Zero Induced EMF: Find Rate of B Change for Problem 9

[CD135]

Homework Statement

Question I need help with: If wire AB of problem 9 moves with a velocity of 50.0 m/s to the right, at what rate should B change when the wire is at a distance of 5.00 cm from the left end of the loop, such that there will be no induced emf in the circuit? Answer: 300 T/s

Problem 9: (I don't know if this information is needed to solve the question above so I'm listing it here anyway)
The wire AB in the diagram moves to the right with a velocity of 25.0 cm/s. If l = 20.0 cm, B = 0.300 T, and the total resistance of the circuit is 50.0 Ω, find (a) the induced emf in wire AB, (b) the current flowing in the circuit, and (c) the direction of the current.Diagram is attatched

Homework Equations

emf=Blv
emf=change in flux/ change in time

The Attempt at a Solution

I don't understand this question. I have been trying for a very long time and I am very lost. If there is a velocity, electric field, and length, how could there ever be zero induced EMF? I was able to get question 9 as emf=Blv, very straight forward and I understand how the equation is derived. I'm clearly missing something here but I cannot figure out what.

 

[CD135]

Any help would be appreciated. I cannot find any information online or in the textbook to help me come up with any sort of solution. Thank you in advance!

 

[cnh1995]

emf=Blv
emf=change in flux/ change in time

These two emfs will be opposing each other such that resultant emf will be 0.
Write mathematical equations for each and simply equate them.

at what rate should B change

This asks you about 'rate of change' of magnetic field. Ring any bells?

Answer: 300 T/s

Right.

 

[cnh1995]

(I don't know if this information is needed to solve the question above

l = 20.0 cm, B = 0.300 T

This is needed.