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 discovery of electrons

Dear Student,

The brief explanation for the discovery of electron is describes as:

Discovery of Electron (Michael Faraday’s Cathode Ray Discharge Tube Experiment)

  • Experimental Setup:

    • Glass tube is partially evacuated (Low pressure inside the tube)

    • Very high voltage is applied across the electrodes

  • Observation:
    Stream of particles move from the cathode (− ve) to the anode (+ ve) −−− known as cathode rays or cathode ray particles

  • Results:

    • Cathode rays move from the cathode to the anode.

    • Cathode rays are not visible. They can be observed with the help of phosphorescent or fluorescent materials (such as zinc sulphide).

    • These rays travel in a straight line in the absence of electric and magnetic fields.

    • The behaviour of cathode rays is similar to negatively charged particles (electrons) in the presence of an electrical or a magnetic field.

    • Characteristics of cathode rays do not depend upon: the material of the electrodes and the nature of the gas present in the tube

  • Conclusions:

    • Cathode rays consist of electrons.

    • Electrons are the basic units of all atoms.

Hope it is clear to you.

Cheers!

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Discovery of the Electron 



J.J. Thomson decided to find out for sure. Thomson was a physics professor at Cambridge University in the UK. He placed cathode tubes in electric and magnetic fields. He knew that these fields will move particles from side to side, but don't have much effect on how a wave moves. In his experiments, the cathode rays bent over to one side, so Thomson knew the cathode rays must be made of some small particle, which he dubbed a "corpuscle." Scientists worked with electricity long before they understood that current was made of electrons. The cathode tube was a prime example. By switching on some voltage, scientists could make fluorescent streams of electricity travel from the bottom part of a glass tube to the top -- but no one knew how it worked. Some thought the rays were a wave traveling through a mysterious "ether" which they thought permeated all space. Others thought the rays were streams of particles. 

Thomson initially thought his corpuscles were much too small to be of interest to anyone outside a science lab. However, people quickly realized that electric current was in fact made of moving electrons. Since electricity is the lifeblood of everything from computers to phones to microwaves, the electron turned out to be interesting to just about everybody. 

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The Discovery of Electron

 
 

In the 19th century there were only few scientists still not believing in atoms' subsistence. There was already much evidence for the hypothesis of an atom. As it was said before the word "atom" derives from the Greek word "atomos" meaning "indivisible". But were the atoms imagined by Dalton and Mendeleev really indivisible, and without internal structure? Scientists of the 19th century thought they were. But already in 1896 it emerged that they were wrong.

 

Michael Faraday

In 1838, Michael Faraday passed current through the glass tube filled with rarefied air. Conducting the experiment he noticed a strange light arc with its beginning at the anode (the positive electrode) and its end almost at the cathode (the negative electrode). The only place where there was no luminescence was just in front of the cathode. The place is called "cathode dark space", "Faraday dark space", or "Crookes dark space". That was the beginning of the long and "turbulent" time of researches on that luminescence. And the luminescence is called "cathode rays".

For long scientists couldn't agree what the nature of that phenomenon was. Some were of the opinion that luminescence in the tube was caused by particles, and some other thought that cathode rays were of wave nature. The dispute between the first ones believing in the corpuscular model of cathode rays and the second ones in the wave model lasted up until 1896.


Cathode rays going out the cathode
J.J. Thomson

The one who put the stop to the dispute was an English scientist named Joseph John Thomson. In his experiment to produce cathode rays he used a tube filled with rarefied gas but he made a bit of modification with the equipment. As it was said before cathode rays are emitted from the cathode and directed to the anode. What Thomson made with the equipment was a little gap in the anode. Through the gap a small beam of cathode rays got out of the area of the cathode and anode influence. Next, the beam passed through a long vacuum tube and fell on a fluoroscopic screen leaving there a fluorescent sign. In the vacuum tube Thomson put also two metal plates connected to a battery. That way he could create voltage between the plates, where the beam had its path. The field was directed perpendicularly to the cathode rays beam. It emerged that under the influence of voltage the beam was deflected (the spot on the screen appeared in a different place than without the voltage turned on). It was the final evidence that cathode rays consisted of charged particles- the other way the beam couldn't be deflected by the electric field. The direction of the deflection has shown of what charge the particles creating the beam are. It emerged to be the negative charge.


Determining q/m of the electrons

 


Determining q/m of the electrons

Knowing that cathode rays were formed of charged particles Thomson decided to measure the velocity of those particles. Except from the electric field he used the magnetic one. The deflection of a particle in the magnetic field depends on the velocity of the particle. Arranging the electric and the magnetic field levelling each other in their influence on the particles, and knowing the intensities of both fields one can calculate the velocity of the particles of cathode rays. That is what Thomson did.

Let's analyse the process of his considerations: Thomson assumed that a beam of cathode rays consisted of small particles having some negative charge q, and some mass m. And there is also some electric field intensity E we must include in the consideration. So the downward force influencing particles q is equal to:


  F1 = E*q  (1)

The magnetic field intensity is equal to B, and so the upward force is equal to:


  F2 = B*q*v  (2)

Equating the right sides of those two formulas (when the beam is not deflected up or down) one gets:


  E*q = B*q*v  (3)

So:


  v=E/B  (4)

Thomson was changing the intensities of the magnetic and electric fields this long until he got the beam not deflected. Yet he could calculate the velocity v of the particles. Then he turned off the electric field. In only the magnetic field negative particles moved on a trajectory, which is a part of a circle. The force influencing them can be presented by two formulas:


  F = B*q*v  (5)

(The force influencing a particle is given by the formula describing magnetic component of Lorentz force.)


  F = (m*v^2)/R  (5)

(Centripetal force in circle motion)

And so:


  B*q*v = (m*v^2)/R  (6)

or:


  q/m = (v)/B*R  (7)

Knowing v he had measured before and measuring B and R Thomson could yet calculate the charge to mass ratio for the particles of cathode rays.

This result was equal to between 1.0 and 1.4*1011 coulombs per kilogram.

In the second part of the experiment electric field can be used instead of magnetic one. Try to analyse such situation by yourself!


CHEERS!!!

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In the beginning of the nineteenth century, chemist John Dalton had, in the very first workable atomic theory, declared one of the fundamental principles of atoms – those tiny little particles that make up all matter in the universe – to be the fact that they were, by definition, indivisible. Before the end of that century, however, even before the entire scientific community had been convinced of the existence of these little things, the nature of atoms as the fundamental building blocks of matter was already being called into question. Atoms, it seemed, were made up of even smaller things.

Thompson’s Experiment

It was the British Physicist J.J. Thompson who is most often given credit for discovering the first subatomic particle – the electron. For the most part, this is true, but it should be known that the word itself had already been coined by Irish physicist George Johnstone Stoney (say his name out loud. It's kind of fun) three years prior to Thompson's discovery. Stoney had originally used the word in order to define a unit of negative electric charge.

Thompson's monumental discovery of the actual particle which would be called the electron came in 1897 while he was experimenting with a cathode ray tube (an electronic device where a beam of particles are sent between a positive and a negative terminal within a vacuum tube, creating a peculiar light).



Read more at Suite101: The Discovery of the Electron: J.J. Thompson and the First Subatomic Particle http://www.suite101.com/content/the-discovery-of-the-electron-a45714#ixzz1Ayvq5QOX
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dafa

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not dafa

michael faraday discovered electrons by experimenting a partialy evacuated cathode ray discharge tube by placing anode+ve charged and cathode -ve charged by giving a low pressure in tube and high voltage then a stream of particles named electrons by faraday which became the unit of all atoms

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