# Problem P. 4211. (December 2009)

**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* (*d**r*), 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 pont)

**Deadline expired on January 11, 2010.**

Sorry, the solution is available only in Hungarian. Google translation

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

### Statistics:

22 students sent a solution. 3 points: 20 students. 1 point: 2 students.

Problems in Physics of KöMaL, December 2009