# Homework Help: NIB Magnet Dropped in a Cu tube - Forces

1. Nov 14, 2012

### MrLiou168

1. The problem statement, all variables and given/known data
I'm trying to figure out and explanation as to why a strong magnet drops so slowly through a non-magnetic but conducting Cu tube. The attached diagram shows the original B field, induced Eddy currents, and secondary B field as a result of those Eddy currents, but I can't figure out my Force vectors. Any help greatly appreciated.

2. Relevant equations
F = qvB
F = qv X B
F = IL X B

3. The attempt at a solution
See attached diagram...
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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• ###### photo (1).JPG
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2. Nov 14, 2012

### Spinnor

I think after studying the following,

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

that your drawing has the induced currents going the wrong way? The induced magnetic field looks right though (note something is wrong as the right-hand rule does not apply to your drawing (induced current and secondary magnetic field)). The induced magnetic field below the falling magnet repels the falling magnet and the induced magnetic field above the falling magnet attracts the falling magnet.

3. Nov 14, 2012

### MrLiou168

Thanks for the response. Those examples are all of coils, and not solid conductors. Does this make a difference?

And in terms of the directions of the Eddy currents, is it correct to use I X B = F?