在研究微型电动机的性能时，应用如图所示的实验电路，图中电表均为理想电表．当调节滑动变阻器R并控制电动机停止转动时，电流表和电压表的示数分别为0.50A和2.0V．重新调节R并使电动机恢复正常运转，此时电流表和电压表的示数分别为2.0A和24.0V．则这台电动机正常运转时输出功率为（ ）
A．32W
B．40W
C．44W
D．48W
当电动机停止转动时，根据欧姆定律得：
电动机的电阻：R=
=U1 I1
Ω=4Ω；2 0.5
当电动机正常运转时：
电动机的总功率：P=U2I2=24V×2A=48W；
电动机自身电阻的发热功率：PR=I22R=（2A）2×4Ω=16W；
电动机正常运转时的输出功率是：P输出=P-PR=48W-16W=32W．故A正确，BCD错误．
故选：A．
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| You are near the front line of a battle. Around you shells are exploding; people are shooting from a house behind you. What are you doing there? You aren't a soldier. You aren't 1 carrying a gun. You're standing in front of a 2 and you're telling the TV 3 what is happening. It's all in a day's work for a war reporter, and it can be very 4 . In the first two years of the 5 in former Yugoslavia (前南斯拉夫), 28 reporters and photographers were killed. Hundreds more were 6 .What kind of people put themselves in danger to 7 pictures to our TV screens and 8 to our newspapers? Why do they do it? "I think it's every young journalist's 9 to be a foreign reporter," says Michael Nicholson, "that's 10 you find the excitement.So when the first opportunity comes,you take it 11 it is a war." But there are moments of 12 . Jeremy Bowen says, "Yes,when you're lying on the ground and bullets (子弹) are flying 13 your ears,you think: 'What am I doing here? I'm not going to do this again.' But that feeling 14 after a while and when the next war starts, you'll be 15 ." "None of us believes that we're going to 16 ," adds Michael.But he always 17 a lucky charm (护身符) with him. It was given to him by his wife for his first war. It's a card which says "Take care of yourself." Does he ever think about dying? "Oh, 18 ,and every time it happens you look to the sky and say to God, 'If you get me out of this, I 19 I'll never do it again.' You can almost hear God 20 ,because you know he doesn't believe you." | ||||
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