An x-ray tube is operated at `20 kV` and the current through the tube is `0*5 mA`. Find (a) the number of electrons hitting the target per second, (b) the energy falling on the target per second as the kinetic energy of the electrons and (c) the cutoff wavelength of the X-rays emitted.

Explanation

(a) `i = ne = 0*5 xx 10 ^ (-3) A`
or, `n = (0*5 xx10 ^(-3)A)/( 1*6 xx 10^(-19)C )= 3*1 xx 10^(15)`.
(b) The kinetic energy of an electron reaching the target is `K= eV`. The energy falling on the target per second ,
`=n eV = iV = (0.5 xx 10 ^ (-3)A) xx (20 xx 10 ^(3) V)`
`10 J s^(-1)`
(c) `(hc)/(lambdamin)= eV`
or, `lambdamin = (hc)/(eV)`
`(1242 eV nm)/(e(20xx10^(3) V)) = (0.62 nm)`.