问题
选择题
通电螺线管中有如图所示方向的电流，其中各小磁针N极所指方向向左的是（ ）
A．甲
B．乙
C．丙
D．丁
答案
根据右手螺旋定则，可得通电螺线管的左端是S极，右端是N极，内部磁感线是S极到N极，外部是N极到S极，
根据小磁针N极指向磁感线的方向，因此甲、丙、丁小磁针N极均指向右，而乙指向左．故B正确，ACD错误；
故选：B
搜题请搜索小程序“爱下问搜题”
通电螺线管中有如图所示方向的电流，其中各小磁针N极所指方向向左的是（ ）
A．甲
B．乙
C．丙
D．丁
根据右手螺旋定则，可得通电螺线管的左端是S极，右端是N极，内部磁感线是S极到N极，外部是N极到S极，
根据小磁针N极指向磁感线的方向，因此甲、丙、丁小磁针N极均指向右，而乙指向左．故B正确，ACD错误；
故选：B
| 完形填空. | ||||
| Bill was a newspaper boy. He went from house to house 1 the day's papers to the neighbors. Each week he went to get his 2 . Bill kept a little book. In the book he wrote down who 3 him and who did not. Bill was keeping a record (记录) Men have been keeping records 4 very early time. How did they do this before they learned to write? Sometimes they made knots (打结) in different colored strings (绳子).Indians in South America kept records in this 5 . Today we have machines for 6 and keeping records. Do you go to the grocery store? Does it have a cash register? The cash register keeps records. It adds up the things you 7 . It tells how much is sold in the store everyday. Many places have machines called computers. They 8 computers to keep all the records. Computers are 9 than knots in strings. Someday men may use _10 even better than computers. | ||||
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