CATHODE RAYS – THE DISCOVERY OF ELECTRON

CATHODE RAYS – THE DISCOVERY OF ELECTRON

The knowledge about the electron was derived as a result of the study of the electric discharge in the discharge tube by J.J. Thomson, 1896.

Discharge tube:

The discharge tube consists of a glass tube with metal electrodes fused in the walls as shown in the figure.

How Cathode Rays Are Produced:

Through a glass side-arm, air can be drawn with a pump. The electrodes are connected to a source of high voltage (10,000 Volts) and the air partially evacuated. The electric discharge passes between the electrodes and the residual gas in the tube begins to glow. If virtually all the gas is evacuated from within the tube, the glow is replaced by faintly luminous ‘rays’ which produce fluorescence on the glass at the end far from the cathode. The rays which proceed from the cathode and move away from it at right angles in straight lines are called Cathode Rays

Charge And Mass (E/M) Of The Cathode Particle:

By counterbalancing the effect of magnetic and electric field on cathode rays. Thomson was able to work out the ratio of the charge and mass (e/m) of the cathode particle. In SI units the value of e/m of cathode particles is – 1.76 × 188 coulombs per gram. As a result of several experiments, Thomson showed that the value of e/m of the cathode particle was the same regardless of both the gas and the metal of which the cathode was made. This proved that the particles making up the cathode rays were all identical and were constituent parts of the various atoms.

Electrons Are Named:

Dutch Physicist H.A.Lorentz named them Electrons. Electrons are also obtained by the action of X-rays or ultraviolet light on metals and from heated filaments. These are also emitted as β-particles by radioactive substances. Thus it is concluded that electrons are a universal constituent of all atoms.

PROPERTIES OF CATHODE RAYS

· They travel in straight lines away from the cathode and cast shadows of metallic objects placed in their path.

· Cathode rays cause mechanical motion of a small pin-wheel placed in their path. Thus they possess kinetic energy and must be material particles.

· They produce fluorescence (a glow) when they strike the glass wall of the discharge tube.

· They heat up a metal foil to incandescence which they impinge upon.

· Cathode rays produce X-rays when they strike a metallic target.

· Cathode rays are deflected by the electric as well as the magnetic field in a way indicating that they are streams of minute particles carrying negative charge.

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