What is the cathode ray tube experiment

introduction

According to ancient Greek and Dalton's atomic theory, the atom was considered to be the smallest possible particle. However, experiments with electricity led to the realization that atoms themselves must be made up of even smaller particles. Michael Faraday carried out important experiments on electrolysis from 1832 to 1833.

M. Faraday (1791-1867)

He discovered the laws of electrolysis, which establish a relationship between the amount of decomposed substance and the amount of electricity used. this lead to Georg Johnstone Stoney 1874 suggested the existence of electric charge carriers. In 1891 he gave this particle its name electron.

Cathode ray experiment

During this time a lot was involved Cathode rays experimented. The cathode rays were made in 1859 by Julius Plücker discovered. He tried to pass an electric current through a vacuum by placing two electrodes in an evacuated glass tube. He then applied a high voltage, with a beam emanating from the negative electrode, the cathode. The cathode rays are negatively charged, move in a straight line and cause a glow when they hit a glass wall. The cathode rays were interpreted as negatively charged particle flows. They were given Stony's name, Elektron. Regardless of the composition of the cathode, electrons of the same type are always emitted.

If two plates are built into the cathode ray tube, between which the beam can pass, the beam is deflected when an electrical voltage is applied. At a given voltage, the extent of the deflection depends on 2 factors:

  • The higher the charge q on the particle, the greater the deflection.
  • The greater the mass m of the particles, the smaller the deflection.

The size q / m is therefore decisive for the extent of the distraction. The value of q / m for the electron was made by 1897 Joseph Thomson certainly:


Cathode rays can also be deflected in magnetic fields, but in a direction perpendicular to the magnetic field.
J. Thomson (1856-1940)

Oil droplets - Millikan's experiment

Up until now we only knew the value of the ratio between charge and mass of an electron. Of more interest, however, is to know the exact value of the charge and the mass of an electron. We owe our knowledge of these values Robert Millikan, who made the first accurate measurements of the charge of an electron:

He generated electrons through the action of X-rays on air. The electrons were then picked up by oil droplets, which sink between two horizontally arranged plates. The mass of a drop can be determined from the rate of descent. The plates were then connected to an electrical voltage with the positive pole on the upper plate. The tension was adjusted so that the oil droplets were kept in suspension. From the mass of the drop and the corresponding voltage, he was able to calculate the drop's charge.

R. Millikan (1868-1953)

A drop can hold several electrons; in all cases the charge is a whole multiple of the charge of the electron. The value e is called elementary charge. The electron is negatively charged. The mass of an electron can be calculated from the values ​​e / m and e.


Charge of an electron:

Mass of an electron: