A stream of electrons travelling with speed of v m/s at right angles to a uniform electric field E is deflected in a circular path of radius r.Prove that e/m =v2 /rE ?

We know that the path of the particles in transverse electric field is parabola.

Here the path of electron is given as a circle; we may assume a small arc of parabola as circular arc of radius r , the necessary centripetal force [ mv^{2}/r] for a circular arc is provided by the electric force (eE) so we have

mv^{2}/r = (eE)

e/m = v^{2 }/rE

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