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## GR0877 Problem 18

PROBLEM STATEMENT: This problem is still being typed.

SOLUTION: (E) Let’s apply Lenz’s law. As the magnet enters the loop, the flux through the loop increases. The induced current generates a magnetic field that opposes the change of flux due to the bar magnet, i.e. it decreases the flux through the loop. Hence, the induced current is counter-clockwise (that is, form $b$ to $a$). As the magnet leaves the loop, the flux due to the bar magnet decreases and the induced current flows in the opposite direction (clockwise). Choice (E).

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Categories: 0877 Tags: , , , ,
1. 12.10.2011 at 04:45

First I’d like to say thank you so much; this is an awesome study aid =)

Okay, so I got this problem wrong when I took this practice exam. All of my reasoning was right, my conclusion was right, but I picked D instead of E. I might have been the only one to do this, but I was thinking about current flowing through the loop while solving the problem, so I thought about current flowing through the loop while picking an answer. The current goes from a to b through the loop while the magnet is approaching, but from b to a through the resistor. I realize in retrospect why you’d most likely call this b to a, but in case anyone else was silly enough to mix this up the way I did and wonder why ETS had a different definition of counter-clockwise than you, here’s your answer.

As a side note, I don’t think my initial labeling is unreasonable; the question really should specify “through the resistor”, or use terms like CW and CCW. The way it’s worded now D and E are both arguably correct.

2. 11.11.2011 at 20:40

Yes, damn you Ben Franklin!

3. 07.10.2012 at 08:27

Grant – the exact same thing got me too. In fact, your comment made me realize that my definition of b to a was not only wrong, but also not the only one. I agree that this is terrible wording on the part of ETS – how could they possibly be testing something as meaningless as knowing that “from b to a” means through the shortest path between the two? But anyways, thanks, and you’re not alone.

4. 26.10.2016 at 03:35

Thank you guys!!! I had the same problem!!! ETS should have been more specific…this is the reason why I hate this kind of test!