Chapter 3

Electromagnetic induction

Light bulb ignition

Take 10 minutes to prepare this exercise.

Then, if you lack ideas to begin, look at the given clue and start searching for the solution.

A detailed solution is then proposed to you.

If you have more questions, feel free to ask them on the forum.

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 .

Light bulb ignition


How many whorls are necessary for the light to turn on ?


  • Can you explain why the light can turn on ?

  • Think about Faraday's law.


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 :

Lenz's law
Electromagnetic levitation