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南开区某中学实验室购买了一捆长度为100m左右的铜导线,苏老师知道小金同学通过实验测定这捆铜线的实际长度.小金首先测得导线横截面积为1.0mm2,查得此类导线每1m的电阻为1.7×10﹣2Ω,再利用图甲所示电路测出整捆铜导线的电阻Rx,从而确定导线的实际长度.

实验室可供使用的器材有:电流表(0~0.6A)、电压表(0~3V)、滑动变阻器R1(“5Ω,1A”)、滑动变阻器R2(25Ω,1A)、电源(电压6V)、开关、导线若干.

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Decision-making under Stress

    A new review based on a research shows that acute stress affects the way the brain considers the advantages and disadvantages, causing it to focus on pleasure and ignore the possible negative (负面的) consequences of a decision.

     The research suggests that stress may change the way people make choices in predictable ways.

    “Stress affects how people learn,” says Professor Mara Mather. “People learn better about positive than negative outcomes under stress.”

    For example, two recent studies looked at how people learned to connect images(影像) with either rewards or punishments. In one experiment, some of the participants were first stressed by having to give a speech and do difficult math problems in front of an audience; in the other, some were stressed by having to keep their hands in ice water. In both cases, the stressed participants remembered the rewarded material more accurately and the punished material less accurately than those who hadn't gone through the stress.

    This phenomenon is likely not surprising to anyone who has tried to resist eating cookies or smoking a cigarette while under stress –at those moments, only the pleasure associated with such activities comes to mind. But the findings further suggest that stress may bring about a double effect. Not only are rewarding experiences remembered better, but negative consequences are also easily recalled.

    The research also found that stress appears to affect decision-making differently in men and women. While both men and women tend to focus on rewards and less on consequences under stress, their responses to risk turn out to be different.

    Men who had been stressed by the cold-water task tended to take more risks in the experiment while women responded in the opposite way. In stressful situations in which risk-taking can pay off big, men may tend to do better, when caution weighs more, however, women will win.

    This tendency to slow down and become more cautious when decisions are risky might also help explain why women are less likely to become addicted than men: they may more often avoid making the risky choices that eventually harden into addiction.