Physical rule: When the current flows through an object in one direction, a magnetic field will be set up with the north and south pole set up one way. When the current flows in the other direction, the north and south pole reverse direction.

An AC voltage rises and falls in a sinusoidal pattern with a frequency of either 50Hz or 60Hz. As the voltage rises from zero to the full voltage, the current flow will increase to a maximum value. As that happens a magnetic field will be established and will reach its maximum in phase with the current flow. But as the current falls back to zero, the magnetic field will fall again to zero. But then the current will begin to flow in the opposite direction. Therefore, the magnetic field will again be established, but this time the north and south pole will be reversed. This happens with every cycle; either 50 times per second or 60 times per second. If we look at that once more, we can see that the magnetic field becomes strong twice per cycle; once with the north and south oriented one way, and then with the north and south oriented the other way. That is why we often see a strong source of vibration that occurs at twice the line frequency; 100 Hz or 120 Hz.

Whenever a magnetic object or ferrous object is subjected to a magnetic field, it reacts to the forces. This phenomenon is called magnetostriction. Magnetostriction is the deformation of a magnetic material in the presence of a magnetic field, and it causes vibration at 100 or 120Hz in electric motors.

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