Properties of cathode rays

FSc

1. Discovery of Electrons (Cathode Rays) A gas discharge tube is fitted with two metallic electrodes acting as cathode and anode. The tube is filled with a gas ,air or vapours of substance at any desired pressure. The electrodes are connected to source of high voltage. The exact voltage required depends upon length of tube and pressure inside tube. The tube is attached to vacuum pump by means of small side tube so that conduction of electricity may be studied at any value of low pressure It is observed that current does not flow through gas at ordinary pressure even at high voltage of 5000 volts. When pressure inside tube is reduced and high voltage of 5000-10000 volts is applied, then an electric discharge takes place through the gas producing a uniform glow inside tube.

2. When pressure is reduced further to about 0.01 torr , the original glow disappears. Some rays are produced which create fluorescence on glass wall opposite to cathode . These rays are called cathode rays. The colour of glow or fluorescence produced on walls of glass tube, depends upon composition of glass.

3. Properties of Cathode Rays Cathode rays are negatively charged. In 1985, J. Perrin showed that when the cathode rays pass between poles of magnet, the path of negatively charged particles was curved downward to point 2 by the magnetic field. In 1897 J. Thomson established their electric charge by the application of electric field. The cathode ray particles were deflected upward. Thomson found that by carefully controlling charge on plates, when the plates and the magnet were both around the tube, he could make the cathode rays strike the tube at point 1 again. In other words he was able to cancel the affect of the magnetic field by applying an electric field that tended to bend the path of cathode rays in opposite direction.

4. They produce a greenish fluorescence on striking walls of glass tube. These rays also produce fluorescence in rare earths and minerals. When placed in path of these rays, alumina glows red and tin stone yellow. Cathode rays cast a shadow when an opaque object is placed in their path. This proves they travel n a straight line perpendicular to surface of cathode. 4. Theses rays can drive a small paddle wheel placed in their path. This shows that these rays posses momentum. From this observation, it is inferred that cathode rays are not rays but material particles having a definite mass and velocity.

5. Cathode rays can produce X-rays when they strike an anode particularly with large atomic mass. Cathode rays can produce heat when they fall on matter e.g. when cathode rays form a concave cathode are focused on platinum foil, it begins to glow. Cathode rays can ionize gases. They can cause a chemical change because they have a reducing effect. 9. Cathode rays can pass through thin metal foil like aluminium or gold foil.

6. The e/m value of cathode rays shows that they are simply electrons JJ.Thomson concluded from his experiments that cathode rays consist of streams of negatively charged particles. Stoney named this particles as electrons. He found that e/m value remained the same no matter which gas was used in discharge tube. He concluded that all atoms contained electrons.