**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 points)

**Deadline expired on 11 January 2010.**