Suggest why some of the kinetic energy of air that passes through the circular area swept out by the blades is not converted into electrical energy.

As with all real devices, wind turbines have their limitations. We can't get it all. The "energy in" is the kinetic energy from the wind's velocity and air density that flows into the area swept by the turbine blades. It is not possible to convert all of the wind's kinetic energy into mechanical energy. Some energy must remain in the air leaving the turbine. The "energy out" is the energy converted by the turbine blades into mechanical energy (which is then usually turned into electricity), plus whatever energy is left in the air after it passes through the turbine rotors.

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