The magnetic field of moving charge I=ngvs ⊕ ⊕v④ Idl dB=。ldlx Anr2 dN nSdl dB=4。ngdls vxp dB=4。an qvx分 4r2 4π2 The magnetic field of moving charge Idl + + + + + v + I S 2 4 ˆ r Idl r dB o       2 4 ˆ r nqdlS v r dB o       2 4 ˆ r dN qv r dB o       I  nqvs dN  nSdl