Electro magnetic Induction problem. Simple one, but check wehre I am wrong.

AI Thread Summary
The discussion centers on a problem involving electromagnetic induction, where a magnet moves through a coil, and the induced electromotive force (emf) is analyzed. The initial interpretation suggested that the induced emf becomes negative as the magnetic field increases and positive as it decreases, forming a V-shaped graph. However, the textbook states that the emf is initially positive, indicating a misunderstanding of the relationship between the magnetic field and the area vector. The key correction involves recognizing that the magnetic field (B) and area vectors are antiparallel at the start, leading to a positive flux and thus a positive induced emf. This clarification resolves the confusion regarding the direction of the induced emf in the scenario.
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Electro magnetic Induction problem. Simple one, but please check wehre I am wrong.

Homework Statement



A magnet moving down, goes through a coil and exits it below. Plot the graph showing variation of induced emf in coil with time.

Homework Equations



emf = - Flux/Time

The Attempt at a Solution


As the magnetic field B increases, emf becomes more and more negative.
Then while leaving, B decreases, emf becomes more and more positive.

Hence

The emf is first negative, forms a V shape in the negative emf region and then an inverted V in the positive emf region when it leaves.

4.Answer in the textbook
The emf is first positive ... in other words just the opposite of what I said.

Where did I go wrong ?
 
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From what two points are you measuring the emf?
 
From either ends of the coil
 
Isn't the "B" in this system a vector?

(Those tricky vectors...)

:cool:

~A137
 
Ya. But how will that matter?
 
I think I get it now.
Initially B and Area vectors are antiparallel, so flux is negative. Hence, the induced emf is positive.
 

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