When spinning under no electrical load (disconnected) the only load wil be the mechanical ones of bearing drag and air resistance of the cooling fins. These are fixed and purely dependant on RPM
When connected to an electrical circuit, the additional loads of overcoming the magnetic field in order to produce electricity, and a thing called "back EMF" ( ElectroMotive Force) come into play ... back EMF is the resistance of a circuit to actually doing anything .. ( think of it as the alternator having to push all those electrons down a wire when they don't want to move from their present steady state)
as a very simple example ...
open circuit alternator has no electrical work to do. Switch on 1 x 15watt bulb .. it must now produce 15 watts of energy ... switch on another .. it must now produce 2 x 15 = 30 watts and so on and so on.
NOW 746 watts = 1 horse power !!!! so .. the amount of load needed to actually feel a difference is beyond the capabilities of most car electrical systems !!
Theoretically it will make a difference .. but in practice you'll never see it
HTH
So if I'm right what your saying is the more demand on the alternator the higher the magnetic resistance the alternator creates :-/
Sort of ... the higher the load the more work the alternator has to do, the electricity is produced by cutting a magnetic field with a coil, as the load increases the regulator increases the strength of the magnetic field, so the alternator has to work harder to force the coil through that magnetic field.
Remember that electricity is only a form of energy, and energy cannot be made or lost, only converted from one form to another, so when you produce energy from an alternator you are converting mechanical energy (rotation) into electricity. The energy for that rotation comes from the engine, which is converting stored energy (petrol) into rotational energy very inefficiently - look how much is wasted in heat etc

The point is this is happening at a very low level .. if you had all your lights on, and your heated windscreen, and your wipers, I doubt you would be using even 300 watts, less than 1/3 of a horse power, and your car produces ( well mine does !! ) 217 BHP, so it is 1/600 ths of your load.. you would use more fuel in 1 acceleration to 30 mph than the alternator would use in a year !!
The calculations on Air Con power useage is very similar BTW
