Mystery of Magnetic Flux: Example 6.2 Revealed

[gsan]

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

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?

 

[atyy]

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

 

[gsan]

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

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?

 

[Defennder]

the direction flow of the current indicates that the induces B field oppose the increasing flux

Yes, correct.

... but the d (magnetic flux linkage) / dt should be less than 0

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

, so the induces B field must point into the plane and the direction of the current should be flow from left to right, right?

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.

 

[gsan]

@Defennder

I m really confuse. what about for below example?

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

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.