Question

Why Lenz's Law? Why must the solenoid make itself as a pole to oppose the pole of the bar magnet going into or out from it?
For example, when a bar magnet approaches a solenoid (that is not connected to any power source), a repulsive force is produced. The repulsive force is caused by the current flow in the solenoid that creates a like pole to repel the bar magnet. Why then? Why can't it just attract it? I found on some websites that it's related to work done and the conservation of momentum. Need some light please.

Answer 1

First things first: The magnetic field "lines" of a bar magnetic extend out from the north pole and into the south pole.

Second: Lenz's law states that the induced current in the loop is such that the induced magnetic field opposes the change in the flux.

So imagine a loop in the plane of the paper. You push the north pole into the loop (into the page), thus increasing the flux into the page. The induced magnetic field will oppose that increase and thus point out of the page. (What direction of induced current would create a magnetic field pointing out of the page?)

If you do the same thing with the south pole of the magnet, the opposite will occur--since now you are increasing the flux out of the page.

Let's take some care to specify directions clearly. Go back to my example of a single loop in the plane of the paper. Call that plane the x-y plane--the center of the loop is at the origin: 0,0,0. The +z axis points out of the page; -z, into the page.

Given that, we can now talk about pushing the north pole of a bar magnet into the loop. We start with the magnet along the +z axis, with the north pole closer to the origin. We move the north pole of the magnet towards the loop (in the -z direction, towards the origin).

Which way is the bar magnet's magnetic field pointing? In the -z direction (into the page).
Which direction is the flux through the loop pointing? In the -z direction (into the page).
Which direction is the flux changing? Since the magnetic field is increasing in the -z direction, the flux is increasing in the -z direction.
Which way must the induced magnetic field point in order to oppose that change in flux per Lenz's law? Since the flux is increasing in the -z direction, the induced magnetic field must point in the +z direction.
Looking at the loop from the vantage point of the +z axis, which way does the induced current flow? Using the right-hand rule for current loops, the current must flow counter-clockwise through the loop to produce a field in the +z direction.

Make sense? Things will be exactly reversed if you turn the bar magnet around and move the south pole into the loop.

Answer 2

So which comes first? The formation of the poles? Or the current flow? Is it that the pushing of the bar magnet (which is equal to pushing the magnetic field into the solenoid) that causes the current to flow that way? Or is it that the current that causes the poles of the solenoid?

Answer 3

And for this question of mine, simply assume that Lenz's Law doesn't exist yet.

Answer 4

In this diagram, a magnet approaches a solenoid with its north pole towards the solenoid.. There is a flux change in the solenoid as the magnet approaches the solenoid.. This induces a current in the solenoid.

Now, the current induced in the solenoid produces a magnetic field. (A current always induces a magnetic field).. The induced current produces a magnetic field such that it opposes the motion of magnet.. (N pole of solenoid is near N pole of approaching magnet). And since the magnets motion is opposed, we have do some extra work to keep the magnet moving and this extra work which we do to move the magnet is equal to the electrical energy produced in solenoid.
so, work done by us = electrical energy
Thus energy is conserved. (law of conservation of energy applies)

Suppose somehow, the magnetic field would be induced in such a way that it would attract the magnet and would not oppose the motion of magnet. what then? We wouldn't have to do any work to move the magnet.. The magnet would be in motion on its own and continue to be in motion.. And electricity would be produced even when we didn't do any external work.
That would totally violate law of conservation of energy..
No input work=some electrical energy.
That is totally absurd.. And makes no sense.. (violates law of conservation of energy)

So, Lenz's law is in accordance with law of conservation of energy..