问题 计算题

如图所示的电路中,各电源的内阻均为零,其中B、C两点与其右方由1.0Ω的电阻和2.0Ω的电阻构成的无穷组合电路相接。求图中10μF的电容器与E点相接的极板上的电荷量。

答案

解:设B、C右方无穷组合电路的等效电阻为RBC,则题图中通有电流的电路可以简化为图1中的电路。B、C右方的电路又可简化为图2的电路,其中是虚线右方电路的等效电阻

由于B'、C'右方的电路与B、C右方的电路结构相同,而且都是无穷组合电路,故有

由电阻串、并联公式可得

由式①、②两式得

解得RBC=2.0Ω ③

图1所示回路中的电流为 ④,电流沿顺时针方向

设电路中三个电容器的电容分别为C1、C2和C3,各电容器极板上的电荷分别为Q1、Q2和Q3,极性如图3所示

由于电荷守恒,在虚线框内,三个极板上电荷的代数和应为零,即Q1+Q2-Q3=0 ⑤

A、E两点间的电势差

又有

B、E两点间的电势差

又有 ⑨

根据⑤、⑥、⑦、⑧、⑨式并代入C1、C2和C3之值后可得Q3=1.3×10-4C ⑩

即电容器C3与E点相接的极板带负电,电荷量为1.3×10-4C

单项选择题
阅读理解

选做题:阅读下面短文,并根据短文后的要求答题请注意问题后的词数要求。

     Called "The School of the Future" and created with help from software giant Microsoft, a recently-

opened public school in Philadelphia, US, is believed to be the first in the world to combine innovation

teaching methods with the latest technology. At this school, students work on wireless laptops, teachers

eschew traditional subjects for real-world topics and parents can track their children‘s work on the Internet.

     The school, which costs the school district $63 million to build, is free and has no entrance exams.

The 170 ninth-grade students were selected at random from l,500 applicants.

     Philadelphia School District Chief Executive Paul Vallas told students they would be observed by other

schools around the world. "You have become instant role models," Vallas said. "People are going to be

watching you."

     Students still sit in classrooms, but lessons rely heavily on information found on the Internet and on

interactive software. Students will be allowed to learn at their own pace. Homework is done on the computer

and sent to the teacher for grading and parents can access to the school‘s network to read the teacher’s

feedback on their children’s progress.

     Traditional education fails to teach students the skills of problem-solving, critical thinking and effective

communication, which they need to succeed in the 21st century, Principal Shirley Grover said in an interview.

"It‘s not about memorizing certain algebraic equations (代数方程式) and then presenting them in 8 test,"

Grover said. "It’s about thinking how maths might be used to solve a quality of water problem or how it

might be used to determine whether or not we are safe in Philadelphia from the bird flu."

     David Terry, 14, said he was hoping to "turn over a new leaf" after discipline problems in his previous

school left him with an "average to really bad" academic record. "This is a great opportunity for me," he said.

"In other schools, I would not _____."

1. What‘s the best title of this passage? (Please answer within 10 words.)

    ______________________________________________________________________________________

2. Which sentence in the passage can be replaced by the following one? Although the students have their

    lessons in classrooms, they mainly learn through the Internet and on some software.

    ______________________________________________________________________________________

3. Please fill in the blank in the last paragraph with proper words or phrases to complete the sentence. (Please

    answer within 10 words.)

    ______________________________________________________________________________________

4. Do you think this kind of school good? Why? (Please answer within 30 words.)

    ______________________________________________________________________________________

5. Please translate the underlined sentence in the fifth paragraph into Chinese.

    ______________________________________________________________________________________