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问题 计算题

(14分) 如图所示，足够长的两根相距为0.5m的平行光滑导轨竖直放置，导轨电阻不计，磁感应强度B为0.8T的匀强磁场的方向垂直于导轨平面。两根质量均为0.04kg、电阻均为0.5Ω的可动金属棒和都与导轨始终接触良好，导轨下端连接阻值为1Ω的电阻R，金属棒用一根细绳拉住，细绳允许承受的最大拉力为0.64N。现让棒从静止开始落下，直至细绳刚被拉断时，此过程中电阻R上产生的热量为0.2J，（g=10m/s2）求：

（1）此过程中棒和棒产生的热量和；

（2）细绳被拉断瞬时，棒的速度。

（3）细绳刚要被拉断时，棒下落的高度。

答案

（1）0.4J   0.9J  （2）1.875m/s （3）3.29m

题目分析：（1）根据Q=I2Rt，电路串并联规律得：Iab：Icd：IR=2:3:1，则Qab=2QR=0.4J，Qcd=9QR=0.9J.

ab棒被拉断瞬间有F＋mg－Tm=0。F=BIL，，，E=BLv得

v=1.875m/s。

（3）cd棒下落过程由能量守恒得 ，则h≈3.296m

判断题

凡是酸中毒血浆pH均<7.36，凡是碱中毒血浆pH均>7.44。
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单项选择题

There are two factors which determine an individual’s intelligence. The first is the sort of brain he is born with. Human brains differ considerably, some being more capable than others. But no matter how good a brain he has to begin with, an individual will have a low order of intelligence unless he has opportunities to learn. So the second factor is what happens to the individual—the sort of environment in which he is reared. If an individual is handicapped environmentally, it is likely that his brain will fail to develop and he will never attain the level of intelligence of which he is capable.

The importance of environment in determining an individual’s intelligence can be demonstrated by the case of the identical twins, Peter and Mark Being identical, the twins had identical brains at birth, and their growth processes were the same. When the twins were three months old, their parents died, and they were placed in separate foster homes. Peter was reared by parents of low intelligence in an isolated community with poor educational opportunities. Mark was reared in the home of well to do parents who had been to college. He was read to as a child, sent to good schools, and given every opportunity to be stimulated intellectually. This environmental difference continued until the twins were in their late teens, when they were given tests to measure their intelligence. Mark’s IQ was 125, twenty five points higher than the average and fully forty points higher than his identical brother. Given equal opportunities, the twins, having identical brains, would have tested at roughly the same level.

The best statement of the main idea of this passage is that （）.

A．human brains differ considerably

B．the brain a person is born with is important in determining his intelligence

C．environment is crucial in determining a person’s intelligence

D．persons having identical brains will have roughly the same intelligence
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