What is Maxwell equation for EM field?

Answer 1

There are four simultaneous differential equations used to describe electromagnetic phenomena. You need some proficiency in vector calculus before you can begin to understand them. This isn't as difficult as it sounds, but you'll get nowhere if you put the cart before the horse.

Answer 2

There's no Maxwell equation for the em field but rather a set of 4 equations called the Maxwell Equations. I won't write them here since you must have a good knowledge of Calculus and Physics to understand them.

Answer 3

See

http://en.wikipedia.org/wiki/Maxwell's_equations