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J.J. Thomson’s Experiment

The model works on the fact that if a beam of electron is subjected to the crossed electric field 99724.png and magnetic field 99740.png, it experiences a force due to each field. In case the forces on the electrons in the electron beam due to these fields are equal and opposite, the beam remains undeflected.
  • When no field is applied, the electron beam produces illuminations at point P (Fig. 1).
  • In the presence of any field (electric and magnetic) electron beam deflected up or down (illumination at Por P)
  • If both the fields are applied simultaneously and adjusted such that electron beam passes undeflected and produces illumination at point P.
Fig. 1
In this case, Electric force = Magnetic force ⇒ eE = evB
⇒ 99752.png v = velocity of electro
  • As electron beam accelerated from cathode to anode its loss in potential energy appears as gain in the KE at the anode. If suppose V is the potential difference between cathode and anode then, loss in potential energy = eV
  • And gain in kinetic energy at anode will be KE 99758.png i.e., 99766.png
⇒ 99775.png
Thomson found, 99781.png
If one includes the relativistic variation of mass with speed 99787.png, then specific charge of an electron decreases with the increase in its velocity.
  • The deflection of an electron in a purely electric field is given by 99793.png, where l = Length of each plate, y = deflection of electron in the field region, v = speed of the electron.
Fig. 2

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