问题 实验题

小明在做“探究什么情况下磁可以生电”实验时,连接了如图所示的实验装置,PQ,MN为两平行金属导轨,水平放置,相距L,两导轨右侧连接定值电阻R,一金属棒垂直搁在两导轨上,与导轨接触良好,接触点分别为a、b。现把该装置放入一大小、方向均不变的磁场区域内,用力F拉动金属棒沿导轨匀速运动时,在闭合回路可产生感应电流,当向右拉动金属棒时,通过金属棒的电流方向为a→b,忽略所有摩擦阻力和除R外其他电阻,则:

(1)当向左拉动金属棒时,通过金属棒的电流

方向为                

(2)当向右拉动金属棒时,金属棒相当于电源给电阻R供电,则此时金属棒的       (选填“a”或“b”)端相当于电源的正极。

(3)小明在做实验时,发现F的大小与PQ、MN间距L、电阻R大小、金属棒运动速度和磁场大小等有关,在保持其他条件不变的情况下,分别改变L、R得到以下实验数据,请根据上述实验数据及你已有的数学、物理知识写出F与L、R大小关系式为:         

答案

(1)b→a;(2)b; (3)

题目分析:(1)当闭合电路的一部分导体在磁场中做切割磁感线运动时,电路中就会产生感应电流,这就是电磁感应现象.感应电流的方向与导体运动的方向和磁场的方向有关.由题意知,当向右拉动金属棒时,通过金属棒的电流方向为a→b,当向左拉动金属棒时,金属棒运动的方向改变,通过金属棒的电流方向与原来相反,为b→a;

(2)当向右拉动金属棒时,通过金属棒的电流方向为a→b,在金属棒的外部电流的方向是从b-a,因为在电源外部,电流的方向从电源的正极流向负极,所以b端相当于电源的正极;

(3)①分析F、L之间关系的数据发现L变大,F也变大,且L变为原来的n倍,F变为原来的n2倍,即F=kL2,任意代入数据:0.1N=(0.2m)2k可得出k=2.5N/m2

②分析F、R之间关系的数据发现R变大时,F变小,且R变为原来的n倍,F变为原来的n分之一倍,即,任意代入数据:,可得出:k=0.8N/Ω

综合可得出F与L、R大小关系式为:

多项选择题
单项选择题

Are teens and young adults more narcissistic (自恋的) today than in the past That’s the view of a California researcher who studies (1) people.

In her new book, The Narcissism (2) : Living in the Age of Entitlement, psychologist Jean Twenge of San Diego State University and (3) W. Keith Campbell of the University of Georgia say research shows (4) young people today have "narcissistic traits" than in (5) generations. Such traits, Twenge says, include a very. (6) and inflated sense of self, which is (7) by a preoccupation with MySpace, Facebook and YouTube.

"We’ve been on this self-admiration cultural (8) for a long time," Twenge says. (9) Twenge’s take on today’s young people isn’t universal. Studies by other researchers, including Canadian (10) Dr. Kali of the University of Western Ontario, have used the same data but found (11) results. "They put a different (12) on it," Kali says.

Twenge’s studies have found more narcissistic traits and a (13) rate of increase among college students today, but Kali found that students’ narcissism was (14) greater in 2006 than in 1976. Twenge’s most recent paper studied the same data as Kali--more than 20 000 college students from 2002 to 2007. (15) researchers used the Narcissistic Personality Inventory to measure narcissistic (16) and findings by both have been (17) in peerreviewed journals.

Twenge’s book (18) just a month after The Mirror Effect : How Celebrity Narcissism Is Seducing America, a book co-written by behavioral (19) Drew Pinsky, (20) suggested that a celebrity-obsessed culture is causing more narcissism.

5()

A.primary

B.original

C.previous

D.initial