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问题 填空题

如图所示电路．电源电压恒定不变，滑动变阻器的最大阻值R1为20Ω，定值电阻R2、R3的阻值分别是10Ω、20Ω，两个电流表的电阻不计．闭合开关S，当滑动变阻器的滑片P置于R1中间位置时，电流表A1和A2的示数之比为______；当滑动变阻器的滑片P置于R1最右端时，电流表A1和A2的示数之比为______．

答案

当滑片P置于R1中间位置时，R1=10Ω；

并联部分的电阻为R并=

R1R2
R1+R2
=
10Ω×10Ω
10Ω+10Ω
=5Ω，

电路中的总电阻为R=R并+R3=5Ω+20Ω=25Ω，

电路的总电流为I=

U
R
=
U
25
，

并联部分的电压为U并=IR并=

U
25
×5Ω=
U
5
，

R1支路的电流为I1=

U并
R1
=
U
5
10Ω
=
U
50
，

所以两电流表的示数之比为I1：I=

U
50
：
U
25
=1：2；

当滑片P置于R1最右端时，R1，R2都短路，电路为R3的简单电路，两电流表测电路中的电流；

根据串联电路各处的电流相等可知，两电流表的示数相等，即1：1．

故答案为：1：2，1：1．

单项选择题

下列哪一种疾病的CSF丙种球蛋白升高（）

A.脑脓肿

B.多发性硬化

C.脑血栓形成

D.蛛网膜下隙出血

E.帕金森病
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单项选择题

I’ve recently turned fifty, which is young for a tree, midlife for an elephant, and ancient for a sportsman. Fifty is a nice number for the states in the US or for a national speed limit but it is not a number that I was prepared to have hung on me. Fifty is supposed to be my father’s age, but now I am stuck with this number and everything it means.
A few days ago, a friend tried to cheer me up by saying, "Fifty is what forty used to be. "He had made an inspirational point: Am I over the hill People keep telling me that the hill has been moved, and I keep telling them that the high-jump bar has dropped from the six feet I once easily cleared to the four feet that is impossible for me now.
"Your are not getting older, you are getting better," says Dr. Joyce Brothers. This, however, is the kind of doctor who inspires a second opinion.
And so, as I approach the day when I cannot even jump over the tennis net, I am moved to share some thoughts on aging with you. I am moved to show how aging feels to me physically and mentally. Getting older, of course, is obviously a better change than the one that brings you eulogies (颂词，悼词). In fact, a poet named Robert Browning considered it the best change of all:
Grow old along with me!
The best is yet to me.
Whether or not Browning was right, most of my first fifty years have been golden ones, so I will settle for what is ahead being as good as what has gone by. I find myself moving toward what is ahead with a curious blend (混合) of both fighting and accepting my aging, hoping that the philosopher(哲学家) was right when he said, "Old is always fifteen years from now. \

The author seems to tell us in Paragraph I that ______.

A．time alone will tell

B．time goes by quickly

C．time will show what is right

D．time makes one forget the past
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