magnetic flux問題

You could consider the metal disk is made up of a number of thin wires lining radially outward from the centre. As such, when the disk is rotating, each of the wire is also rotating and cutting magnetic firld lines.

Induced emf will be produced when a metallic wire cuts magnetic field lines. Hence, induced emf is produced between the rim and centre of the rotating disk. If given the direction of rotation of the disk, the direction of magnetic field lines, you caould apply Fleming's Right Hand Rule to find the direction of the induced emf.

[For example, if the disk is rotating clockwise under a vertically downward magnetic field, the rim of the disk will be at potential higher than the centre of the disk]

Similar situation can be applied to a rotating rod, a fan-like mettalic blade etc.

2011-08-02 23:07:16 補充：
Lenz' law can only be used when there is an induced current flowing. If the magnetic field covers the while disk, there is only induced emf, but no induced current.

2011-08-02 23:17:28 補充：
If the magnetic field covers part of the disk, there is induced eddy current. You could then use Lenz's Law to find the direction of the current flow.

2011-08-02 23:19:03 補充：
Notice that Lenz'a Law is a rule enables the direction of induced current to be determined. It is NOT a law to explain why there happens to be an induced emf.