问题 问答题

小明家新买了一条标有“220V,50W”的电热毯,他想用电流表和电压表测量电热毯的电阻值,于是连接了下图所示的电路,其中R为待测电热毯的电阻,电源两端的电压为6V,滑动变阻器的最大阻值为20Ω.但是,小明在测量过程中发现,无论怎样调整滑动变阻器,电表的示数都几乎没有变化,且查得各元件完好,电路连接无误.请分析产生这一现象的原因是什么?如果仍然使用这些器材,怎样连接电路才能有效地改变电热毯两端的电压?在小明电路图的右侧画出你的电路图.

答案

(1)现象产生的原因:通过计算可知电热毯的电阻约为R=

U2
P
=
(220V)2
50W
=968Ω,滑动变阻器电阻最大为20Ω,电热毯的电阻远大于滑动变阻器的电阻,电热毯两端的电压远大于滑动变阻器两端的电压,所以无论怎样调整滑动变阻器,电表的示数都几乎没有变化.

(2)因为并联电路两端的电压相等,当滑片在左端时,电压表的示数等于电源电压6V,当滑动变阻器的滑片在右端时,电热毯被短路,电压表示数为O,因此使电热毯与滑动变阻器并联,改进的电路图是:

单项选择题
阅读理解

Birthdays often involve surprises. But this year's surprise on the birthday of the great British playwright William Shakespeare is surely one of the most dramatic.

On April 22, one day before his 441st birthday anniversary, experts discovered that one of the most recognizable portraits of William Shakespeare is a fake. This means that we no longer have a good idea of what Shakespeare looked like. "It's very possible that many pictures of Shakespeare might be unreliable because many of them are copies of this one," said an expert from Britain's National Portrait Gallery.

  The discovery comes after four months of testing using X-rays, ultraviolet light, microphotography and paint samples. The experts from the gallery say the image —commonly known as the "Flower portrait"— was actually painted in the 1800s, about two centuries after Shakespeare's death. The art experts who work at the gallery say they also used modern chemistry technology to check the paint on the picture. These checks found traces of paint dating from about 1814. Shakespeare died in 1616, and the date that appears on the portrait is 1609.

 "We now think the portrait dates back to around 1818 to 1840. This was when there was a renewed interest in Shakespeare's plays," Tarnya Cooper, the gallery's curator (馆长), told the Associated President.

  The fake picture has often been used as a cover for collections of his plays. It is called the Flower portrait because one of its owners, Desmond Flower, gave it to the Royal Shakespeare Company.

  "There have always been questions about the painting," said David Howells, curator for the Royal Shakespeare Company. "Now we know the truth, we can put the image in its proper place in the history of Shakespearean portraiture."

Two other images of Shakespeare, are also being studied as part of the investigation (调查) and the results will come out later this month._________________

小题1:What makes the birthday of Shakespeare dramatic this year?

A.It was found that he painted a portrait in 1814 instead of in 1609.

B.The Flower portrait has been found to be a fake.

C.Three portraits of Shakespeare are being tested to identify a real one.

D.It was found that there was a renewed interest in Shakespeare's plays around 1818 to 1840.小题2:Which statement is True according to the passage?

A.Portraits of Shakespeare are all unreliable.

B."Flower portrait " was a portrait of Shakespeare given to Desmond Flower.

C.1814 might be when the portrait was drawn.

D.The Flower portrait is not often used as a cover for Shakespeare's play.小题3:How many methods were used to test the portrait?

A.Not mentioned.

B.Two

C.Three

D.At least four.小题4:The best title for this passage is _________.

A.Birthdays often involve surprises 

B.The surprise on the 441st birthday of Shakespeare

C.One portrait of Shakespeare is a fake

D.How can we know Shakespeare's appearance?小题5:Which is the best sentence to fill in the blank in the last paragraph?

A.Soon we'll know which portrait is reliable.

B.Maybe we cannot find a real portrait of Shakespeare.

C.If the two portraits are found to be false, they will test more.

D.For now what Shakespeare really looked like will remain a mystery.