The history of the atomic structure and quantum mechanics dates back to the times of Democritus, the man who first proposed that matter is composed of atoms. These theories could not gain much importance due to the lack of technology. The experiments conducted during the nineteenth century and early twentieth century revealed that even atom is not the ultimate particle. The continued efforts of the scientists led to the discovery of subatomic particles like an electron, proton, and neutron. In the nineteenth century,J.J Thomson discovered electron to mark an inception to the world of subatomic particles. Once electron was discovered, he continued his experiments to calculate the charge and the mass of the electron. With the help of his experiments, he derived a formula for the calculation of charge to mass ratio of the electron.
Experimental setup for the determination of charge to mass ratio of electron:
While carrying out discharge tube experiment, Thomson observed that the particles of the cathode deviate from their path. He noticed, the amount of deviation in the presence of electrical or magnetic field depends on various related parameters. They are:
- Particles with a greater magnitude of the charge experienced greater interaction with the electric or magnetic field.Thus,they exhibited a greater deflection.
- Lighter particle experienced greater deflection. Thus, deflection is inversely proportional to the mass of the particle.
- Deflection of the particle from their path is directly proportional to the strength of the electrical and the magnetic field present.
Let us now understand this with the help of his experimental observations. The electrons deviated from their path and hit the cathode ray tube at point ‘x’ in the presence of a lone electric field. Similarly, electron strikes the discharge tube at point ‘z’ when only the magnetic field was present. Thus, to make electrons continue on the same path we need to balance the electric and magnetic field acting on them. Finally, based on the deflection of the electron, Thomson calculated the value of charge to mass ratio of the electron. The charge to mass ratio of electron is given by:
e/m = 1.758820 × 1011C/kg
Where, m = the mass of the electron in kg.
e = the magnitude of the charge of an electron in coulombs.
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