问题 问答题

长0.5m的直导线,通过4A的电流,将直导线放入匀强磁场中,当导线与磁场方向垂直时,导线所受磁场力为0.8N,求:

(1)匀强磁场的磁感应强度B的大小.

(2)若将通电导线中的电流减为2A,则这时磁感应强度B的大小是多少?导线所受安培力是多大?

答案

(1)当导体棒长为L=0.5m,电流为I=4A,并且导体棒和磁场垂直,所以B=

F
IL
=
0.8
0.5×4
T
=0.4T

(2)当电流减少为2A,该磁场的磁感应强度不变,其由磁场本身决定,导体棒受到的安培力大小为

F′=BI′L=0.4×2×0.5N=0.4N,

答:(1)匀强磁场的磁感应强度B的大小为0.4T.(2)若将通电导线中的电流减为2A,则这时磁感应强度B的大小仍为0.4T.导线所受安培力0.4N.

完形填空
Professor Green, known to the world as a scientist, is not only absent-minded but short-sighted as well. His mind is always busy 41 scientific problems and seldom notices what is going on 42  him.
One fine day recently, he went  43 a walk in the countryside, but as 44  he has a book in his hand. When he went out , he began to read his book . He hadn’t gone far 45 he run into a big cow and fell down. In the fall, he had lost his glasses, without which he couldn’t see anything. He thought he had hit his head  46 a fat lady. “I’m sorry, Madam.” He said politely 47 searching for his glasses. As soon as he had 48 , he realized his mistake.
Soon he was fixing his mind on his book 49  and paid no attention to anything else. He had scarcely been walking for five minutes when he fell over again, _50 both his book and his glasses. This time he got very angry, seizing his umbrella, he gave the “cow” a wild blow. Then, after finding his glasses, he realized with horror that he made a second mistake. A large fat woman was fleeing from him in a horror.
小题1:
A.to thinkB.thinkingC.thinking aboutD.to think of
小题2:
A.withB.aroundC.forD.at
小题3:
A.forB.toC.towardsD.over
小题4:
A.alsoB.oftenC.everD.usual
小题5:
A.whenB.whileC.just thenD.at that time
小题6:
A.toB.againstC.aboutD.onto
小题7:
A.afterB.inC.beforeD.during
小题8:
A.put it onB.taken them offC.put them onD.taken them out
小题9:
A.alwaysB.againC.usuallyD.very much
小题10:
A.holdingB.gettingC.findingD.losing
单项选择题