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## Homework Statement

http://www.jyu.fi/kastdk/olympiads/2005/Th2.pdf [Broken]

http://www.jyu.fi/kastdk/olympiads/2005/Th2%20Solution.pdf [Broken]

(Question 3)

My first doubt is why the magnetic field at the center of the coil is:

[tex]|\vec B|=\frac{\mu_0 NI}{2a}[/tex] and not

[tex]|\vec B|=\mu_0 nI[/tex], where n=N/l, the density of turns. The problem does not give any value for l.

**Second,**

They calculate the emf on each ring by doing [itex]\epsilon=\int E dr[/itex], where [tex]E=\omega Br[/tex]. Am I right?

And finally why do they sum the emfs of the rings?

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(Question 2), Why is that for the stationary regime I will calculate the mean values of the induced magnetic field? I think the magnetic needle will never stay put, there will never be a stationary regime, I think.

Thank you for the help.

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