问题 问答题

如图所示,一对平行光滑轨道放置在水平面上,两轨道间距L=0.20m,电阻R=8Ω,有一电阻r=2Ω,质量m=1kg的金属棒ab垂直平放在轨道上,轨道电阻可忽略不计,整个装置处于垂直轨道平面向下的匀强磁场中,磁感应强度B=5T,现用一外力F沿轨道方向拉金属棒,使之做初速为零的匀加速直线运动,加速度a=1m/s2.试求:

(1)2s内通过电阻R的电量Q大小;

(2)外力F与时间t的关系;

(3)求当t=5s时电阻R上的电功率PR和F的功率PF的大小,并用能量守恒的观点说明两者为何不相等?

答案

(1)t=2s时,金属棒通过的位移为 x=

1
2
at2=2m

回路磁通量的变化量为△Φ=BxL=2Wb

感应电流为 I=

BLv
(r+R)

则电量  Q=I△t=

△φ
R+r

代入解得  Q=0.2C

(2)安培力表达式为FA=BIL=B

BLv
R+r
L=
B2L2at
R+r

代入解得,FA=

B2L2at
(r+R)
=0.1t

根据牛顿第二定律得  F-FA=ma

则得  F=1+0.1t   

(3)当t=5s时,I=

BLat
(R+r)
=0.5A,

则PR=I2R=2W,

因F=1.5N,v=at=5m/s,则PF=Fv=7.5W.  

外力F的功率转化为用于导体棒动能增加的机械功率和电阻上的发热功率,而发热功率还包括电阻R上的功率和导体棒电阻r的功率,所以有PR<PF.  

答:(1)2s内通过电阻R的电量Q大小是0.2C;

(2)外力F与时间t的关系是F=1+0.1t;

(3)当t=5s时电阻R上的电功率PR是2W,F的功率PF的大小是7.5W,外力F的功率转化为用于导体棒动能增加的机械功率和电阻上的发热功率,而发热功率还包括电阻R上的功率和导体棒电阻r的功率,所以有PR<PF

阅读理解

After a 15-year ban on the sale of fireworks in Beijing, Kang Guoliang, 51, was able to start his old trade again yesterday.

As a salesman in Xinhui store in Dongcheng District, he is happy about the increasing number of buying fireworks wrapped in red paper — a color standing for happiness and good luck.

“Fireworks are available for the first time in town for more than a decade,” Kang said happily.“People will buy them.”

The store has 300 boxes of fireworks piled up and is open 24 hours at the moment.Residents are buying the fireworks and firecrackers for the upcoming Spring Festival, which falls on February 18.

Citywide, 2,116 registered stores and retailers, 585 in the centre and 1,600 on the outskirts are trading fireworks in the Chinese capital.Sales of fireworks within the Fifth Ring Road started yesterday and will last until March 4, Xinhua reported.Among the stores, 117 stores are permitted to operate 24 hours.About 600,000 boxes of firecrackers worth more than 100 million yuan have been carried to registered stores.

The Chinese capital began a ban in 1993 to reduce fireworks-related deaths and fires during the holiday season.Last September the ban was removed in response to residents’ fondness for fireworks when celebrating Spring Festival.

According to new rules, Beijing residents are allowed to set off fireworks within the Fifth Ring Road all day on Lunar New Year’s Eve and Lunar New Year’s day, from 7 a.m.to midnight every day from February 19 to March 4.

“Spring Festival without fireworks is not Spring Festival,” said Sheng Hefei, who was buying fireworks in the store.“It was fun to light the firecrackers when I was little,” he said.“The sound and view of fireworks make a real holiday.” However, not all residents welcome the return of firecrackers because of injuries, pollution and noise.“My child is scared of the sudden sound of firecrackers, and it is annoying to hear it all night long,” complained Lu Jun, a local resident.

(China Daily02/11/2008)

小题1:The passage is likely to belong to a(n)__________?

A.description

B.argument

C.advertisement

D.news report小题2:What’s the main idea of the passage?

A.Beijing fireworks sales start after a 15-year ban.

B.People go to buy fireworks and firecrackers every where in Beijing.

C.Beijing residents all welcome the return of firecrackers.

D.Many stores began to sell fireworks and firecrackers.小题3:We can learn from the passage that ____________.

A.every Spring Festival falls on February 18

B.Beijing residents can set off fireworks everywhere .

C.fireworks are allowed to be sold because people like them.

D.from February 19 to March 4, Beijing residents can set off fireworks twenty four hours every day.小题4:What is the writer’s attitude towards the sales of fireworks in Beijing?

A.Negative

B.Not mentioned

C.Positive

D.Satisfactory

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