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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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