I do not need a course in rudimentary physics, I'm prefectly aware of the basic laws of the universe.

As for magic it self it defies the normal laws of physics, I think that's already estblished already. When magic transfer to a spell, say fire, that fire is not normal fire, it's magical fire. It still interacts with the normal world, just in its own odd way.

And if you could just transfer energy from the results of a spell it competetly and udderly break the Equestrians advatange. Any suffently advanced civilization could build something capable of transferring that energy, making the Equestrians to weak to do anything.