The Lenz's law explains the negative sign in the Faraday's flux rule,
It states that the polarity of induced emf is such that it tends to produce a current opposing the change in magnetic flux that produced it.
Lenz's law is in accordance with the conservation of energy :
As the induced emf opposes the cause that produces it, therefore, the mechanical work need to be done to continue the process. It is this mechanical energy which is converted into electrical energy, in accordance with the law of conservation of energy.
Electromagnetic induction converts the mechanical energy into electrical energy.
