Question

1. You may notice that when you are near an electrical transformer station, you hear a low frequency hum coming from the transformers. This is due to the 60 Hz AC electric current flowing the coils and around the core of the transformer. However, it turns out that the actual main frequency of the sound heard by the human ear is at 120 Hz. Using your knowledge of fundamental and harmonic overtones, present a possible explanation for this effect.

Answer 1

In the case of a transformer, currents & voltages creates harmonics by non-linear loads that generate non-sinusoidal waveforms. These current and voltage wave forms operate on frequencies that are in multiples of the fundamental 60hz frequency. That is, the fundamental frequency is at 60 hertz, the 2nd harmonic is at 120hz frequency (60 x 2) which we can hear.