Q. 20

(a) Estimate the

Given:

Potential difference between collector and emitter = 500V

Specific charge of electron (charge per unit mass e/m) = 1.76 × 1011 C

Kinetic energy of an electron is given by:

…(1)

Where,

M = mass of electron

v = velocity of electron

e = charge of electron

V = potential difference (accelerating potential)

(a) From equation (1), we can write

…(2)

By putting the values in equation (2) we can find electron velocity.

v = 1.327 × 107 ms-1

(b) Accelerating potential, V = 10MV = 106V

Let speed of electron be v1

Again putting the values in equation (2),

v1 =

v1 = 1.8 × 109ms-1

This result is wrong as we understand that speed of light

(i.e. 3 × 108 ms-1) is the theoretical limit of the speed.

Such problems can be dealt using relativistic mechanics,

Relativistic mass is given by:

m =

Where,

m = relativistic mass

m0 = rest mass

v = velocity of particle

c = speed of light

At relativistic speeds, kinetic energy is given by,

KE = mc2-m0c2

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