An electron is accelerated from rest by a potential difference of 350 V. It then enters a uniform magnetic field of magnitude 200 mT, its velocity being at right angles to the field. We have to calculate (a) the speed of the electron and (b) radius of its path in the magnetic field.

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Mass of an electron,

Kinetic energy of 350 eV that electron acquires a potential difference of 350 V is much less than its rest mass energy, which is 0.5 MeV. We, therefore, will treat this problem as non-relativistic. The speed v of the electron can be calculated from its kinetic energy.

Therefore,

Let R be the radius of the circular orbit of the electron as it moves in a uniform magnetic field at right angles to the direction of its velocity. Equating the centripetal acceleration with the force on the moving electron due to the magnetic field, we get