Light bulb ignition
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A plane coil of square-section of side is put near a high voltage wire, travelled by the effective intensity .
Its frequency is .
A light-bulb is added to one end of the coil and can only be turned on for a voltage .
How many whorls are necessary for the light to turn on ?
The magnetic field created by the wire is : (Use Ampere's circuital law)
The flux of this field through the coil is :
Faraday's law gives the electromotive force that appears in the frame :
The effective voltage at the edges of the light bulb is :
Hence the minimum number of whorls in the coil can be deduced from the formula :