Lenz law

Lenz law determines the direction of induced electric current in the loop (minus sign in the Faraday law).

An induced current is always in such a direction as to oppose the motion or change causing it.

• If the flux increases, the induced magnetic flux is to decrease the total flux.
  
• If the flux decreases, the induced magnetic flux is to increase the total flux.

The Lenz law is easily understood by means of moving metal rod lying on the U-shaped conductor. Use right hand rule to deduce the direction of e.m.f. The inudced flux is here to out from the page, to oppose the increase of magnetic flux in to the page.

Deduce the direction of induced current in the following cases:

1. Bar magnet, 2. Metal ring, 3. Metal rod

Lenz law is an extension of Newton 3. law applied to electromagnetism. The induced B-field tries to oppose the ultimate reason to the change of the flux. For example:

1. Non-magnetic copper ring induces the magnetic force that bounces back the falling bar magnet and act as counter force to gravity.
2. Magnetic force due to induced eddy currents brakes the fall of a magnet inside the copper tube. Play a video.