Poorly worded question?

[jtrexler]

A conducting loop of wire is placed in a magnetic field that is normal to the plane of the loop. Which one of the following actions will not result in an induced current in the loop?

1)Decrease the area of the loop.

2)Increase the strength of the magnetic field.

3)Decrease the strength of the magnetic field.

4)Rotate the loop about an axis that is parallel to the field and passes through the center of the loop.

5)Rotate the loop about an axis that is perpendicular to the field and passes through the center of the loop.
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Now, I know the basics of magnetic fields- as long as there is relative motion between a magnet and a loop there will be current in the loop. But I am having issues with this question. Is changing the size of a loop "relative motion"?
Thanks for your help.

[pallidin]

Originally posted by jtrexler
A conducting loop of wire is placed in a magnetic field that is normal to the plane of the loop. Which one of the following actions will not result in an induced current in the loop?

1)Decrease the area of the loop.

2)Increase the strength of the magnetic field.

3)Decrease the strength of the magnetic field.

4)Rotate the loop about an axis that is parallel to the field and passes through the center of the loop.

5)Rotate the loop about an axis that is perpendicular to the field and passes through the center of the loop.
-------------------------------------------

Now, I know the basics of magnetic fields- as long as there is relative motion between a magnet and a loop there will be current in the loop. But I am having issues with this question. Is changing the size of a loop "relative motion"?
Thanks for your help.


Increasing or decreasing the strength of the magnetic field will cause the magnetic field to interact with the loop, causing induction, so, 2 & 3 are out.
5 obviously causes induction so 5 is out.
This leaves 1 & 4.
Not sure where to go from there.

[Doc Al]

Originally posted by jtrexler

Now, I know the basics of magnetic fields- as long as there is relative motion between a magnet and a loop there will be current in the loop. But I am having issues with this question. Is changing the size of a loop "relative motion"?

Yes. Changing the size of the loop changes the magnetic flux and induces a current.

Careful with the idea that relative motion always induces a current in the loop. If the relative motion does not impact the flux (as in #4) there will be no induced current in the loop.

[jtrexler]

Okay, thanks for the help. I thought that changing the area of the loop would induce a small amount of current, but wasn't sure if that was what the question was going for.