题干

如图为一种握力计和它的工作原理示意图.电源电压恒定不变,握力显示表是由电流表改装而成,定值电阻R2起保护电路作用,其电阻值为5Ω,弹簧一端的滑片P在电阻R1的最上端时不施加力,移动到最下端时施加的力最大(弹簧的电阻不计).该握力计测量范围为0~300N,电路中电流变化范围0.1~0.6A.则下列判断正确的是(   )

A:握力300N时电路中电流为0.6A

B:握力计工作电路的电源电压为3V

C:R1连入电路阻值变化范围0﹣30Ω

D:R2消耗的最小功率为0.5W

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答案(点此获取答案解析)

B

同类题3

Are you shy? If you are, you are not alone. In fact, close to 50 percent of people are shy. Almost 80 percent of people feel shy at some point in their lives. These days, shyness is becoming more and more common. Now, scientists are trying to understand shyness. They have some interesting ideas about why people are shy.

    Is it possible to be born shy? Many scientists say yes. They say 15 to 20 percent of babies behave shyly. These babies are a little quieter and more watchful than other babies. Interestingly, these shy babies usually have shy parents. As a result, scientists think that some shyness is genetic.
    Family size might causepeople to be shy as well. Scientists at Harvard University studied shy children. They found that 66 percent of them had older brothers and sisters. As a result, they became shy. At the same time, children with no brothers and sisters may be shy as well. Growing up alone, they often play by themselves. They are not able to learn the same social skills as children from big families.

    You may also be shy because of where you were born. When scientists studied shyness in different countries. They found surprising differences. In Japan, most people said they were shy. But in Israel, only one of three people said so. What explains the difference? One scientist says the Japanese and Israelis have different opinions of failure. In Japan, when people do not succeed, they feel bad about themselves. They blame(责备)themselves for their failure. In Israel, the opposite is true. Israelis often blame failure on outside reasons, such as family, teachers, friends, or bad luck. In Israel, freedom of opinion and risk taking are strongly supported. This may be why Israelis worry less about failure and are less shy.

    For shy people, it can be difficult to make friends, speak in class, and even get a good job. But scientists say you can get over your shyness. They suggest trying new things and practicing conversation. And don't forget—if you are shy, you are not the only one.