I'm a little late on the discussion here, and as a moped newby my opinion may not be worth as much as Don's or Fred's, but as an electrical engineer, I have some expertise here.
An an alternator works on the principle that when a conductive material moves through a magnetic field, it induces current flow. (the coils spinning in a magnetic shell)
A motor works on a similar premise where when a current flows through a magnetic field, it produced a physical force on the conductive material. (charged coils wrapped around a shaft)
These two premises counteract each other.
The current generated by the alternator creates a force which acts in the opposite direction of the force that is driving the alternator (in essence, the alternator turns itself into a motor which wants to turn the opposite direction of the mechanical force that is spinning the alternator).
Because of this, the more current being produced by the alternator, the more physical force is required to turn the alternator (similarly, the more power that is required from an electrical motor, the more current must be supplied to it). If we were dealing with a 100% lossless system (no friction, no electrial resistance, perfect magnetic flux) we would get exactly as much power out (electrical) as is put in (mechanical). But, since this is the real world, you will ALWAYS need to put more mechanical energy into the system than you get out as electrical energy.
I'm not sure how much sense that makes, but it's the result of the first law of thermodynamics (if we want to get really picky, we can throw the 2nd law in as well but it isn't needed at this point)