Direction of the Induced emf: Lenz's Law
General statement:
- The effect of the induced emf is such as to oppose the change in magnetic flux that causes the induced emf.
Appling Lenz' Law: Method One - Determining the Direction of Secondary B-Field
- The induced emf creates a current that itself creates a secondary magnetic field. This secondary magnetic field also changes with time and thus creates a changing secondary magnetic flux. The secondary flux changes in such a way to opposes the change in flux creating the emf.
Normally this means that the secondary magnetic field increases or decreases in such a way as to oppose the change in the magnetic field creating the induced emf.
Permanent Magnet Example:
The magnetic field of a permanent magnet emanates out of its north pole and enters its south pole. The B-field is also stronger closer to its poles. |  |
When the permanent magnet is not moving, its B-field creates a flux through the loop (Fig a). However, this flux is not changing and consequently no current is induced.
When the north pole of a permanent magnet is pushed into a loop (Fig b) the flux increases. An upwards secondary magnetic field is created that opposes the downward B-field of the magnet. As viewed from above, the current in loop must flow counterclockwise in order to create this secondary B-field. Don't be mislead, it is the changing flux created by the changing secondary B-field which opposes the changing flux created by the permanent magnetic being pushed down and not just the creation of secondary B-field in the opposite driection.
When the magnet is removed from the loop (Fig c), the decreaing B-field in the loop creates a decreasing flux. To oppose this decrease, the current in the loop flows in such a way that tries to sustain the magnetic field. The current now has to flow clockwise in order to create a positive secondary flux that tries to counter acts the decreasing flux due to the with drawl of the permanent maget. This means that the secondary magnetic field has to be in the same direction as the magnets B-field.
Applying Len'z Law: Method Two - Determining the Sign of the various Terms
- Choose a positive direction for area of the loop A. The direction of A is normal to the surface - either up or down in the example below.
- The right hand rule: With your thumb pointing along the direction chosen for A , the curl of your fingers gives the positive direction for e.
E-Store
lenz law
