Балкански 2009, Теория — Задача 3. Exercise 2

Concours Général de Physiques « Minko Balkanski » · 16 май 2009 г. · 5 т.

Указания

The main problem and the two exercises are completely independent and can be solved in any order.

All the answers must be given in English or French. The clarity and precision will be taken into account for the final note.

The exam is 4 hours long! The calculators are authorized.

Условие

Consider a perfectly conducting disk of radius $r_0$ in a constant magnetic field $B$ perpendicular to the plane of the disk. Sliding contacts are provided at the edge of the disk ($C_1$) and at its axle ($C_2$) ( cf. Figure 2 ). This system is Faraday’s “homopolar generator.” When turned at constant angular velocity, it provides a large direct current with no ripple. A torque is produced by a mass $M$ hung on a long string wrapped around the perimeter of the disk.

(a) Explain. how and why a current flows. Give a quantitative expression for the current as a function of angular velocity.

(b) Given a long enough string, this system will reach a constant angular velocity $\omega_f$. Find this $\omega_f$ and the associated current.

Решение

---

Оригинал в Архива: CGP_2009_en.pdf