i have design a simple generator, i want to get approximately 12 v from this generator. how should i design this generator in term of number of turn, area of coil. what formula can i use in this problem. in this figure that i attached, the two discs that are magnet-attached and coil-attached are facing each other.
You need electrical power to make magnetic field, to avoid that he use permanent magnets I suppose Neodymium magnets which will also eliminate resistance on load. Often people modify car alternator for this, removing copper windings from rotor and installing Neodymium magnets, this can eliminate resistance on load, but you lose voltage control. Voltage control can be done on other ways, such as rotating speed, usage of voltage regulators,....
I suggest wind generator construction calculators. For this kind generators you can find a lots of materials on Internet. Google "construction wing generators". In one number of "Home Power" magazine I remember they give complete explanation of calculations, but I forgot number of edition.