作者ddwin (AntiChina)
看板Physics
标题[问题] 电磁solenoid
时间Mon Jan 5 20:25:07 2009
The long solenoid has 220 turns/cm and carries a current i=1.5A;
its diameter d is 3.2cm. At its center we place a 130-turn close-
packed coil of diameter 2r=2.1cm.The current in the solenoid is
reduced to zero and then increased to 1.5A in the other direction
at a steady rate over a period of 50ms.
a.What is the magnetic field B at the center of the solenoid ?
Ans:0.04
b.What is absolutely value of the induced emf that appears in
the central coil while the current in the solenoid is being changed?
Ans:1.4 * 10^-4
我的问题在b小题，照题目来看磁场的大小是随时变的cos函数
所以可将a中求得的代入 B(t)=0.04*cos(wt)
磁通量 Φ=Bπr^2=0.04*cos(wt)*πr^2
ε=-dΦ/dt= w*0.04*sin(wt)*πr^2 = 17.39* 10^-4 *sin(wt)
这和答案不相等，解答是直接把磁通量乘以2再除以周期
不知道我这样算有什麽错呢?解答为何那样写呢?
谢谢
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1F：推 rogerfd:你把问题想太复杂了吧,就直接改变的磁通量除以经过的时间 01/06 11:58
2F：→ rogerfd:不就是他的解答了? 01/06 11:58
3F：推 rogerfd:而且你也不能知道他是cos function吧，他说steady rate 01/06 12:02
4F：→ rogerfd:也许是斜线的变化呀，应该是不大确定是cos 01/06 12:03
5F：→ ddwin:谢了 01/06 12:42