Ok Ahmed, I misspoke, but getting into the details is complicated.
Ahmed wrote: the gravitational field is a force
according to Newton but according to Einstein and "general relativity" it is the effect of the space-time curvature
http://www.astronomes.com/la-fin-des-et ... -generale/. This theory applies in astronomy with the extremely massive objects, but on earth that of Newton remains valid.
To be concrete I chose to compare an object subject to gravity and a magnet subject to the magnetic field excluding its gravity.
The stone placed on top of a mountain undergoes a force of gravitation, the magnet on the fridge, a force of attraction.
Stable states from which no energy comes out.
The falling stone accumulates kinetic energy, which is transformed into mechanical energy in shock with the ground.
In the case of hydropower, this is the case of water falling into a penstock. The energy is converted into mechanics by the turbine, then into electrical ...
To tear the magnet from its support, it is necessary to apply a force opposite to its attraction. The magnet, if it is not too far from its support will have the strength to return there as soon as it will be released but no energy comes out of it.