问题 问答题

如图所示,是示波管工作原理示意图,电子经加速电压加速后垂直进入偏转电场,离开偏转电场时的偏转量为,两平行板间的距离为,电势差为,板长为,为了提高示波管的灵敏度(单位偏转电压引起的偏转量)可采取哪些措施?

答案

增大、减小U1或d均可提高示波管的灵敏度

电子经U1加速后,以的速度垂直进入偏转电场,

由动能定理得:     ①   --------2分

电子在偏转电场中运动的时间为:  ②    --------2分

电子在偏转电场中的加速度为:    ③    --------2分

电子在偏转电场中的偏转量为:  ④    --------2分

由以上四式联解得到示波管的灵敏度为:,

可见增大、减小U1或d均可提高示波管的灵敏度。   -------------2分

单项选择题 A型题
单项选择题

Weak dollar or no, $ 46,000-the price for a single year of undergraduate instruction amid the red brick of Harvard Yard-is (1) But nowadays cost is (2) barrier to entry at many of America’s best universities. Formidable financial-assistance policies have (3) fees or slashed them deeply for needy students. And last month Harvard announced a new plan designed to (4) the sticker-shock for undergraduates from middle and even upper-income families too.

Since then, other rich American universities have unveiled (5) initiatives. Yale, Harvard’s bitterest (6) , revealed its plans on January 14th. Students whose families make (7) than $60,000 a year will pay nothing at all. Families earning up to $ 200,000 a year will have to pay an average of 10% of their incomes. The university will (8) its financial- assistance budget by 43%, to over $ 80m.

Harvard will have a similar arrangement for families making up to $180,000. That makes the price of going to Harvard or Yale (9) to attending a state-run university for middle-and upper-income students. The universities will also not require any student to take out (10) to pay for their (11) , a policy introduced by Princeton in 2001 and by the University of Pennsylvania just after Harvard’s (12) . No applicant who gains admission, officials say, should feel (13) to go elsewhere because he or she can’t afford the fees.

None of that is quite as altruistic as it sounds. Harvard and Yale are, after all, now likely to lure more students away from previously (14) options, particularly state-run universities, (15) their already impressive admissions figures and reputations.

The schemes also provide a (16) for structuring university fees in which high prices for rich students help offset modest prices for poorer ones and families are less (17) on federal grants and government-backed loans.

Less wealthy private colleges whose fees are high will not be able to (18) Harvard or Yale easily. But America’s state-run universities, which have traditionally kept their fees low and stable, might well try a differentiated (19) scheme as they raise cash to compete academically with their private (20) . Indeed, the University of California system has already started to implement a sliding-fee scale.

7()

A.less

B.more

C.richer

D.poorer