PyQt5 and virtualenv problem

[D. Xenakis]

I tried to install SIP and PyQt5 using the pip install command but it didnt work on both cases (i was getting errors), so i finally installed them using the windows installers provided in riverbankcomputing website.
My problem though here is that whenever i try to create a new virtualenv enviroment, those packages are not included and i cant import them. How can i add PyQt5 to my new virt enviroment? What is the logic behind this problem so i understand whats going on here?

Thx in advance