Isn't educational use the same as fair use?
Unfortunately, no. Using an item for an educational only counts towards one factor of fair use; you must consider all four of the factors to make a fair use determination.
Is it always fair use to use 10% or less of a work?
No. Copyright law does not provide a percentage that would constitute fair use. Generally, the smaller the amount used, the more likely it is that the use is fair. However, the other factors must also be considered.
If something is freely online is it in the public domain?
No, it likely still has some sort of copyright protection. YouTube videos, blog posts, and even your class notes are automatically protected by copyright. That protection lasts for the life of the author plus seventy years! It's good practice to assume items you're using - whether print, audio, or video - have copyright protection.
Copyright law doesn't provide specific numbers of pages that are okay (or not okay) to copy or scan. Instead, there are some broad guidelines. If you follow these guidelines, it's likely that you will be protected by the fair use doctrine:
Some authors and publishers are interested in loosening the restrictions that copyright law places on their readers. They may choose to offer their work under an open license, such as a Creative Commons license. These licenses allow people to re-use an original work, as long as certain criteria are met. Some authors may require re-users to use the materials only for non-commercial purposes, or to re-use it only without modifications. Many authors require re-users to attribute the material to the original author, which is a good scholarly practice anyway.
Some authors don’t want to place any restrictions on how their work is used. They may choose to dedicate the work to the public domain, for anybody to use as they see fit. The Creative Commons Public Domain Dedication is a popular way to do this.