问题 计算题

如图所示的电路中各元件值分别为:R1=R2=10Ω,R3=R4=20Ω,电容C=300μF,电源电动势E=6V,内阻不计,原先双掷开关S与触点2接触,则当开关S从与触点2接触改为与触点1接触,且电路稳定后,试求:

(1)求电容器C所带电荷量;

(2)若开关从与触点1接触改为触点2接触,直到电流变为0时为止,求通过电阻R1的电荷量

答案

(1)9×10-4 C   (2)4.5×10-4

题目分析:(1)在直流电路中,电容器相当于断路.开关S接触触点2时,电容器不带电.开关S接触触点1时,4个电阻串联,根据闭合电路欧姆定律得:A

电容器与电阻R、R并联,电压为V

故带电量为

即当开关S从触点2改接通触点1,且电路稳定后,电容C的带电量为9×10-4C.

(2)开关S从触点1改为接触点2后,电容器相当于电源,电阻R1与R串联,而电阻R与R串联,两个支路再并联,两支路电流之比为:

故通过两个支路的电量之比为:

根据电荷守恒定律,有

故通过电阻R1的电荷量

即开关S从与触点1接触改为触点2接触,直到电流变为0时为止,通过电阻R1的电荷量为4.5×10-4 C.

单项选择题

A good modern newspaper is an extraordinary piece of reading. It is remarkable first for what it contains the range of news from local crime to international politics, from sports to business to fashion to science, and the range of comment and special features (特定) as well, from editorial page to feature articles and interviews to criticism of books, art, theatre and music. A newspaper is even more remarkable for the way one reads it never completely, never straight through, but always by jumping from here to there, in and out glancing at one piece, reading another article all the way through, reading just a few paragraphs of the next.

A good modern newspaper offers a variety to attract many different readers, but far more than any one reader is interested in. What brings this variety together in one place is its topicality (时事性), its immediate relation to what is happening in your world and your locality now. But immediacy and the speed of production that goes with it mean also that much of what appears in a newspaper has no more than transient (短暂的) value.

For all these reasons, no two people really read the same paper: what each person does is to put together out of the pages of that day’s paper, his own selection and sequence, his own news paper. For all these reasons, reading newspapers efficiently, which means getting what you want from them without missing things you need but without wasting time, demands skill and self-awareness as you modify and apply the techniques of reading.

It can be conclude from the passage that newspaper readers ().

A. apply reading techniques skillfully

B. jump from one newspaper to another

C. appreciate the variety of a newspaper

D. usually read a newspaper selectively

单项选择题