K.C.S.E Physics Q & A - MODEL 2012PP2QN018

K.C.S.E Physics Q & A - MODEL 2012PP2QN018

​ Figure 18 shows parts of an x-ray tube.

(a) Explain why:
(i) A potential difference is applied to the filament.
(ii) A high potential difference is applied between the cathode and the anode.
(iii) Most of the tube is surrounded by lead.
(b) State how the resulting x-rays are affected by increasing the potential difference between the anode and the cathode.
(c) Light of frequency 7.5 x 10¹⁴ Hz strikes a metal surface whose work function is 4.0 x 10^-19J. Determine the kinetic energy of the emitted photoelectrons. (take planks constant h = 6.63 x 10^-34 Js)

answers

​(a) (i) To produce electrons; by thermionic emission; 
(ii) To accelerate the electrons to give them enough K.E. t produce X-rays at the anode;
(iii) To absorb stray X-rays, thus protecting the operator from those rays; 
(b) Increases K.E. of electrons and hence causes X-rays of higher frequency; 
  OR
 X - ray are more penetrative
 X - rays of shorter wavelength.