It's not a metaphor.
http://en.wikipedia.org/wiki/Metaphor
No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.
http://en.wikipedia.org/wiki/Metaphor
No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.