Thomson’s Experiment to Determine Specific Charge of Electrons

Fig: Thomson’s Experiment to Determine the specific charge(e/m)

Let us consider the Thomson’s Experiment contains the apparatus that consists of cathode ‘C’ and anode A.the narrow beam of Electrons emerges out from a small hole that passes through the plates P1 and p2. The high potential difference is applied to these plates and an electric field is created which is surrounded by the magnetic field. Then the different regions are obtained on the screen.

Now,

When the magnetic field is turned off and the electric potential difference is applied then,

1/2MV2=ev

where,

m=mass of electron
v=velocity of electron
e=charge of electron
v=potential different

Calculation of ‘V’

When the electric field and magnetic field applied at a time then,

Lorentz force =electric force
BeV=eE

or, V=E/B ……………………………(II)

Where,
E=V/d is an electric field intensity.

B=magnetic field and is calculated by flux meter.

Calculation of ‘V’

V is a potential applied between plate using eq’n II in me.

J. J. Thomson first determined the specific charge (charge to mass ratio e/m) of the electron in 1887. In his experiment, J. J. Thomson had found a charged particle that had a specific charge two thousand times that of the hydrogen ion, the lightest particle known at that time. Once the charge on the particles was measured he could conclude with certainty that these particles were two thousand times lighter than hydrogen.