Latest Blog about Discovery of Electron (Cathode Rays) 2022
Discovery of Electron
Gas Discharge Tube
• It is a glass tube having two metallic electrodes sealed into it.
• It may contain a gas, air or vapours of any substance at any pressure.
• The tube can be connected to a vacuum pump to maintain any low pressure.
• The electrodes are connected to a high voltage battery. The exact voltage required depends upon the length of the tube and the pressure inside it.
• A slit can be used in it to get a sharp beam of radiation.
Example: A ‘neon sign’ is also a discharge tube, which contains neon gas at a pressure of about 10 torrs. Television. and computer monitor screens are also gas discharge tubes called cathode-ray tubes (CRT).
William Crooks Experiment
In the late 19th century, William Crooks studied the passage of electric anode current through gases. For this purpose, he used a gas discharge tube.
At normal pressure, gases do not conduct electricity even if the voltage is as high as 5000 volts.
• However, when the pressure inside the gas discharge tube is reduced and a high voltage of 5000-10000 volts is applied, the gases begin to conduct electricity and a uniform glow appears inside the tube.
When pressure is further reduced to 0.01 torr, the uniform glow disappears and fluorescence appears on the glass walls opposite the cathode. This is due to the striking of some rays on the glass wall. These rays are called Cathode rays.
• The colour of the fluorescence depends upon the composition of the glass.
• Different gases and vapours of different substances were used in the discharge tube. Also, different metals were used as electrodes. But always same rays were produced.
Properties of Cathode Rays
1. Travel in a Straight Line
In 1869, Hittorf showed that Cathode rays travel in a straight line. He found that cathode rays produce a sharp shadow of an opaque object placed in their path. It shows that these rays travel in a straight line, perpendicular to the surface of the cathode.
2. Possess Momentum.
These rays can drive a small paddle wheel placed in their path. Cathode rays strike against the paddles of the paddle wheel and make it move.
This shows that cathode rays are actually beamed of particles which have momentum (i.e. mass and velocity).
3. Negatively Charged
Cathode rays are negatively charged.
In 1895. J Perrin showed that when cathode rays are passed through a magnetic field, these are curved downward by the magnetic field to point P3.
Moreover, in 1897, J.J. Thomson showed that when cathode rays are passed through an electric field, these are deflected towards a positively charged plate. He subjected a beam of cathode rays to the simultaneous effects of electric and magnetic fields as shown in the figure:
✓ In the absence of an electric & magnetic field, the cathode rays strike at P₁.
✓ When only an electric field is applied, the cathode rays strike at P₂.
✓ When only a magnetic field is applied, the cathode rays strike at P3.
The strength of an electric and magnetic field is so cancelled out that the cathode rays strike at P₁ These experiments show that cathode rays are negatively charged.
4. Produce Fluorescence
They produce greenish fluorescence on striking the walls of the glass tube.
They also produce fluorescence in rare earth and minerals. e.g. alumina glows red and tin stone glows Yellow.
5. Produce X – rays Cathode ray
cathode produces X – rays, when strikes an anode, particularly with high atomic mass.
6. Possess Energy (Energetic Rays)
When cathode rays strike an object, it becomes heated up showing that cathode rays are energetic rays. When cathode rays from a concave cathode fall on a platinum foil, it begins to glow.
7. Ionic gasses
These can ionize gasses by removing electrons from them. Thus positive ions are produced.
8. Causes Chemical Changes
These are negatively charged. So, their addition causes a reduction of a substance. Thus, these can cause a chemical change.
9. Pass through a thin foil
These can pass through a thin foil-like aluminium or gold foil.
10. Charge to mass (e/m) ratio
Their (e/m) ratio shows that they are simply electrons.
Conclusions of Discovery of Electron
J.J. Thomson proved that cathode rays are actually a stream of negatively charged particles. He calculated the e / m ratio. He found that the thee / m ratio remains the same for every gas used in the discharge tube. He concluded that these are fundamental particles of atoms.
Stoney called these rays ‘electrons’.