问题 选择题

如图所示,在电容器C的两端接有一个圆环形导体回路,在圆环回路所围的面积之内存在着垂直纸面向里的匀强磁场B,已知圆环的半径r=5cm,电容器的电容C=20μF,当磁场B以2×10-2T/s的变化率均匀增加时,则电容器的(  )

A.a板带正电,电荷量为π×10-9C

B.a板带负电,电荷量为-π×10-9C

C.a板带正电,电荷量为π×10-6C

D.a板带负电,电荷量为-π×10-6C

答案

根据楞次定律知,感应电动势的方向是逆时针方向,则a极板带正电.根据法拉第电磁感应定律得:

E=

△B•S
△t
=2×10-2×π×52×10-4V=5π×10-5 V,

则:Q=CU=CE=2×10-5×5π×10-5=π×10-9C.故A正确,B、C、D错误.

故选A.

阅读理解

阅读短文,判断正误。正确的在题前括号内写T,错误的写F。

     James wrote a play for television about a family who came to England from India, and the play was

very interesting. It was bought by an American TV company. James was then invited to go to New York

to help them. He lived in Washington, which is an hour away from New York by air. The plane was going

to take off at 8:30 in the morning. So he had to be at the airport at about 7:30. He ordered a taxi for 6:30

and went to sleep. He forgot to wind (上发条) the clock, and it stopped after midnight. Also the driver of

the taxi had to work very late that night and he got up very late the next morning.

    James woke with the feeling that something was wrong. He looked at his clock. It stood there silently

with the hands pointing to ten past twelve. He turned on the radio and knew it was ten. He was late for the

plane.

    He was just preparing his coffee when the radio sent out another news, "Reports are coming in of a plane

crash (飞机坠落) near Washington airport. A Boeing 707 fly to New York crashed shortly after taking off

this morning. Plane number 2234…" James suddenly turned pale (苍白).

    "My plane," he said aloud. "If I haven't been late, "I'd have been on the plane!"

( )1. James was a writer from England.

( )2. He had to reach the airport at half past eight.

(     )3. James would like to take a taxi at six thirty.

( )4. When he turned on the radio he heard the speaker saying that he missed the plane.

(     )5. James was lucky because he was not able to get on the plane.

单项选择题