如图所示电路，电源电压保持不变。闭合开关S，当滑动变阻器的滑片P从a端滑到b端的过程中，两个电阻的U－I关系图像如图所示。则下列判断正确的是( )
A．图线甲是电阻R1的“U－I”关系图像
B．电源电压为9V
C．滑动变阻器R2的最大电功率为3.6W
D．滑动变阻器R2的最大阻值为20Ω
答案：C
题目分析：A、由图象可知，图象甲电压随电流的增大而减小，甲是滑动变阻器的图象，图象乙电压随电流的增大而变大，因此图象乙是电阻R1的图象，故A错误；
B、由图象可知，电流I=0.3A时，滑动变阻器电压U甲=9V，电阻R1的电压U乙=3V，电源电压U=U甲+U乙=9V+3V=12V，故B错误；
C、由图象可知，甲是滑动变阻器的图象，最大电压时，其电流也最大，滑动变阻器R2的最大电功率为3.6W故C正确；
D、当电路电流最小为I=0.3A时，滑动变阻器接入电路的阻值最大，此时滑动变阻器电压U甲=9V，滑动变阻器最大阻值R滑=U甲 /I ="9V/" 0.3A =30Ω，故D错误；故选C．
根据短文内容，从下框A～F选项中选出能概括每段主题的最佳选项，并在答题卡上将该选项涂黑。选项中有一项为多余项。
小题1:_______________ Studies show that even in infancy(幼儿期)，music has an effect on the human brain. They have also shown that classical music can lower the blood pressure and calm an upset infant. Singing simple songs to your children is the beginning of enjoying music. 小题2:_______________ Fun little rhyming(押韵的) songs are very easy for children to learn, even with a very limited vocabulary. They will pick up the words and tune quickly and find that singing is a great way to lift their own spirits. 小题3:_______________ Some kids enjoy watching music videos. There are also music videos that are inspiring, uplifting and just plain entertaining, such as Disney’s Fantasia 2000. Kids are attracted by the images they see moving to even the smallest differences of music. Many teachers use these videos to teach about music terms. 小题4: _____________ If you make friends with an instrument you will have a friend for life. Learning to play an instrument can lead to a life-long love of music. When possible, create opportunities for your child to see a soloist(独奏者) playing her or his particular instrument for inspiration. 小题5:_____________ A long trip can be made shorter and more fun by singing. Teaching your kids some of the songs you grew up with can also be fun. There are a lot of sing-a-long CDs and downloads available(可用的)，so create a travel mix, and not only will your children enjoy the music more, they will also enjoy the fond memories to the family singing together on the trip to Grandma’s house. |