如图所示电路，电源电压不变，R1=40Ω，R2=60Ω，通过R1的电流I1=0.3A，求：
（1）电路的总电阻
（2）电源电压
（3）R2消耗的电功率
（4）通电1min，电阻R1上产生的热量．
（1）两电阻并联，则有
=1 R
+1 R1 1 R2
则可得总电阻R=
=R1R2 R1+R2
=24Ω； 40Ω×60Ω 40Ω+60Ω
（2）由欧姆定律得：
电源电压U=U1=I1R1=0.3A×40Ω=12V；
（3）R2消耗的电功率P2=
=U2 R2
=2.4W；(12V)2 60Ω
（4）R1上产生的热量：
Q1=I2Rt=（0.3A）2×40Ω×60s=216J；
答：（1）总电阻为24Ω；（2）电源电压为12V； （3）R2消耗的功率为2.4W；
（4）R1上产生的热量为216J．
IQ（intelligence quotient智商）is a score that shows a person’s level of intelligence. People used to believe that some people are born with a high IQ and some aren’t. 32 , a study by scientist at University College London has challenged（挑战）this idea. According to an article last October on the Journal Nature, scientists are beginning to think that our IQ is not a constant（不变的）score. The scientists tested 33 healthy young people in 2004 between the ages of 12 and 16. Then they did tests again four years later, when the same people were between 16 and 20. Scientists found big changes in the IQ scores between 2008 and 2004. Some 33 and some fell 34 as many as 21 points. To test whether these scores were meaningful, the scientists compared them with results from brain scans（扫描）. They found that the IQ changes matched changes in the structure（结构）of the subjects’ brains. “A change in 20 points is a huge 35 ,” said Professor Cathy Price, who led the research. He said it could mean the difference between an average and a 36 person. The team has not found a clear cause for these changes. However, they say it is 37 that education plays a role in changing IQ. “Here we have shown that children’s 38 is likely to be still developing, ” says Price. “We have to be careful not to write off 39 performers at an early stage. In fact, their IQ may improve in a few more years. ”
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