[Corvettaholic]

So you have a magnetic field. Its just sitting there having a good time. So you start rotating the field through a coil of wires through it really fast! Wow, a alternator. No surprises there. But! Doesn't doing something with the magnetic field induce drag of some kind? If not, it would be really easy to generate power with the spinning of wheels on a car. So why does power generation using a magnetic field cause spinny things to slow down?

[Donaldson Tan]

drag is observed because part of the mechanical energy input is converted into electrical energy. so there is less mechanical energy converted into kinetic energy, thus the spinning coil moves slower than it is when all the mechanical energy input is converted to kinetic energy, hence it seems that drag is induced.

drag is a more appropriate word when you have fluid flow acting against the motion of the body of interest. if the density of fluid is very small, eg. air, and the area of surface normal to the fluid flow is small, drag can be neglected.

[ATMyller]

it would be really easy to generate power with the spinning of wheels on a car.

That how hybrid cars charge their batteriers during engine breaking.

[Donaldson Tan]

electromagnetic braking is technically not dragging