14.6 Lenz Law

In the diagram, if the bar magnet moves towards the solenoid, the induced current (left to right through the ammeter) will be in a direction that produces a similar pole (north on the left of the solenoid), repelling the approaching magnet.  If the bar magnet moves away, the induced current reverses direction (right to left through the ammeter), reversing the poles of the solenoid (south on the left of the solenoid) to attract the leaving magnet.  You can observe this in the video clip on Faraday's research into electromagnetic induction in the Watch and Listen activity below.

As you have learned, you can assign the magnetic poles of a solenoid based on Lenz's law.  Using the left-hand rule for solenoids, grasp the coil with your thumb pointing in the direction of the magnetic field within the coil (which is from the south pole to the north pole).  Your fingers then wrap around the coil in the direction of electron flow.

Lenz's law is a direct consequence of the conservation of energy.  Consider the falling magnet in a conductor, which you saw in Inquiry Lab 12...7.  If the induced current in the conductor, caused by the motion of the falling magnet, were orientated in such a way that it produced a magnetic field supporting the movement of the magnet, it would cause it to accelerate downward at a greater rate.  That would increase the current in the conductor, which would lead to a further increase in the downward acceleration of the magnet, leading to an even greater induced current and magnetic field, producing greater accelerations, and so forth.  This would mean that a magnet dropped into a metal pipe would accelerate like a bullet and shoot out of the lower end!  A positive feedback loop such as this would be creating energy, violating the universal law of the conservation of energy.