Example:coaxial cable shown below carries a current of I,Find the energy stored by a segment of the cable with length L. solution: B= 4I B24。 dW=2πrld 2πr Wm 244.8元2rI Example: coaxial cable shown below carries a current of I，Find the energy stored by a segment of the cable with length L. solution： R2 R1 r r I B o   2  2 2 2 2 8 r B I w o o m      dV  2 rldr