When the wire frame rotates in a magnetic field, the induction electric current in the frame changes direction as many times per second as the frame rotates per second. The polarity of the voltage between the ends of the frame changes with the same frequency. If the ends of the frame are connected to the rings, then an alternating voltage is applied to the brushes.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.