V 

 102 ELECTRONS AND IONS [CH. ix. 



No wonder that violent radiation -effects are pro- 

 duced. The "power" required to stop an electron, 

 flying with one-thirtieth of the speed of light, inside 

 a molecular thickness, can be estimated thus 



1 u 



energy -f time = - DMT . ^7 



* ' ff *^ 



= 10- 27 (10 9 ) 3 10 8 = 10 8 ergs per second ; 

 or thus 



= 10 again, 

 tt 



which is equivalent to ten watts. (Though the time 

 it lasts is only the 10~ 17 part of a second.) 



But only a small fraction of this goes into 

 radiation. The radiating power can be estimated 

 thus, from Larmor's expression for it, as deduced in 

 Appendix G, 



/xe 2 10~ 40 



-(u) 2 = 10 x 10 62 = 100 ergs per second. 



The rest therefore, it would appear, must take the 

 form of heat. 



It is worth considering what circumstances would 

 give radiation an advantage over heat, and vice versd. 

 Because sometimes conspicuously the target gets 

 heated, and sometimes X-rays are emitted. Let u 

 be the speed and I the distance of stoppage, then 



u z 



so the force required to stop it is 



