3. 可觀測到的星體、銀河等等，都在旋轉。請問為什麼？

2. A long coaxial cable (as shown in Fig.) consists of two thin walled concentric conducting cylinders with radii a and b. The inner cylinder carries a steady current i, and the outer cylinder provides the return path for that current. The current sets up a magnetic field between the two cylinders. Calculate the energy stored in the magnetic field for a length l of the cable.