Engineering electromagnetics field theory

[sarwansagar]

I am taking a course in Electromagnetic Fields Theory, and the prescribed book for us is Engineering Electromagnetics by William Hayt. We are referring to its 7th edition, in one numerical Example 2.3 we have to find total charge contained in 2 cm length electron beam where as we are given the volume charge density of it hence by integrating that we come to know the required total charge. my question is that we have the find total charge contained in 2 cm length then how can we have its volume density despite of its line charge density? a beam of electrons is sharp line.

 

[Spinnor]

The electron beam still has a width, a little googling and numbers like .5mm came up. Do you have any clues as to what the beam width might be?