# Lenz's Law/Conservation of Energy

1. Nov 19, 2005

### KatieKangaroo

Show that Lenz's Law is consistent with the principle of conservation of energy.
Lenz's Law states that an induced voltage will always act to appose the change that caused it. I'm just struggling to link this with the principle of conservation of energy. Can anyone help me out?

2. Nov 19, 2005

### Chi Meson

Lenz's law says a little more than that; stretch it out and it becomes:

An increasing voltage flows through a coil. This coil creates an increasing magnetic field (increasing flux). This change in flux will induce a voltage in a second coil. The induced voltage ( if it can) will cause an induced current. THis induced current will create an increasing magnetic field, and therefore a secondary changing flux. THis secondary change in flux will oppose the original change in flux that caused the induced voltage. The induced change in flux from the second coil will then induce another voltage in the first coil. This voltage will then oppose the original voltage.

This way you can't get one current to create a second current without losing the first one.

3. Nov 19, 2005

### Chi Meson

KatieK: (in response to your PM)

If you are talking about a bar magnet being put through a coil, Lenz's Law is simpler (not simple, just simpler than two coils).

If a current is induced in the coil when a magnet is put through it, then the energy of this current could do work. If the force of pushing the magnet were somehow not resisted, then you could keep the mechanical energy (the KE of the moving bar magnet) while gaining electrical energy (of a current moving due to a voltage).

The induced magnetic field is what opposes the moving magnet and "takes away" the KE. IF the induced current caused a magnetic field that attracst the magnet, then you would get the electrical energy, plus an increase in KE (since the attraction would pull the bar magnet into the coil). Hence, the fact that the induced current creates a magnetic field that opposes the motion of the magnet means that no free energy is created.