Lenz's Law: Determine Current A-B/B-A

[anifau98]

Homework Statement

Use Lenz's law to determine whether the induced current in the resistor R is from A to B or from B to A in the following configurations:

(1) a magnet with south side pointing down is entering a coil attached to a resistor with A coming before B if you go clockwise

(2) a magnet with north side leaving a coil attached to above resistor

(3) a magnet with north side entering the above coil

(4) a magnet with south side entering the coil like above but it is flipped (like it is starting to wind from bottom instead of top)

2. The attempt at a solution

I tried using the Right Hand Rule but I'm not sure exactly what to do. Like for the first one, since the south side is entering the coil, the magnetic field would be from south to north (?) so then the current would be counterclockwise..since it is entering, it would want to oppose and the induced current is clockwise?? so it is from A to B?

For the second one since it is leaving, it would want to stay the same so it would be counterclockwise and be from B to A?

and the third one since it is north entering, the field is pointing down so it is clockwise, and would want to oppose so becomes counterclockwise??

and last one I'm not sure how the change in which side the coil starts to wind even effects the current at all? so it would be same as the first one A to B??

 

[Defennder]

Is there supposed to be a picture for this question? It's hard to visualise the problem.

 

[baba89]

Your reasoning for the first three scenarios is correct. Lenz's law states that the direction of the induced current will be such that it opposes the change in magnetic flux. In the first scenario, the south side of the magnet entering the coil creates a magnetic field that is increasing in a clockwise direction. The induced current in the coil will produce a magnetic field that opposes this change, so it will flow counterclockwise (from A to B).

In the second scenario, the north side of the magnet leaving the coil creates a magnetic field that is decreasing in a clockwise direction. The induced current in the coil will produce a magnetic field that opposes this change, so it will flow counterclockwise (from B to A).

In the third scenario, the north side of the magnet entering the coil creates a magnetic field that is increasing in a counterclockwise direction. The induced current in the coil will produce a magnetic field that opposes this change, so it will flow clockwise (from B to A).

For the fourth scenario, the change in which side the coil starts to wind does not affect the direction of the induced current. The key factor is still the change in magnetic flux. In this case, the south side of the magnet entering the coil creates a magnetic field that is increasing in a clockwise direction. The induced current in the coil will produce a magnetic field that opposes this change, so it will flow counterclockwise (from A to B), just like in the first scenario.