有一横截面积为S的粗细均匀的铝导线，流经其中的电流为I，电子的定向移动速率为v，设单位长度中有n个自由电子，电子的电荷量为-e（e为元电荷量），在t时间内，通过导线横截面的自由电子数可表示为（ ）
A．nvSt
B．nvt
C．It e
D．It ne
A、B在t时间内，通过导线横截面的自由电子的电量为q=nvte，自由电子数目为n=
=nvt．故A错误，B正确．q e
C、D通过导线的电流为I，在t时间内，通过导线横截面的自由电子的电量为q=It，自由电子数目为n=
=q e
．故C正确，D错误．lt e
故选BC
| We can make mistakes at any age. Some mistakes we make are about money. But most mistakes are about people. One might say, "Did Jerry care when I broke up with Helen " "When I got that great job, did Jim really feel good about it, as a friend " Or "Did he envy my luck " "And Paul — why didn’t I pick up that he was friendly just because I had a car " When we look back, doubts like these can make us feel bad. But when we look back, it’s too late. Why are we wrong about our friends or our enemies Sometimes what people say hides their real meaning. And if we don’t really listen, we miss the feeling behind the words. Suppose someone says to you, "You’re a lucky dog." Is he really on your side If he says, "You’re a lucky guy" or "You’re a lucky gal," that’s being friendly. But if he says, "You are a lucky dog." There’s a bit of envy in those words. Maybe he doesn’t see it himself. But bringing in the dog he put you down a little. What he may be saying is that he doesn’t think you deserve your luck. "Just think of all the things you have to be thanking for" is another noise that says one thing and means another. It could mean that the speaker is trying to get you to see your problem as part of your life as a whole. But is he Wrapped up in this phrase is the thought that your problem isn’t important. It’s telling you to think of all the starving people in the world when you have not got a date for Saturday night. How can you tell the real meaning behind someone’s words One way is to take a good look at the person talking. Do his words fit the way he looks Does what he says square with his tone of voice, his posture, the look in his eyes Stop and think. The minute you spend thinking about the real meaning of what people say to you may save another mistake. Therefore, when you hear someone saying, please try to know what he really means. Don’t just listen to what he says with your ears but feel the words he uses with your head. In this way, you may make less mistakes. |
In order to know what he says, the writer wants us to ______ his words.
A．feel
B．repeat
C．listen to
D．ignore