e=E
(unless of course you mean 10e⁹; in which case, e is the neperian e. 19e⁹=81030.84...)
But note that neither E nor e is a scientific notation anyway. That is only an usage, a de facto standard introduced by calculators. Calculators (at least the first ones) were using only 7 segment displays. So it was impossible for them to use the scientific notation 6.02×10²³
Instead they invented a symbol (defined in their user manual... until after decades of this usage, this de facto standard was so common, that there was no need anymore to even explain it in a manual).
This symbol was E, which cannot be confused with any of the 10 digits, was easy to remember (since it is a letter), and was drawable with a 7 segments display.
6.02×10²³ is displayed 6.02E23 on 7 segments display
This notation was then adopted by computer languages. Generally in a case insensitive way. So 6.02e23 is also 6.02×10²³
And now, the notation is popular on forums (including yahoo qa), because it is not that easy to type 6.02×10²³ (unless you use plenty of utf-8 maths shortcut, as I do)
Exactly like LaTeX ^ is now on forums (more than the **, more common in programming language) a notation understood as "power" (6.02E23 = 6.02*10^23)
So, in short, this is not a scientific notation, but a classic computer notation : 6.02E23 = 6.02e23 = 6.02*10^23 = 6.02×10²³
(on forums I would avoid "e" since it can create a confusion with the logarithm base. Especially if you take into account the fact that some people don't bother to indicate what is a power and what is a factor. We read quite often 4x2-2x+5=0 for 4x²-2x+5=0)