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P. 4211. A current of I is flowing in a wire which is in the centre of an infinitely long straight metal tube. The radius of the tube is r the width of its wall is d (dr), and the young modulus of its material is E. By what amount does the radius of the tube change if a current of magnitude I is started in the tube which current is opposite to the current in the wire. Does the radius increase or decrease?

(5 points)

Deadline expired on 11 January 2010.

Google Translation (Sorry, the solution is published in Hungarian only.)

Megoldás. A sugár megnő $\displaystyle \Delta r=\frac{\mu_{0}I^2}{8\pi^2d\,E}$ értékkel.

Statistics on problem P. 4211.
 22 students sent a solution. 3 points: 20 students. 1 point: 2 students.

• Problems in Physics of KöMaL, December 2009

•  Támogatóink: Morgan Stanley