Answer: A changing magnetic flux establishes a current in a circuit.
Faraday's Law of Electromagnetic Induction was formulated from the experiments made by him and states that:
The voltage induced in a closed circuit is directly proportional to the speed with which the magnetic flux that crosses any surface with the circuit as edge changes in time
[tex]\oint\limits_C {\vec{E}}\,d\vec{l}=-\frac{d}{dt} \int\limits_S {\vec{B}} \, d\vec{A}[/tex]
where:
[tex]\vec{E}[/tex] is the Electric Field
[tex]d\vec{l}[/tex] is the infinitesimal element of the C contour
[tex]\vec{B}[/tex] is the magnetic field density
[tex]S[/tex] is an arbitrary surface, whose edge is C
The negative sign indicates the direction of the induced current and refers to the opposition between the fields induced by the magnetic flux and the electromotive force.
So, it is the change in the magnetic flux that establishes a voltage in the circuit, hence current.
