Answer to Question 4


A coil is connected to a 50 Hz AC supply.  Explain why you can hear it buzzing.  What frequency is the buzzing?  Explain your answer.

The adjacent turns are attracting because the current is going in the same direction.

Therefore they vibrate.

The frequency is 100 Hz.

This is because they attract each half-cycle and relax when the voltage is 0 V.  There are two half-cycles in every whole cycle.


(If the coil was made perfectly, and was of infinite length, then the forces between adjacent turns would sum to zero, so there would be no hum.  A very well made real coil would only hum from the turns at the very ends.  The manufacture of coils is done very carefully if hum is a problem, e.g. in audio applications.)