问题 问答题

一辆电动自行车(如图)的铭牌上给出了如下的技术参数表.

规格后轮驱动直流永磁轻电机
车型26M电动自行车额定输出功率120W
整车质量30kg额定转速240r/min
最大载重120kg额定电压40V
续行里程大于40km额定电流4A
(1)此车的电动机在额定电压下正常工作时,消耗的电功率是多大?

(2)此电动机的内阻是多少?电动机正常工作时的效率(输出功率与输入功率之比)是多少?

(3)总质量70kg(包括人及所带物品)的人骑此车沿平直公路行驶,若行驶过程中所受的阻力为车和人总重的0.02倍,且只靠电动机提供动力,g取10m/s2,求此人骑车行驶的最大速度.

答案

(1)电动机在额定电压下正常工作时消耗的电功率为:

P=UI=40×4W=160W,

(2)根据能量守恒定律,有:

P=P+P

即:UI=I2r+120

代入数据,有:

40×4=42×r+120

解得:r=2.5Ω

电动机正常工作时的效率:

η=

P
P
×100%=
120
160
×100%=75%

(3)自行车阻力:

f=0.02mg=0.02×(30+70)×10=20N

匀速行驶,牵引力与阻力平衡,为:

F=f=20N

故输出功率为:

P=Fv=fv

解得:

v=

P
f
=
120
20
=6m/s

答:(1)此车的电动机在额定电压下正常工作时,消耗的电功率是160W;

(2)此电动机的内阻是2.5Ω;电动机正常工作时的效率为75%;

(3)此人骑车行驶的最大速度为6m/s.

阅读理解

阅读理解

     It is hardly surprising that clothing manufacturers (生产商)follow certain uniform standards for various

features(特征)of clothes.What seems strange,however,is that the standard adopted for women is the

opposite of the one for men.Take a look at the way your clothes button.Men's clothes tend to button

from the right, and women's from the left.Considering most of the world's population-men and women-are

righthanded,the men's standard would appear to make more sense for women.So why do women's clothes

button from the left?

      History really seems to matter here.Buttons first appeared only on the clothes of the rich in the 17th

entury, when rich women were dressed by servants.For the mostly righthanded servants,having women's

shirts button from the  left would be easier.On the other hand, having men's shirts button from the right

made sense, too.Most men dressed themselves,and a sword drawn from the left with the right hand would

be less likely to get caught in the shirt.

      Today women are seldom dressed by servants,but buttoning from the left is still the standard for them.

Is it interesting? Actually, a standard, once set, resists change.At a time when all women's shirts buttoned

from the left,it would have been risky for any single manufacturer to offer women's shirts that buttoned

from the right.After all, women had grown so used to shirts which buttoned from the left and would have

to develop new habits and skills to switch.Besides, some women might have found it socially awkward to

appear in public wearing shirts that buttoned from the right,since anyone who noticed that would believe

they were wearing men's shirts.

1.What is surprising about the standard of the clothing industry?

A.It has been followed by the industry for over 400 years.

B.It is different for men's clothing and women's.

C.It works better with men than with women.

D.It fails to consider righthanded people.

2.What do we know about the rich men in the 17th century?

A.They tended to wear clothes without buttons.

B.They were interested in historical matters.

C.They were mostly dressed by servants.

D.They drew their swords from the left.

3.Women's clothes still button from the left today because____.    

A.adopting men's style is improper for women

B.manufacturers should follow standards

C.modern women dress themselves

D.customs are hard to change

4.The passage is mainly developed by________.

A.analyzing causes

B.making comparisons

C.examining differences

D.following the time order

判断题