikan to measure several important properties of 'the electron' over several experiments during the years from 1909 to 1913.
The results and conclusions of this work were first published Phil. Mag., 34, p1, 1917
and later in a series of books which started out with the title The Electron and was revised a couple of times to

The electron: its isolation and measurement and the determination of some of its properties.  (1919) and later editions

Electrons (+ and -), Protons, Photons, Neutrons, Mesotrons and Cosmic Rays  (1936)

As a result of this work and also work on the photoelectric effect Millikan was awarded the 1923 Nobel prize.

The result of this ongoing work over an extended time period, during which other people also added discoveries has resulted in variations in modern accounts in more modern texts.

However the importance of this work is that it enabled the drawing together of several branches of Physics and Chemistry towards the more coherent whole we have today.

 

Before Millikan, Faraday had discovered the laws of electrolysis and Avogadro had presented his hypothesis, both in the 1830s.
Then, however the molecule was not at all established, Dalton's atoms were still on pretty shaky ground and ions and ionisation were yet to come.
Between then and the late 1800s the particulate nature of matter became more and more established, but electricity was seen as quite a different subject.
The idea that the particles of matter were held together by electric forces was yet to arrive. Electricity was known to come in two polarities, positive and negative, but details were not known.

Then in 1897 Thomson discoverd 'particles' of electricity. He had discovered the electron.
Furthermore he measured the ratio of the charge to mass, e/m for this particle.
Then in 1909 Perrin came up with a good value for the Avogadro constant or number.
That is the number of particles in a mole.
It was seen that this tied in with Faraday's work since 96500 coulombs were required to deposit 1 mole of a monovalent element.
Since the proposition was that 1 mole contained a large (Avogadro's) number if identical particles it followed that an identical charge must be supplied to deposit each one and that these might be tied in with Thomson's electrons.
Thomson and independently Wilson were experimenting with the production of ions in gases and measuring their charge, both positive and negative. (This part is not normally taught in chool Physics these days) but it was their methods that Millikan drew upon and extended so that:

It was at this point that Millikan entered with his series of experiments that were able to determine not only the values of both the mass and charge on the electron but that the charge was equal to the 96500 coulombs divided by Avogadro's number and that it was negative.
So electrons were particles that were carriers of a fixed amount of negative charge that also possessed a small amount of mass compared to any atom of any element.

The way was now open for Physicists to develop atomic models and Chemists to develop electron exchange models of ions and valency (chemical bonding).
Both of which developed rapidly in the early 1900s.

 

In his actual experiments Millikan changed Wilson's 'condenstaion of water' method to a fine spray of oil.
This fine spray did not evaporate like water and could be controlled and came ready with a small charge due to friction in the atomiser nozzle.
Since this was a small charge and many droplets were not charged at all, in later experiments he followed Wilson in irradiating the air in the chamber with X rays.
This first ionised some of the air and then the air particles transferred this to the droplets by collision.

A swansont notes, he was able to control the potential on his plates so the he could measure for both positive and negatively charged droplets as he did not initially know which would occur.
Today we sometimes use alpha rays (positive) insted of X rays. These steal electrons from the gas, creating positive gas ions, which in turn regain electrons from the oil droplets, creating positive oil droplets. A rays, being neutral will separate electrons from the gas particles, creating posotv gas ions and free electrons, some of which attach to the oil droplets forming negative ions.

So Millikan's original equation was

If a droplet aquires a charge q, then the resultant force on the droplet will be mg ± Eq depending upon the sign of the charge q. (E is the strength of the electric field between the plates)

I assume you have an idea of the method but I can provide more detail if you like.

 

 

 

 

 

[Arnav]

15 hours ago, studiot said:

Sorry I wasn't completely clear.

The thing is that 'Millikan's Experiment' was not one single experiment at all.
It was a determined effort by Millikan to measure several important properties of 'the electron' over several experiments during the years from 1909 to 1913.
The results and conclusions of this work were first published Phil. Mag., 34, p1, 1917
and later in a series of books which started out with the title The Electron and was revised a couple of times to

The electron: its isolation and measurement and the determination of some of its properties.  (1919) and later editions

Electrons (+ and -), Protons, Photons, Neutrons, Mesotrons and Cosmic Rays  (1936)

As a result of this work and also work on the photoelectric effect Millikan was awarded the 1923 Nobel prize.

The result of this ongoing work over an extended time period, during which other people also added discoveries has resulted in variations in modern accounts in more modern texts.

However the importance of this work is that it enabled the drawing together of several branches of Physics and Chemistry towards the more coherent whole we have today.

 

Before Millikan, Faraday had discovered the laws of electrolysis and Avogadro had presented his hypothesis, both in the 1830s.
Then, however the molecule was not at all established, Dalton's atoms were still on pretty shaky ground and ions and ionisation were yet to come.
Between then and the late 1800s the particulate nature of matter became more and more established, but electricity was seen as quite a different subject.
The idea that the particles of matter were held together by electric forces was yet to arrive. Electricity was known to come in two polarities, positive and negative, but details were not known.

Then in 1897 Thomson discoverd 'particles' of electricity. He had discovered the electron.
Furthermore he measured the ratio of the charge to mass, e/m for this particle.
Then in 1909 Perrin came up with a good value for the Avogadro constant or number.
That is the number of particles in a mole.
It was seen that this tied in with Faraday's work since 96500 coulombs were required to deposit 1 mole of a monovalent element.
Since the proposition was that 1 mole contained a large (Avogadro's) number if identical particles it followed that an identical charge must be supplied to deposit each one and that these might be tied in with Thomson's electrons.
Thomson and independently Wilson were experimenting with the production of ions in gases and measuring their charge, both positive and negative. (This part is not normally taught in chool Physics these days) but it was their methods that Millikan drew upon and extended so that:

It was at this point that Millikan entered with his series of experiments that were able to determine not only the values of both the mass and charge on the electron but that the charge was equal to the 96500 coulombs divided by Avogadro's number and that it was negative.
So electrons were particles that were carriers of a fixed amount of negative charge that also possessed a small amount of mass compared to any atom of any element.

The way was now open for Physicists to develop atomic models and Chemists to develop electron exchange models of ions and valency (chemical bonding).
Both of which developed rapidly in the early 1900s.

 

In his actual experiments Millikan changed Wilson's 'condenstaion of water' method to a fine spray of oil.
This fine spray did not evaporate like water and could be controlled and came ready with a small charge due to friction in the atomiser nozzle.
Since this was a small charge and many droplets were not charged at all, in later experiments he followed Wilson in irradiating the air in the chamber with X rays.
This first ionised some of the air and then the air particles transferred this to the droplets by collision.

A swansont notes, he was able to control the potential on his plates so the he could measure for both positive and negatively charged droplets as he did not initially know which would occur.
Today we sometimes use alpha rays (positive) insted of X rays. These steal electrons from the gas, creating positive gas ions, which in turn regain electrons from the oil droplets, creating positive oil droplets. A rays, being neutral will separate electrons from the gas particles, creating posotv gas ions and free electrons, some of which attach to the oil droplets forming negative ions.

So Millikan's original equation was

If a droplet aquires a charge q, then the resultant force on the droplet will be mg ± Eq depending upon the sign of the charge q. (E is the strength of the electric field between the plates)

I assume you have an idea of the method but I can provide more detail if you like.

 

 

 

 

 

Thank you studiot for the in depth explanation

15 hours ago, studiot said: