理想变压器与电阻R及交流电流、电压表V和A按图示方式连接，已知变压器原副线圈的匝数比为n1：n2=10：1，电阻R=11Ω，原线圈两端输入电压U随时间变化的图象如图所示，下列说法中正确的是（ ）
A．V表的读数为220V
B．通过R的电流为2A
C．A表的读数为2A
D．变压器的输入功率为44W
由原线圈两端输入电压U随时间变化的图象可知，U1=220V，T=0.02s
A、根据原副线圈的电压比等于匝数之比，可知U2=
U1=22V，所以电压表的示数为22V，故A错误；1 10
B、根据欧姆定律得：I=
=2A，故B正确，C错误；U2 R
C、副线圈功率P2=I2U2=44W，所以变压器的输入功率P1=P2=44W，故D正确．
故选BD
| 阅读理解。 | |
| Most people think of racing when they see greyhounds(灰狗) and believe they need lots of exercise. They can actually be quite lazy! Greyhounds are good at fast races but not long-distance run- ning. They do need regular exercise but they like to run for a short burst and then get back on the bed or a comfortable seat. Another misunderstanding is that greyhounds must be aggressive (好斗的) because they are big in size. In fact greyhounds love people and are gentle with children. Greyhounds can live for 12～14 years but usually only race for two or three years, and after that they make great pets. They don't need a lot of space,don't make a lot of noise,and don't eat a lot for their size. Normally, greyhounds can be as tall as 90cm. There is, however, a small-sized greyhound, which stands only 33cm. Greyhounds come in a variety of colors. Grey and yellowish-brown are the most common. Others include black, white, blue, red and brown or a mix of these. Greyhounds have smooth body coats ,low body fat and are very healthy. Because they're slim (苗条的)they don't have the leg problems like other dogs with the same height. But they do feel the cold ,especially since they would much rather be at home in bed than walking around outside. | |
| 1. The text is written mainly ________. | |
| A. to tell people how to raise greyhounds B. to let people know more about greyhounds C. to explain why greyhounds are aggressive D. to describe greyhounds of different colors | |
| 2. It can be inferred that greyhounds _______. | |
| A. love big dog houses B. like staying in bed all day C. make the best guard dogs D. need some exercise outdoors | |
| 3. Why does the author say that greyhounds.make great pets7 | |
| A. They are big in size. B. They live a very long life. C. They can run races for some time. D. They are quiet and easy to look after. | |
| 4. If you keep a pet greyhound,it is important _______. | |
| A. to keep it slim B. to keep it warm C. to take special care of its legs D. to take it to animal doctors regularly |