This question caused a debate in the class: EMF from a changing magnetic field

[Physical_Fire]

Hello,
So recently our class have learnt about E.M.F induction entirely with it's equation and a classmate found a question from the internet. I honestly believe the answer is A as the magnetic field is changing rapidly, but some of my classmates believe it's B. While others believe it's C. This has caused a debate to ensue and I shared my reasoning. What does the community think is the correct answer?

 

[berkeman]

What is the Relevant Equation relating EMF to the magnetic flux through the coil? Can you sketch a plot of the magnetic flux versus time while the magnet is spinning (and label the points P, Q, R on that plot)?

And based on the Relevant Equation, what do you need to do to that plot to show the EMF induced in the coil versus time?

 

[Physical_Fire]

-d phi(b) / change in t. phi(b) is the change in magnetic flux. and the graph would be as attached. However, I am not sure how to label as I don't understand which has the highest emf

 

[berkeman]

Good. The first vertical dotted line on the left is in the wrong place, though, and should be at the Origin (move it to the left).

Where do you think point Q could be on the graph?

EDIT -- the 3 middle points on the 2 graphs look okay, but the outer points are a bit misaligned (the bottom sine wave is asymmetrical)

 

[Physical_Fire]

Q should be the second vertical line as it has the magnetic flux is changing rapidly so emf is at its max. So does that mean it's A, and I was secretly right the entire time?

 

[berkeman]

Q should be the second vertical line as it has the magnetic flux is changing rapidly so emf is at its max. So does that mean it's A, and I was secretly right the entire time?

It could be either of the zero crossings of the flux sine wave, so yes, I believe the correct answer is A.

 

[Physical_Fire]

Thanks for resolving.

 

[Tom.G]

See this post and its embedded video of the experimental results.
https://www.physicsforums.com/posts/6503403

Cheers,
Tom