# Lenz's law

1. Sep 20, 2008

### gsan

http://img.photobucket.com/albums/v235/gsan_bot/magnetic1.jpg" [Broken]

Refer to example 6.2 shown in the picture, the magnetic flux = -1.2t and the deriative time of magnetic flux should be -1.2, thus, the flux is decreasing and the direction of the induced current should be reverse. but why it say the flux is increasing?

Last edited by a moderator: May 3, 2017
2. Sep 20, 2008

### atyy

It only says that the magnitude of the flux is increasing.

3. Sep 20, 2008

### gsan

the direction flow of the current indicates that the induces B field oppose the increasing flux.... but the d (magnetic flux linkage) / dt should be less than 0, so the induces B field must point into the plane and the direction of the current should be flow from left to right, right?

4. Sep 20, 2008

### Defennder

Yes, correct.

It is greater than zero because Lenz law is -d(phi)/dt and not just d(phi)/dt.

No, as you said above, the current inducing induced must itself generate an opposing B-field to counteract increasing flux. Since flux is increasing, this means the resultant B-field must point upwards. And if it point upwards, it means the current must be anti-clockwise, not clockwise.

5. Sep 20, 2008

### gsan

@Defennder

I m really confuse. what about for below example?

http://img.photobucket.com/albums/v235/gsan_bot/magnetic2.jpg" [Broken]

if you say that Lenz law is -d(phi)/dt, then for this case, -d(phi)/dt should be less than zero, the flux is decreasing and the current should flow couterclockwise.

Last edited by a moderator: May 3, 2017