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Overview         (For age - group : Above 16 )

Educational atomic physics animation : Discovery of electron took a long journey over hundreds of years from lightning to discharge tubes to cathode rays and finally discovery of electron by J. J Thomson, this 50 minute animation looks at every aspect of this discovery.

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Science : Atomic theory
Type :
Animation Type :
Total animation length: 50 minutes

The animation covers:

  • Perfecting the CRT?
  • Thomson's experiment in detail
  • Calculations to determine e/m
  • Thomson's Atomic model - Plum pudding
  • How electron was found to be the subatomic particle?


More about Cathode rays in explanatory notes:
a) Description of the discharge tube- what is happening inside the discharge tube, why high voltage is essential to produce glow, why low pressure is required for glow……….
b) History of Cathode rays
c) Properties of cathode rays
d) Uses of cathode rays
e) Another method to determine e/m

Discovery of electron from lightning
Discovery of electron from lightning

Thomson saw electrons where others saw sparks

Thomson saw electrons where others saw sparks

Modifications were made in discharge tube by Thomson

Modifications were made in discharge tube by Thomson

Plum pudding or chocolate cookies model

Plum pudding or chocolate cookies model
Details of the animation/ movie /software

amber rubbed on fur gets electrostatically charged

Electrons are the negatively charged subatomic particles. They have negligible mass of about 9.1 * 10(-31) Kg and a charge of 1.6*10(-19) C.

Sir John Joseph Thomson first discovered the electron in 1897 while studying the nature of cathode rays.

The cathode rays were a subject of interest from over almost two centuries.

early model of cathode ray tube
It was observed that whenever the glass tube filled with air at low pressure and sealed with electrodes at both the ends was subjected to about 10,000 volts of electricity, a strange glow was produced around the edges of the wide end of the glass tube, irrespective of the type of gases present in it. (The glass tube with sealed electrodes is called discharge tube.)

Many Scientists like Faraday, Goldstein, Perrin, Crooke, and Hertz investigated on the properties of the cathode rays.

behaviour of a charge in electric and magnetic  field
The properties of the cathode rays that they discovered during the experimentation was that:

  • They were produced by the negative electrode, or cathode, in an evacuated tube, and traveled towards the anode.
  • They travel in straight lines and cast sharp shadows.
  • They have energy and can do work.
  • They were deflected by magnetic fields

Though the properties were known, the nature of the particles was still unknown. In fact the scientific community was divided into two groups one group believed that cathode rays as nothing but corpuscles (particles) while the other group believed it to be an electromagnetic wave.

In 1897, SirJ.J. Thomson, drawing on work of his colleagues, studied the nature of cathode rays.

thomson's modifications to the cathode ray tube
For studying the properties of cathode rays, he improved the discharge tube by completely evacuating it and performed three sets of experiments on the beam of cathode rays:

He first subjected the cathode rays to an electric field and then to a magnetic field individually. The remarkable achievement for him was that he could not only bend the rays in the presence of magnetic field but also could successfully bend the rays in electric field.

moving charge in an electric field

This led to the conclusion that cathode rays are not electromagnetic waves because electromagnetic waves are not affected by either magnetic field or electric field. He concluded that they are nothing but charged corpuscles (particles).

effect of electric field  upward force in a parabolic path
Further the deflection of rays towards the positive plate led him to the conclusion that they are beam of same kind of particles with a negative charge.

the path of charge in  electromagnetic field
To determine the charge to mass ratio of the particles he then subjected the beam to simultaneous crossed electric and magnetic fields perpendicular to the direction of the motion of the particles.

He initially switched off both the fields and noted the equilibrium position.
He then switched off the magnetic field and noted down the deflection point of the cathode rays in the presence of only electric field. He noted down the deflection.Then he applied the magnetic field in such a way that the cathode rays were brought back to the equilibrium position.

behaviour of charge in magnetic field
Now at equilibrium position, the electric and magnetic force on the charged particle are equal. Using the equations of motion he calculated the velocity of the particle.
He then carried out the calculations to determine the specific charge (e/m) ratio of the particles.

He named these particles as electrons. (The word 'electron' first used by G. Johnstone Stoney in 1891 had been used to denote the unit of charge found in experiments that passed electric current through chemicals.)

J. Thomson confirmed that an almost constant value of e /m was always obtained under various experimental conditions and even for different gases and metals. He therefore concluded that the cathode rays are a collection of the same kind of particles.

The e/m value was found to be 1.76 * 10(11) C/Kg.

He compared this value with the value of hydrogen atom and found that the mass of these particles were less than that of the hydrogen atom (lightest element on earth).
This led him to the conclusion that the electrons are constituents of the atoms and thus atoms are divisible.

If you are still unclear about the concepts, please refer the animation

Besides the above details the animation file also contains the following in detail

The animation also contains history of discharge tube, behavior of charged particle in electric and magnetic field

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Q & A
Q1: how did electrons travel toward cathod joseph malinao 25/9/2013
Q2: abcd arman 6/6/2013
Q3: how the discovery of the sub-atomic particle took place? jimerson 16/11/2011
Q4: why millcans cant not use large size of oil sandeep 22/7/2010
Q6: show me the animations related to cathode ray tube sonali agrawal 28/10/2009
Q7: how create virus to the c# programing chandan 20/2/2009
Q8: explain about motion of electronics in electric and magnatic field with equations? ramkumar 3/11/2008
Q9: send me animation software on discovery of electron. karma 3/10/2008
Ans: You will have to purchase to download the animation
Q10: What is the limit of frequency up to which signals using a sky wave can be transmitted? jay 30/9/2008
Q11: what is so significant about e/m ratio? Why not calculate the mass driectly? rash 14/8/2008
Q12: how to getting this animation SMPN2 BOJONEGORO 14/6/2008
Ans: Please buy and download these animations or subscribe and view them online
Q13: What is the meaning of discharged tube? Ankit K 7/6/2008
Ans: A gas-filled tube, also known as a discharge tube is a device in which a gas conducting an electric current emits visible light. It is usually a glass tube from which virtually all the air has been removed (so that it ‘contains’ a near vacuum), with electrodes at each end. When a high-voltage current is passed between the electrodes, the few remaining gas atoms in the tube (or some deliberately introduced ones) ionize and emit colored light as they conduct the current along the tube. Fluorescent lighting, CFL lamps, mercury and sodium discharge lamps and HID lamps are all gas-filled tubes used for lighting. Neon lamps and neon signage (most of which is not neon based these days) are also low-pressure gas-filled tubes.
Q14: what is the energy of the free, particle and does it have adefinite value? abeer 17/4/2008
Q15: in millikansoil drop method why droplets were in form of mist? kiran 14/4/2008
Q16: can i ask you questions about quantum mechanics abeer 4/4/2008
Ans: yes you can
Q17: please describe the discovory of electrons tofike 6/1/2008
Ans: We have described it in the details section, please go through that and if there is any specific question we will be glad to answer it.
Q18: I am doing a project for chemistry over the discovery of electrons. I believe that you should not have to pay to watch the full video. You should be able to watch it for free. Tiffany 10/11/2007
Ans: We wish that we could provide it for free, there are some of them freely viewable in the downloads section, why not try them out?
Q19: Please add sound to the remaining video animations. candace A. Muhammad-Taylor 7/11/2007
Ans: Yes we are in the process of adding sound to all animations, only that it is taking time
Q20: when Rutherford's perform the experiment first sidra 16/9/2007
Ans: The only connection 1894 – Rutherford was research student under JJ Thomson. 1906-Ernest Rutherford, alpha particles scatter in air, 1907 with H. Geiger, finds a method of detecting a single alpha particle and counting them. 1908-Rutherford receives Nobel prize for disintegration of atoms, he also finds alpha particle is helium atom and beta are electrons. 1909-Geiger and Marsden gold foil scattering experiment. 1911: Ernest Rutherford, Infers the nucleus from the alpha scattering result
Q21: animation of cathode rays ramulu 31/8/2007
Ans: Please be more specific about your requirement, you can watch this animation online for as low as $2
Q22: The e/m ration of anode ray particle same or different when different gases are used in the discharge tube? sara 30/7/2007
Ans: E/M ratio of 'Anode' rays differs from gases to gases as these are positive ions of the gases, infact Goldstein’s canal rays experiment failed because he believed that he had discovered the proton. But his ratio of charge to mass of this new particle was found to differ from gas to gas as these were ions, not protons
Q23: what is multipole expension imran ashraf 15/2/2007
Ans: Multipole expansion is a mathematical tool used to study phenomena whose behavior depends on powers of the reciprocal distance. To study physical shape or structure (or density distribution in mathematical terms) of small things like nuclei, DNA base pairs, colloids etc., we obtain experimental curves like intensity, potential and current distributions by scattering. These are fitted onto a theoretical model of the object shape using multipole expansion to get an idea about its actual shape
Q24: please give me listing of full program for teaching. thanks you raisujanem 12/1/2007
Ans: You can download a flash based catalog of all animation on our site from Downloads section on oursite, please let us know if you need more assistance
Q25: what is the important in the structure of atom ray an afdal 10/11/2006
Ans: could you please re-phrase the question, I cant figure out what is it that you want to know.
Q26: how did J.J Thomson claimed that the charged particles were relly 'electrons'? Iqra Sajid 6/11/2006
Ans: During those times two theories abounding about cathode rays were that they were corpuscles (particles) or electromagnetic waves. JJ Thomson concluded that cathode rays are not electromagnetic waves because electromagnetic waves are affected by neither magnetic field or electric field secondly deflection of rays towards the positive plate indicated their negative nature, computing the same e/m for different metals he could prove it was charged particle instead of a ray
Q27: from where cathode rays are emmited qadeer 20/7/2006
Ans: From the cathode (negative electrode) material.
Q28: your animations are really very useful harsha 19/7/2006
Ans: Thanks for appreciating the animation, your negative feedback are also welcome as they help us to improve.
Q29: why neutrons are discovered lately harsha 19/7/2006
Ans: It took a long time to discover neutron because hunt for charged particles was relatively easier (they were visible as flashes) and only after the charges balanced that the hunt for balancing masses began. Neutron remained largely hidden because it did not produce a flash and it was not deviated by magnetic field. It was thought to be photon or a gamma ray for some time but differentiated itself (thanks to Curie and Joliot ) by being non-ionizing and was finally given mass value by Chadwick.
Q30: in the crt experiment, were the electrons from the gas filled inside the crt also ejected? aziz 10/7/2006
Ans: Yes, ejection of electrons by gases resulting into their ionization is common in-fact, Goldstein and others failed to deflect cathode rays, due to excessive ionization of the gas present in the discharge tube, by electric field. Thomson reduced the pressure, minimized ionization, and was able to see the anticipated deflection.
Q31: CRT ANIMATIOM OR VIDEO sahar 24/2/2006
Ans: This is available inside the animation, please subscribe to view it online or purchase and download it

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