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问题 计算题

有两只定值电阻R1与R2，每只允许通过的最大电流都是0.25A，它们的阻值分别为R1=20 Ω，R2=40 Ω，试分别求出串、并联两种情况中允许加在电路两端的最大电压．

答案

（1）串联时，电流相同，允许通过两电阻的电流最大为0.25A，

则两电阻两端电压U最=I（R1+R2）=0.25A×（20Ω+40Ω）=15V；

（2）两电阻并联，由于电压相同，要保证元件不被烧坏，要取电压较小的，由于允许通过两电阻的最大电流为0.25A，则电阻Rl两端电压最大Ul=I1Rl=0.25A×20Ω=5V；

电阻R2两端电压最大U2=I2R2=0.25A ×40Ω=10V；

因此，两电阻并联时，电路两端的最大电压U=5V．

单项选择题

“察举”制，是( )实行的一种自上而下推选人才为官的制度。

A．西周

B．战国

C．秦代

D．汉代
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填空题

Exercising to music pumps up brain power


If music makes you smarter, and exercise helps brain function, can exercising to music really boost brainpower Some researchers said it can. Volunteers who listened to Vivaldi’s "Four Seasons" while working out on a treadmill did much better on a test of (1) ability than when they exercised without music, a team (2) Ohio State University found.
"Evidence suggests that exercise improves the (3) performance of people with coronary artery (4) ," said psychologist Charles Emery, who led the study.
"And listening to music is (5) to enhance brainpower. We wanted to put the two results together," Emery (6) in a statement.
Writing in the latest issue of the journal Heart & Lung, Emery and (7) said they studied 33 men and women taking (8) in a cardiac rehabilitation program after having bypass (9) , angioplasty or other procedures to treat clogged arteries.
The (10) said they felt better emotionally and mentally after working out (11) or without the music. But their improvement on the (12) fluency test doubled after listening to music on the treadmills.
"Exercise seems to (13) positive changes in the nervous system, and these changes may have a (14) effect on cognitive ability," Emery said.
"Listening to music may (15) cognitive function through different pathways in the brain. The combination of music (16) exercise may stimulate and increase cognitive arousal (17) helping to organize cognitive output."
Emery said he now wanted to (18) people using music of their own choice. "We used the ’Four Seasons’ because of its (19) tempo and positive effects on medical patients in previous (20) ," Emery said. "But given the range of music preferences among patients, it’s especially important to evaluate the influence of other types of music on cognitive outcomes.\
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