问题 实验题

某同学欲设计一个电路,自制一台电子秤,通过查阅资料发现电子秤的主要部件为一个压敏电阻,没有压力时阻值为20Ω,允许通过的最大电流为0.5A,现有下列器材:压敏电阻、质量为m0的砝码、电流表、滑动变阻器、干电池各一个、开关及导线若干、待测物体(可置于压敏电阻的受压面上)。请完成对该物体质量的测量。

A.直流电源:电动势约4.5V,内阻很小;

B.电流表A1:量程0~0.6A,内阻0.125Ω;

C.电流表A2:量程0~3.0A,内阻0.025Ω;

D.滑动变阻器R1:最大阻值10Ω;

E.滑动变阻器R2:最大阻值50Ω;

F.开关、导线等。

(1)在可供选择的器材中,应该选用的电流表是__________,应该选用的滑动变阻器是____________。

(2)根据所选器材设计一个电路,用来描绘压敏电阻的电流随压力变化的关系图象,要求压敏电阻两端电压调节范围尽可能大,在方框中画出完成的测量电路,并根据设计的电路图连接实物图,如图所示;

(3)请说明具体的测量步骤。

答案

(1)A1;R1

(2)

(3)①调节滑动变阻器,使通过压敏电阻的电流为某一数值;

②保持滑片位置不动,在压敏电阻上方放一砝码,读出此时电流表的示数;

③不断增加砝码个数,分别读出电流表对应的电流值;

④建立m-I坐标系,根据记录数据进行描点,用平滑的曲线连接,让尽可能多的点落在这条曲线上;

⑤根据物体放在压敏电阻上所对应的电流值,对应图象读出该物体的质量。

单项选择题
单项选择题

Parenting was never a piece of cake in any age, but probably the greatest source of headache for parents today in Japan is the ubiquitous cellphone. Today, 96 percent of senior high school students and 58 percent of junior high school students have cellphones. Even among primary school children, 31 percent have them.

By enabling youngsters to stay connected with their parents at all times, these gadgets help to keep children safe. For the kids, they are fun toys, too, that let them text to or chat with their pals whenever they want, play Internet games, and enjoy blogging for their own profile and diary purposes.

But terrible dangers lurk beneath all that fun and convenience.

Every year about h 000 children become involved in rape and other crimes through dating service sites. Violent and obscene images are only a couple of clicks away. On gakkoura saito, or so-called unofficial school websites where kids can post whatever they want, anyone can fall victim to brutal "verbal mob lynching" by their peers.

Amid today’s urgent need to address these problems, the government’s Meeting on Education Rebuilding has issued a report. In response to the Prime Minister’s recent comments — "I cannot think of one good reason for (letting youngsters) have a cellphone" and "I would like everyone to discuss whether cellphones are really necessary" — the report recommended that "parents, guardians, schools and all parties concerned should cooperate among themselves, so that elementary school pupils and junior high school students do not have a cellphone unless there is a compelling reason for them to do so."

But since many parents believe in the necessity of cellphones as a safety tool, it is unrealistic to expect everyone to do away with them. Rather, it would make more sense for guardians, schools and cellphone companies to consider, from their respective standpoints, how cellphones should be used by children.

We suggest that parents sit down with their offsprings and talk about their "house rules" for cellphone use. For instance, set the hours allowed, so the kids won’t be texting to their friends late into the night, remind them never to give away personal information online, and so on,

But there are limits to what individual families can do, and this is where we also suggest that schools should educate their pupils on the dangers of cellphone use. One way to go about this, for instance, may be for each class to set its own rules on sending e-mail messages.

What is the main idea of the passage?()

A.Parents neglect to protect their kids from cellphones.

B.Parenting with cellphones is a source of headache.

C.Cellphones should be banned from campus.

D.Kids need lessons on the uses of cellphones.