关于螺线管磁场计算的问题 求教了The cross section above shows a long solenoid of length l and radius r consisting of N closely wound turns of wire.When the current in the wire is I,the magnetic field within this solenoid has magnitude Bo.A solenoid with the same number of turns N,length l,and current I,but with radius 2r,would have a magnetic field of magnitude most nearly equal to (A) Bo/4 (B) Bo/2 (C) Bo (D) 2Bo (E) 4Bo是说通电螺线管是2R吧 一楼 ..
问题描述：
关于螺线管磁场计算的问题 求教了
The cross section above shows a long solenoid of length l and radius r consisting of N closely wound turns of wire.When the current in the wire is I,the magnetic field within this solenoid has magnitude Bo.A solenoid with the same number of turns N,length l,and current I,but with radius 2r,would have a magnetic field of magnitude most nearly equal to
(A) Bo/4 (B) Bo/2 (C) Bo (D) 2Bo (E) 4Bo
是说通电螺线管是2R吧 一楼 ..
答
对于“无限长”的螺线管,（半径R与长L相比可忽略）,内部磁场为：B=(N/L)*μo*I.（外部磁场为零）
如果不是无限长,那么就按“毕奥—萨伐尔定律”去算.