Final answer:
An electrical cycle of an alternating current generator is completed by the full rotation of a coil within a magnetic field, inducing an alternating current emf.
Step-by-step explanation:
The completion of an electrical cycle in an alternating current (AC) generator is achieved when the coil completes one full rotation within the magnetic field. This rotation causes the generator to produce an alternating current emf that varies sinusoidally in time, thus completing one full cycle of AC power. As the coil rotates, due to external mechanical energy supplied, it cuts through the magnetic lines of force, inducing an emf in the coil according to Faraday's law of electromagnetic induction. The number of cycles per second, which is the frequency of the output voltage, depends on the rotation rate of the armature coil and other factors such as the strength of the magnetic field.
The AC generator operates on the same basic principle as a motor, but in reverse. While a motor uses electrical energy to produce motion, a generator uses motion to produce electrical energy, converting kinetic energy from a moving fluid or other sources into electrical energy fed into our power grids.