In physics, "ea n" doesn't typically stand for anything. It's possible you're thinking of a different abbreviation or a specific context where those letters are used together.

Could you please provide more context? For example:

* What is the full phrase or sentence you saw "ea n" in?

* What topic or subject are you studying?

* Is there any surrounding information that might be helpful?

Knowing more about the situation will allow me to help you figure out what "ea n" refers to.