问题 问答题

请你仔细阅读下列资料:蓄电池充电后电压变化很小,可以认为恒定不变.除电压外,容量是电池的另一个重要指标.如蓄电池容量为“15A•h”,它表示:若该蓄电池以1A的电流为用电器供电,可以工作15h,即若用字母Q表示电池容量,有Q=It.

整车净重40kg最高车速20km/h
蓄电池的电压48V轮胎能承受

的最大压强

2×105Pa
蓄电池的容量10A•h行驶噪声62dB
电动自行车因轻便、无污染而成为人们喜爱的代步工具,下表是某种型号的电动自行车的相关数据.

若该电动自行车在行驶过程中轮胎与地面接触的总面积为1.0×10-2m2,一个质量为60kg的人骑该电动自行车在公路上匀速行驶,电动自行车工作电流为5A.求:

(1)电动自行车行驶时对地面的压强;

(2)一次充电后,电动自行车最多可以行驶的时间;

(3)若电动自行车受到的阻力为车人总重的0.04倍,电能转化为机械能的效率为80%,电动自行车的行驶速度是多少?

答案

(1)电动自行车对地面的压力:

F=G=(m+m)g=(40kg+60kg)×9.8N/kg=980N,

对地面的压强:

p=

F
S
=
980N
1.0×10-2m2
=9.8×104Pa;

(2)∵Q=It,

∴行驶时间为:

t=

Q
I
=
10A•h
5A
=2h;

(3)车受到的阻力为:

f=0.04G=0.04×980N=39.2N,

∵η=

W机械
W
×100%=
Fs
UIt
×100%=
Fv
UI
×100%,

∴行驶速度为:

v=

UIη
f
=
48V×5A×80%
40N
=4.8m/s.

答:(1)电动自行车行驶时对地面的压强为9.8×104Pa;

(2)一次充电后,电动自行车最多可以行驶的时间为2h;

(3)电动自行车的行驶速度是4.8m/s.

完形填空

It was only October, but my students were already talking about Christmas plans. As each day passed, everyone became more excited waiting for the final school bell. As soon as everyone heard the     , they would run for their coats and go home, everyone except David.

David was a small boy in old clothes. I had often      what kind of home life David had, and what kind of mother could send her son to school dressed so improperly for the cold winter months, without a coat, boots, or gloves, But something made David     .I can still remember he was always     a smile and willing to help, He always stayed after school to straighten chairs and mop the floor. We never talked much. He would just simply smile and ask what else he could do, then thank me for letting him stay and slowly head for home.

Weeks passed and the     over the coming Christmas grew into restlessness(烦躁)until the last day of     before the holiday break. I smiled in relief(安慰)as the last of them hurried out the door. Turning around I saw David     standing by my desk.

“I have something for you, " he said and pulled from behind his back a small box, Handing it to me, he said anxiously, "Open it." I took the box from him, thanked him and slowly unwrapped it. I lifted the lid and to my     .I saw nothing. I looked at David's smiling face and back into the box and said, "The box is nice, David, but it's     .”

"Oh no it isn't, " said David. "It's full of love. My mum told me before she died that love was something you couldn't see or touch unless you know it's there."

Tears filled my eyes as I looked at the proud dirty face that I had rarely given attention to. After that Christmas, David and I became good friends and I never forgot the meaning     the little empty box set on my desk.

小题1:

A.warning

B.calling

C.ringing

D.shouting小题2:

A.admired

B.wondered

C.realized

D.learned小题3:

A.special

B.kind

C.popular

D.funny小题4:

A.bringing

B.enjoying

C.wearing

D.sharing小题5:

A.argument

B.arrangement

C.movement

D.excitement小题6:

A.school

B.year

C.attention

D.program小题7:

A.weakly

B.sadly

C.quietly

D.helplessly小题8:

A.anger

B.joy

C.appreciation

D.surprise小题9:

A.cheap

B.empty

C.useless

D.improper小题10:

A.from

B.behind

C.over

D.towards

单项选择题