Cathode rays
In 1859, Julius plucker started the study of conduction of electricity through gases at low pressure in a discharge tube. When a high voltage of the order 10, 000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the –ve electrode to the +ve electrode. Since the –ve electrode is referred to as cathode, these rays were called cathode rays.
Properties of Cathode rays
(1) They travel in straight lines away from cathode with very high velocity ranging from 107 to 109 m/sec.
(2) A shadow of metallic object placed in the path is cast on the wall opposite to the cathode.
(3) They produce a green glow when strick the glass wall matter. Light is emitted when they strike the zinc-sulphide screen.
(4) When a small pin wheel 10.0is placed in their path, the blades of the wheel are set in motion. Thus the cathode rays consist of material particles which have mass and velocity.
(5) They are deflected by the electric and magnetic fields. When the rays are passed between two electrically charged plates, these are deflected towards the positively charged plate. It shows that cathode rays carry -ve charge. These particles carrying negative charge were called negatrons by Thomson.
The name negatron was changed to ‘electron’ by Stoney
(6) They produce heat energy when they collide with the matter. It shows that cathode rays posses Kinetic energy which is converted into heat energy when stopped by matter.
(7) These rays affect the photographic plate.
(8) Cathode rays can penetrate the thin foil of solid materials.
(9) Cathode rays can ionize the gases through which they pass.
(10) The nature of cathode rays is independent of
(a) The nature of cathode and
(b) The gas in discharge tube.
Measurement of e/m for electron :
- In 1897, J.J. Thomson determined the e/m value (charge/mass) of the electron by studying the deflection of cathode rays in electric & magnetic fields.
- The value of e/m has been found to be –1.7588 ´ 108 coulomb/g.
- By performing a series of experiments, Thomson proved that whatever gas be taken in the discharge tube and whatever be the material of the electrodes the value of e/m is always the same.
- Electrons are thus common universal constituents of all atoms.
JJ Thompson's Atomic Model
According to JJ Thompson , Atom is Uniformly Positively Charged with equal Amount of Negatively Charged electrons embedded in it. The positive and negative charges in an atom are equal in magnitude due to which atom is electrically Neutral.
it is also referred as “Plum Pudding Model” or ” Watermelon Model ”
