Cathode Rays Experiment

A discharge tube is a cylindrical hard glass tube fitted with two metallic electrodes. The tube has a side-tube with a stopcock. The discharge tube can be evacuated or filled with any gas to any desired pressure through this side-tube.

In this diagram
a) A discharge tube containing air under normal pressure. Electricity does not flow through the gas.

b) When the pressure inside tube is reduced to about 10-2 atm, the gas starts conducting electricity, and it starts emitting light.

c) When the pressure is reduced to about 10-4 atm, the emission of light by the gas ceases, and the glass wall of tube opposite cathode starts glowing with a faint greenish light.

The two electrodes can be connected to a source of high voltage. Gases are ordinarily poor conductor of electricity. Under normal pressure, gases do not conduct electricity even when a potential of 10,000 volts is applied.

William Crookes discovered that the gases start conducting under reduced pressure. When the pressure inside the discharge tube was reduced gradually, the following points were noted –

1) At about 10-2 atmosphere, a glow surrounding the cathode (negative electrode) detaches from the electrode surface. Thus a space is left between it and the electrode which is called Crookes Dark Space. At this stage electric current begins to flow from one electrode to the other.

2) At sufficiently low pressures, this glow fills while of the tube.

3) When pressure is lowered to about 10-4 atmosphere, light emission by the residual air in the discharge tube stops. But the walls of discharge tube opposite to cathode start glowing. At this stage, a stream of rays called Cathode Rays are emitted from the cathode.

It was discovered letter, that glass wall of tube glows due to the bombardment of glass wall by certain radiation emitted from the cathode. These radiations are named Cathode Rays.

Cathode Rays

When a very high electrical potential (~ 10,000 volts) is applied across a gas taken in a discharge tube at a very low pressure (~ 0.001 Torr) some radiations are emitted from the cathode. These radiations are called Cathode Rays.

At this stage, the glass walls of discharge tube opposite to cathode starts glowing with a faint greenish light. It is now known that this greenish glow on walls is due to bombardment of glass wall by cathode rays.

Properties of Cathode Rays

  1. Cathode Rays travel in Straight Lines
  2. Cathode Rays produce Mechanical Effects
  3. Cathode Rays consist of negatively charged particles
  4. These travel with high speeds approaching that of light
  5. These rays heat the object on which they fall due to transfer of kinetic energy to the object
  6. Cathode rays cause fluorescence
  7. Cathode rays ionize gas through which they pass
  8. Nature of these rays does not depened upon nature of gas and cathode material used in discharge tube
  9. These rays can penetrate through thin metallic sheets
  10. When cathode rays fall on solids like copper, X-rays are produced
  11. Charge-to-mass ratio for cathode rays was found to be same as that for an electron (1.76 × 108 coulomb per gram). This observation show that the cathode rays consist of negatively charged particles having same charge-to-mass ratio as that of an electron. Thus cathode rays are just stream of electrons.

Determination of charge-to-mass ration of an electron using Cathode Rays

Cathode rays were found to consist of negatively charged particles. These negatively charged particles were found to be electrons.

J.J. Thomson studied combined effect of electric and magnetic fields on the cathode rays. The two fields were applied in such a way that deflection due to electric field is cancelled out by deflection due to magnetic field and the net deflection was zero.

Thus from strengths of electric and magnetic fields ratio of charge to mass can be determined. Thus e/m for an electron was found to be 1.76 × 108 C g-1

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