EMP'S Quick Over View
EMP is of course an intense electro-magnetic pulse that will instantly destroy electrical circuits which is a by product of nuclear detonations but also several countries have been experimenting with EMP weapons designed specifically against the electrical infrastructure as opposed to physical destruction of everything.
For example a 1.4 megaton EMP bomb detonated 250 miles above Kansas would destroy most electronics in the U.S. See map below of anticipated effects radius or area. This of course will cause complete chaos and subsequent death from the total destruction of our electrical infrastructure as the entire U.S. economy is dependent upon this grid.
Actually, nobody knows the magnitude of infrastructure failure and destruction that an EMP attack would cause. All of the conjecture is based on modeling. Electrical grids going down for months - if not years; communications gone.