Lenz's law

It is a qualitative law that specifies the direction of induced current, but states nothing about its magnitude.

Lenz's law predicts the direction of many effects in electromagnetism, such as the direction of voltage induced in an inductor or wire loop by a changing current, or the drag force of eddy currents exerted on moving objects in the magnetic field.

[4] Lenz's law states that: The current induced in a circuit due to a change in a magnetic field is directed to oppose the change in flux and to exert a mechanical force which opposes the motion.Lenz's law is contained in the rigorous treatment of Faraday's law of induction (the magnitude of EMF induced in a coil is proportional to the rate of change of the magnetic flux),[5] where it finds expression by the negative sign:

[6] This means that the direction of the back EMF of an induced field opposes the changing current that is its cause.

When a voltage is generated by a change in magnetic flux according to Faraday's law, the polarity of the induced voltage is such that it produces a current whose magnetic field opposes the change which produces it.

When net positive work is applied to a charge q1, it gains speed and momentum.

When q2 is inside a conductive medium such as a thick slab made of copper or aluminum, it more readily responds to the force applied to it by q1.

The energy of q1 is not instantly consumed as heat generated by the current of q2 but is also stored in two opposing magnetic fields.

However, the situation becomes more complicated when the finite speed of electromagnetic wave propagation is introduced (see retarded potential).

[9] Famous 19th century electrodynamicist James Clerk Maxwell called this the "electromagnetic momentum".

A cheatsheet for remembering Lenz law
Aluminium ring moved by electromagnetic induction, thus demonstrating Lenz's law.
Experiment showing Lenz's law with two aluminium rings on a scales-like device set up on a pivot so as to freely move in the horizontal plane. One ring is fully enclosed, while the other has an opening, not forming a complete circle. When we place a bar magnet near the fully enclosed ring, the ring is repulsed by it. However, when the system comes to a rest, and we remove the bar magnet, then the ring is attracted by it. In the first case, the induced current created in the ring resists the increase of magnetic flux caused by the proximity of the magnet, while in the latter, taking the magnet out of the ring decreases the magnetic flux, inducing such current whose magnetic field resists the decrease of flux. This phenomenon is absent when we repeat the experiment with the ring that isn't enclosed by inserting and removing the magnet bar. The induced currents in this ring can't enclose themselves in the ring, and have a very weak field that cannot resist the change of the magnetic flux.