Explain why an induced current must flow in such a direction so as to oppose the change producing it.
If a change in the magnetic field of current i1 induces another electric current, i2, the direction of i2 is opposite that of the change in i1. If these currents are in two coaxial circular conductors i1 and i2 respectively, and both are initially 0, then the currents i1 and i2 must counter-rotate. The opposing currents will repel each other as a result. Lenz's law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change in flux or to exert a mechanical force opposing the motion.
This mechanical force that is exerted is converted into electrical energy. Hence, more the mechanical force, more the electrical energy. That is why, an induced current must flow in such a direction so as to oppose the change producing it.