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Discovery of the Electron-Cathode Rays

DISCOVERY OF THE ELECTRON CATHODE RAYS

Most of the information about electrons is obtained from the study of cathode rays, which were discovered during the experiments with gas discharge tubes. A discharge tube is long glass tube, such as shown in Fig. 5.2. This tube is fitted with metal electrodes on either end across which high voltage can be applied. The electrode which is connected to the negative terminal of the power source is called cathode while the electrode which is connected to the positive terminal is called anode. The tube is also connected to a vacuum pump for controlling the pressure of gas inside the discharge tube.

Fig. 5.2. Cathode rays.

When the gas pressure inside the discharge tube is one atmosphere, no electric current flows through the tube. If the gas pressure is reduced to about 10-2 atmospheres and a potential difference of about 10000 volts is applied to the electrodes, an electric current flows and at the same time light is emitted by the gas. The colour of the light depends on the nature of the gas taken in the discharge tube. The colour of light is magenta red, if the tube is filled with normal air in the beginning. The neon lights (reddish orange) used in advertising signs operate in this way. As the gas pressure in the discharge tube is reduced further, a dark space appears in the vicinity of cathode and alternate light and dark bands can be seen between the two electrodes. If the gas pressure is reduced to 10 -6 atmospheres, the emission of light ceases, instead the end of the glass tube opposite to cathode glows (fluoresces) with a faint greenish light. Further investigations revealed that the fluorescence was caused due to the bombardment of the walls of the tube by rays emanating from cathode. These rays were called cathode rays. These rays were found to consist of negatively charged material particles, called electrons.

PROPERTIES OF CATHODE RAYS

The cathode rays possess the following properties:

1.      Cathode rays travel in straight lines. An object placed in the path of cathode rays casts a sharp shadow. It shows that cathode rays travel in straight lines.

2.      Cathode rays consist of material particles. This was indicated by the fact that a light paddle wheel placed in the path of cathode rays starts rotating (Fig. 5.3).

3.      Effect of electric field. When electric field is applied to a stream of cathode rays, they get deflected towards positive plate (Fig. 5.3). It showed that cathode rays themselves are negatively charged.

 

Fig 5.3 Effect of electric field on cathode rays.

 4.      Effect of magnetic field. When magnetic field is applied, the cathode rays get deflected. The direction of deflection again indicates that cathode rays are negatively charged.

5.      On striking against walls of the discharge tube cathode rays produce faint greenish fluorescence.

The above mentioned properties of cathode rays indicate that the cathode rays consist of a fast-moving stream of negatively charged material particles. These particles were named electrons. 

 

DETERMINATION OF CHARGE TO MASS RATIO (E /M ) OF ELECTRONS

Sir J. J. Thomson studied the deflection of cathode rays under the simultaneous application of electric and magnetic fields, applied perpendicular to each other (Fig. 5.4). It is possible to adjust the two fields so that the cathode rays strike the fluorescent screen at the same position as they do when neither field is applied. From the magnitudes of electric field and magnetic field thus applied it is possible to calculate charge (e) to mass (m) ratio of the electrons. The value of elm was found to be 1.76 x 108 coulombs/g or 1.76 x 10 11 coulombs/kg. The elm ratio for the particles in the cathode rays was found to be same irrespective of the nature of cathode or the nature of the gas taken in the discharge tube. It showed that the electrons are the universal constituent of matter. It may be mentioned here that charge per unit mass (elm), for a charged particle, is known as specific charge of the particle.

Fig. 5.4. Cathode rays deceased through magnetic and electric fields.

J,J, Thomson(1856-1940), a British physicist, discovered the electron in 1897. He won the Nobel prize for physics in 1906

 

CHARGE ON THE ELECTRON

J.J. Thomson’s experiment led to the precise determination of elm ratio for an electron. The charge (e) on an electron was determined by Robert Milikan in 1909 by ‘oil drop’ experiment.

The charge on the electron is found to be 1.60 x I0-19 coulombs. This is taken as one unit negative charge.

 

MASS OF THE ELECTRON

By combining the elm ratio and charge (e) of the electron, it i:s possible to calculate mass of the electron.

Thomson’s experiment: elm = 1.76 x 1011 coulombs/kg

Milikan’s experiment: e = 1.60 X 10-19 coulombs/electron

Mass of the electron, m

= e / e /m  1.60 x 10-19 coulombs/electron /  1.76 x 1011 coulombs/kg

= 9.1 x 10-31 kg/electron.

The mass of electron is very small and is approximately 1/1840 times the mass of an atom of hydrogen.

In general, an electron is represented as e-:

An electron is a fundamental particle of atom carrying one unit negative charge and having mass nearly equal to l/1840th of mass of an atom of hydrogen.