AH Exp – Electrical Phenomena 05 – e/m for an electron

Aim – To determine the charge to mass ratio of the electron

Instructions

▪ Switch on the EHT supply.
The beam of electrons is deflected by the electric field, E.
E=V/d where V is the voltage across the plates and d is the distance between the plates.
V is also the accelerating p.d. (2kV) and d is 8cm.
▪ Switch on the current through the Helmholtz coils.
The coils produce a uniform magnetic field and deflect the electron beam down.
The downward force on the electrons due to the magnetic field equals the upward force due to the electric field. Hence Ee=Bev (1) where e is the charge on an electron and v is the velocity of the electrons and B is the magnetic flux density.
▪ Now 1/2mv²=eV so substituting in (1) gives e/m=E²2/2B²V
▪ B can be found from B=0.72 μ0NI/r where μ0 is 4πx10^-7, N is the number of turns in each coil (50), I is the current in the coils and r is the radius of the coils.
▪ Note the current in the coils.
▪ Use the rule to measure the radius of the coils.
▪ Determine a value for e/m for the electrons.

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