VCAA 2012 Exam 2, Q8c, Lenz's Law

[Brytz]

I just did this exam paper but when I attempted this question, I was really stumped. What I understand is there is a decrease in the change of flux. Does the magnetic field direction be the same as the magnetic flux?

How do you approach this style of question?

http://www.vcaa.vic.edu.au/Documents/exams/physics/2012/2012physics2-w.pdf

[lzxnl]

At position two, there is max flux B*A through the square loop. At position three, it is not in the magnetic field anymore, so the field is zero. As the loop begins to leave the magnetic field, the area that the field interacts with the loop decreases, so the magnetic flux decreases, inducing an emf and hence current.

[Brytz]

The current goes in which direction and why? How does the decrease in change in flux affect the direction of the current?

[lzxnl]

OK, so as the loop leaves the magnetic field, the flux through the loop decreases. An induced current appears to create a magnetic field to reinforce the magnetic field. In this case, the induced magnetic field would be from left to right, so counterclockwise current by right hand rule.