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

• Physical_Fire
Physical_Fire
Thread moved from the technical forums to the schoolwork forums
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?

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?

-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)

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?

Physical_Fire said:
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.

Thanks for resolving.

berkeman

Replies
11
Views
2K
Replies
3
Views
1K
Replies
31
Views
1K
Replies
3
Views
2K
Replies
5
Views
2K
Replies
6
Views
2K
Replies
2
Views
3K
Replies
7
Views
2K
Replies
10
Views
2K
Replies
2
Views
1K