11. What are the Kepler's laws of planetary motion? Please derive the Kepler's third law.

13. A long cylindrical conductor, of radius a, has two cylindrical cavities of diameter a through its center, as shown in Fig. 1. A current I is directed out of the page and is uniform through a cross section of the conductor. Find the magnetic field in terms of \( \mu_0 \), I, r and a at (a) point \( P_1 \) and (b) point \( P_2 \).