Can somebody explain to me, someone who has no idea of EE, what a unipolar inductor is? Im trying to understand the process by which the rotational energy of a black hole is converted into energy to power relativistic jets. In the book i read (https://1lib.eu/book/2339827/854387 [pages 157, 158, 159]) it is said that the black hole in the magnetic field of the surrounding gas behaves like a spinning conductor in a magnetic field, therefore like a unipolar inductor (apparently). I already asked this on the physics site but that has for now been left unanswered, so I thought I`d ask here about the analogy the book uses.
I will take a shot. What in physics is called a unipolar inductor is not any kind of device that in electronics is called an "inductor". Rather, it is is a homopolar generator.
When an electron (or any charged particle) moves in a magnetic field, a force acts on it that is perpendicular to its direction of motion, and perpendicular to lines of the magnetic field. If the electrons are bound, that is "attached" to some atom, this force will distort the shape of the electron "cloud" around the atom. However, if the electron is "free", the force will cause the electron to veer from the direction it is travelling in.
Some of the electrons in metals are free. In a "homopolar generator", a metal disk rotates with it's axis parallel to the lines of a magnetic field. The free electrons in the metal disk will tend to either move toward the center, or away from the center depending upon the orientation of the magnetic field and the direction of rotation of the disk. In either case, the accumulation of electrons in one part of the disk, and a corresponding dimunition of electrons in another part of the disk create an electric potential. In the homopolar generator, "brushes" provide an electrical contact between the disk and wires, and transfer this electric potential to a circuit.
Electrons, as well as ions, move freely in plasma. If plasma is spinning, in a magnetic field, the electrons will tend to move inward or outward, just like in the human invented homopolar generator. Positively charged particles will move in the opposite direction as the electrons. This will create an electric potential in the plasma.
How all this relates to rotating black holes, and the jets that emanate from them, I am not competent to say. But you can take away that 1) rotation, plus 2) freely moving charged particles, plus 3) magnetic field, generates 4) an electric potential.