问题 问答题

(1)有一根横截面为正方形的薄壁管(如图1所示),用游标为20分度的游标卡尺测量其外部的边长l的情况如图2甲所示;用螺旋测微器测得其壁厚d的情况如图6乙所示.则此管外部边长的测量值为l=______cm;管壁厚度的测量值为d=______mm.

(2)一种电池标称电动势为9V,内电阻约50Ω,允许的最大输出电流为50mA.为了较准确的测量这个电池的电动势和内电阻,可以设计出如图3所示的实验电路,已知实验中所使用的电压表内电阻足够大,可以忽略其对电路的影响;图中R为电阻箱,阻值范围为0~999.9Ω,R0为保护电阻.

①实验室里备用的定值电阻有以下几种规格:

A.10Ω      B.50Ω      C.150Ω      D.500Ω

实验时,R0应选用______较好(填字母代号).

②按照图3所示的电路图,将图4所示的实物连接成实验电路.

③在实验中,当变阻箱调到图5所示位置后,闭合开关S,电压表的示数为8.70V,此时通过电阻箱的电流为______mA.

④断开开关S,调整电阻箱的阻值,再闭合开关S,读取并记录电压表的示数.多次重复上述操作,可得到多组电压值U和通过电阻箱的电流值I,利用多次读取和计算出的数据,作出如图6所示的图线.根据图线可知,该电池的电动势E=______V,内电阻r=______Ω.

答案

(1)此管外部边长的测量值为l=22mm+7×0.05mm=22.35mm=2.235cm

管壁厚度的测量值为d=1.037mm.

(2)当电阻箱的电阻调为零时,电路中电流最大,根据闭合电路欧姆定律得

    Im=

E
R0+r

得R0=130Ω,所以保护电阻应大于130Ω,但不能太大,故选150Ω.

②本题用闭合电路欧姆定律列式求解电源内阻,电阻箱和保护电阻串连接入电源,电压表接在电阻箱两端,电路图如图所示:

③当变阻箱调到图5所示位置后,闭合开关S,电压表的示数为8.70V,根据欧姆定律得

I=

U
R+R0
=17.6mA.

④由U-I图可知,电源的电动势E=9.5V;

由闭合电路欧姆定律可知图象的斜率代表内阻:r=

△U
△I
=50.0Ω;

故答案为:(1)2.235;1.037

(2)①C;②如图所示;③17.6; ④9.5,50.0

阅读理解

阅读理解

     Fear and its companion pain are two of the most useful things that men and animals possess,if they are

properly used.If fire did not hurt when it burnt,children would play it until their hands were burnt away.

Similarly,if pain existed but fear did not,a child would burn itself again and again,because fear would not

warn the child to keep away from the fire that had burnt him or her before.A really fearless soldier-and

some do exist-is not a good soldier because he is soon killed;and a dead soldier is of no use to his army.

Fear and pain are therefore two guards without which men and animals might soon die out.

     In our first sentence we suggested that fear ought to be properly used.If,for example,you never go out

of your house because of the danger of being knocked down and killed in the street by a car,you are letting

fear rule you too much.Even in your house you are not absolutely safe:a plane may crash on your house,or

ants may eat away some of the wood in your roof,and they later may fall on you,or you may get cancer!

     The important thing is not to let fear rule you,but instead to use fear as your servant and guide.Fear will

warn you of dangers;then you have to decide what action to take.

     In many cases,you can take quick and successful action to avoid the danger.For example,you see a car

coming straight towards you;fear warns you,you jump out of the way,and all is OK.

     In some cases,however,you decide that there is nothing that you can do to avoid the danger.For

example,you cannot prevent a plane from crashing into your house,and you may not want to go and live

in a desert where there are no plants.In this case,fear has given you its warning;you have examined it and

decided on your course of action,so fear of this particular danger is no longer of any use to you,and you

have to try to overcome it.

1.Children would play with fire until their hands were burnt away if________.

A.they were given no warning beforehand

B.they didn't have any sense of pain at all

C.they had never burnt themselves

D.they felt afraid of the fire

2.A really fearless soldier________.

A.is of a little use to his army

B.is not afraid of battles at all

C.is nothing but a dead soldier

D.is easy to get killed in a battle

3.People sometimes succeed in timely avoiding dangers because________.

A.they are warned of the danger and take quick action

B.they are quick both in mind and in action

C.they are calm in face of danger

D.they have gained much experience

4.It is implied but not stated that________.

A.fear is always something helpful

B.too much fear is harmful

C.fear should be used as a servant and guide

D.fear is something strange and particular

选择题