4. 一質點在作圓周運動，位置向量可以用極座標寫成。請算出向心加速度。

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.