用一根硬导线弯成半径为R的半圆形，将其置于磁感应强度为B的均匀磁场中，以频率f旋转，如图所示，这个导体回路中产生的感应电动势ε等于：（）
A.（6R2+（1／2）πR2）2πfBsin2πft
B.（6R2+（1／2）πR2）fBsin2πft
C.（1／2）πR2fBsin2πft
D.（πR）2）fBsin2πft
参考答案：D
解析：用电磁感应定律，当通过线圈的磁通量变化时，在线圈中产生感应电动势ε。
A man was bitterly envious of his neighbour．He spent long hours in prayer，wanting to be to his neighbour in every aspect of life． One night，God appeared before him in a dream and said，“I am pleased with your ．Ask for a gift． your neighbour will get the same gift in double amount．” He was glad but sad that his enemy would get a greater gift．He requested，“God， the sight in one of my eyes．” With his single eye，he peeped at his neighbour but was to see his neighbour's sight was not lost! Confused，he to God，“Why don’t you keep your word?” God said，“Son，I always keep my word． is not a gift，but a curse．You have the gift of sight in only one eye now，as asked by you．But your neighbour has sight in both of his eyes．I the gift．” He suddenly from his dream and was sure that it was a message from God．Englightened by the dream，he went to his neighbour and begged his pardon for thinking and acting him．They embraced each other and were good friends thereafter．
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