Answer :


Magnetic field strength, B = 6.5 G = 6.5 × 10-4 T

Initial velocity of electron = 4.8 × 106 ms-1

Angle between the initial velocity of electron and magnetic field, θ = 900

We can relate the velocity of the electron to its angular frequency by the relation,

V = rω …(1)


V = velocity of electron

r = radius of path

ω = angular frequency

We understand that, magnetic force on the electron is equal to the centripetal force on it, hence we can write,

Fe = Fc

e × V × B = mV2/r …(2)

From equation (2) we can write,

eB = mV/r …(3)

Now, by putting value of V from equation (1) in equation (3)

e × B = m × ω …(4)

We know that, ω = 2πν

Putting in equation (4), we have


Now, by putting the values in equation (5) we get,

v = 18.2 × 106Hz

Hence, the frequency of rotation is 18.2 × 106Hz.

Note: The frequency of rotation is independent of the initial velocity of the electron.

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