问题 计算题

如图所示,L上标有“6 V  3 W”字样,电流表量程为0~0.6 A,电压表量程为0~15 V,变阻器R的最大电阻为100 Ω。只闭合S1,滑片置于a点时,变阻器连入电路中的电阻为Ra,电流表示数为Ia。只闭合S2,移动滑片,变阻器两端电压与其连入电路的电阻关系如图所示;当滑片置于b点时,电压表示数U= 8 V,电流表示数为Ib。已知Ra∶R= 12∶5,Ia∶I= 3∶5。(灯丝电阻不随温度变化)

求:(1)小灯泡的电阻;

(2)定值电阻R0和电源电压;

(3)在电表的示数不超过量程,灯泡两端的电压不超过额定值的情况下,只闭合S1时,变阻器连入电路的最小电阻为多少;只闭合S2时,电路消耗的最小功率为多少。

答案

(1)因为,则小灯泡的电阻: 

(2)只闭合S2,R与R0串联,滑片P置于b点时, U= 8 V,由图可知电阻Rb =16 Ω,则电流

电源电压:U = Ib (Rb+ R0) = 0.5 A×(16 Ω+ R0)      ①

只闭合S1,R与L串联,当滑片P置于a点,由于Ia∶I= 3∶5,则I=0.3 A

电源电压:U = Ia (Ra+ RL) = 0.3 A×(Ra+ 12 Ω)      ②

已知Ra∶R= 12∶5,再由①②式,可得: R= 20 Ω,U = 18 V

(3)只闭合S1,电流表的量程大于灯的额定电流,则电路中最大电流等于灯的额定电流0.5 A。

得变阻器接入电路的最小电阻R最小= 24 Ω  

只闭合S2,变阻器两端电压为15 V时,连入电路中的电阻最大

由于,则变阻器接入电路的最大电阻R最大= 100 Ω)

电路消耗的最小功率:

(1)已知灯泡的额定电压和额定功率,利用公式得到灯泡的电阻;

(2)只闭合S2,R与R0串联;只闭合S1,R与L串联。根据欧姆定律分别列出公式,联立求解即可;

(3)只闭合S1,R与L串联,保证灯泡不被烧坏,可得最大电流,进一步求的滑动变阻器最小电阻;

只闭合S2,R与R0串联,滑动变阻器接入电阻最大时,电路消耗的最小功率,利用公式P=UI得到电路最小功率

点评:此题考查了电路连接关系的识别、串并联电路的特点、欧姆定律以及电功率的计算和根据图象获得有用信息,是电学典型的综合题,考查全面,难度较大,需要熟练掌握电路特点、计算公式和规律.

阅读理解

It is often said that politeness costs nothing.In fact,it seems that a little more courtesy could save businesses£5 billion every year.

Frequently hearing the phrase “thank you” or “well done” means the same to staff as a modest pay rise,researchers say.

Praise and encouragement also makes employees more likely to work hard and stay in their jobs,saving on the cost of finding replacements.

A third of 1,000 workers surveyed by consulting firm White Water Strategies said they did not get thanked at all when they did well—and a further third said they were not thanked enough.

In both cases,staff said they felt undervalued,meaning they were less likely to exert themselves and were more likely to look for employment elsewhere.

The net result is around £5.2 billion in lost productivity from employees who would raise their game if they felt more appreciated,White Water claimed According to the company,praising staff has the same motivational kick as a 1 per cent pay rise—and works out much cheaper for bosses.

Three out of four employees said that regular acknowledgement by their boss was important to them,but only a quarter said they were actually given as much praise as they felt they needed.

The survey found that those in blue-collar and manual jobs were less likely to be given any recognition for doing well.

In regional terms,Scottish staff felt most undervalued.Four out of ten workers said they were never thanked and eight out of ten said they would like more praise.However,workers in the North-East are less impressed by being buttered up by the boss,as only 69 per cent said they felt the need to be told “well done” regularly.

Older employees and women need the most reassurance,according to psychologist Averil Leimon,a director of White Water Strategies.She said that words of praise did more than create a pleasant place to work—they could even boost profits.

51.The second paragraph means that_____________.

A.employees ask for high pay instead of hearing “thank you” or “well done”

B.bosses always think highly of their employees’ work

C.bosses’ praise and encouragement are important to workers

D.bosses should praise their workers from time to time

52.Why praise and encouragement are needed according to the passage?

A.Most bosses feel it necessary

B.Most workers didn’t work hard enough.

C.Most bosses can make money from praise and encouragement

D.Most workers will work harder and stay in their jobs from praise and encouragement.

53.According to tiffs passage,the majority of staff felt______________.

A.there were thanked enough        B.they were undervalued

C.they got satisfactory pay        D.they didn’t need encouragement

54.Which of the following is TRUE according to the passage?

A.Blue-collar and manual workers need more regular acknowledgement by their bosses.

B.A third of the workers surveyed by White Water Strategies never got thanked at all when

they did well.

C.Old employees and women do not need to be appreciated as much as the young.

D.Fewer than 20% of Scotiish felt that they never got thanked.

55.The main idea of the passage is that___________

A.praise and encouragement may help employees work better

B.workers are always demanding more praise and encouragement

C.bosses in Scotland usually praise and encourage their staff enough

D.if undervalued employees will certainly look for employment elsewhere

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