DISCOVERY OF ELECTRON: 

CATHODE RAYS

During the latter half of the nineteenth century, it was found that while normally dry gases do not conduct an electric current, they do so under very low pressure and then patches of light are seen. The passage of electricity through gases as studied by a number of physicists, particularly by Faraday, Davy, Crookes and J.J. Thomson.

When a current of high voltage (10,000 volts) is passed through a gas of air kept at a very low pressure (0.01 – 0.03 mm) blue rays are seen emerging from the case. These rays are called “Cathode Rays”.

 2294_CATHODE RAYS.JPG

 

Some of the important properties of the cathode rays studied by Sir J.J. Thomson and others are given below:

  •  Cathode rays come out at right angles to the surface of the cathode and move in straight lines.

  •  Their path is independent on the position of the anode.

  •  They produce phosphorescence on certain salts like ZnS and fluorescence on glass.

  •  They blacken photographic plates.

  •  The rays pass through thin sheet of metals. If the metal sheet is too thick to be penetrated the rays cast a shadow.

  •  They produce X-ray when they strike a metal.

  •  The rays ionize a gas through which they pass.

  •  They heat a substance on which they fall.

  •  They rotate a light wheel placed in their paths. This shows that cathode rays contain material particles having both mass and velocity.

  •  The mass of a particle present in cathode rays is found to be 1/1837 of H-atom. This shows that the particle is of sub-atomic nature.

  •  Cathode rays are deflected by a magnetic or an electric field showing the particle to be electrically charged, the direction of deflection  shows that they are negatively charged.

  •  Cathode rays contain the smallest unit of negative charge.

  •  No cathode ray was produced when the tube was completely evacuated.

  •  Different gases produce same cathode rays as they have the same e/m (charge/mass) ratio. This indicates that the particles present in cathode rays are fundamental constituent of all matter.

Sir J.J. Thomson named these negatively charged sub-atomic particles as electron.

“A sub-atomic particle which is a fundamental constituent of all matter having a mass 1/1837th of a H-atom and which carries the smallest unit of negative charge is called an electron”.

Determination of Velocity and Charge/mass (e/m) ratio of Electrons:

Sir J.J. Thomson (1897) extended the cathode ray experiment for the determination of velocity of electrons and their charge/mass ratio, The value of e/m for an electron = 1.76 x 108 C/g. For the H+ ion (proton), 
e/m = 96500/1.008 C/g.

Millikan’s Oil Drop Method: Determination of Charge on an Electron:

In 1909, Millikan measured the charge on an electron by his oil drop method. In this method a spray of oil droplets is produced by an atomizer, some of which pass through an opening into a viewing chamber, where we can observe them with a microscope. Often these droplets have an electric charge, which is picked up from the friction forming the oil droplets. A droplet may have one or more additional electrons in it, giving it a negative charge.

As the droplet falls to the bottom of the chamber, it passes between two electrically charged plates. The droplet can be suspended between them; we adjust the voltage in the plates so that the electrical attraction upward just balances the force of gravity downward. We then use the voltage needed to establish this balance to calculate the mass - to charge ratio for the droplet. Because we already know the mass of the droplet we can find the charge on it.

Millikan's found that the charge on all droplets could be expressed as whole number multiples of e, where the value of e is 1.602 x10-19 C. By combining e/m. ratio and 'e' we calculate mass of the electron

Me  = e / (e/m) = 1.6022 x 10-19 / 1.76 x 108   

       = 9.104´10-31 kg

This very small value shows that the electron is a subatomic particle. Thus charge on an electron = 1.602 x 10–19C. 

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