On which principle is electricity generated in a power plant?

An electric generator is a device that converts electrical energy into mechanical energy.

What is the function of a field magnet in an $AC$ generator?

How is the magnetic field produced in a solenoid used?

By increasing the number of turns $3$ times in a toroidal solenoid,the magnetic field:

The diagram below shows a circuit containing a coil wound over a long and thin hollow cardboard tube. Copy the diagram and:
$(i)$ Show the polarity acquired by each face of the solenoid.
$(ii)$ Draw the magnetic field lines of force inside the coil and also show their direction.
$(iii)$ Mention two methods to increase the strength of the magnetic field inside the coil.