如图，为理想变压器原线圈所接电源电压波形。原、副线圈匝数比n1: n2=10:1，串联在原线圈的电路中电流表的示数为1A。下列说法正确的是
A．变压器输出两端所接电压表的示数为20
V
B．变压器输出功率为200 W
C．变压器输出的交流电的频率为50Hz
D．若n1=100匝，则变压器输出端穿过每匝线圈的磁通量变化率的最大值为2
W b/s
CD
根据
n1: n2,U
为20
v,电压表读数为20v,A错。
变压器输出功率等于输入功率，P=UI=200W,B错；
由图像可以看出周期为0.02s,频率为50赫兹。C对。
由感应电动势
，所以当磁通量变化率最大时，感应电动势最大；所以磁通量变化率的最大值为2 W b/s。D对。
| 阅读理解。 |
| Ronald Reagan was the only actor to become president of the United States, but other movie stars have also been successful in public office. One of them was Shirley Temple, who is remembered throughout the world as a little girl. She danced and sang in many movies. Shirley began to make movies at the age of five, and she was only nine years old when she played the role of Heidi. In almost every part of the world where movies are seen on television, children and their parents still enjoy Heidi and the other movies of Shirley Temple. Although she has not appeared in a film for over thirty years, her name is still familiar to old and young alike. When Shirley Temple was twenty-one years old, she was already retired from her career as an actress. Twenty years later, many people still thought her as a little girl that Americans had always loved in the movies, but others knew her as an intelligent woman with knowledge of world problems. One of the people who recognized these important abilities was President Richard M. Nixon. In 1969, he appointed (委派) her as a representative (代表) to the United Nations, and this marked the beginning of a new career for the former child star of the movies. One of Shirley's most popular dancing and singing partners was George Murphy, who was twenty-six years older than Shirley. He stopped making movies at about the same time as his young partner. In 1969, he was elected United States senator (参议员) from California, and he served in this office for six years. |
| 1. Some of the American actors or actresses like Ronald Reagan, Shirley Temple and George Murphy . |
[ ] |
| A. were later elected senators B. later became more successful in the movie circle (电影界) C. were later active in politics D. later became presidents of the United States |
| 2. Shirley Temple spent ______ years on her career as an actress. |
[ ] |
| A. 21 B. 9 C. 11 D. 16 |
| 3. Quite a lot of people throughout the world as well as Americans remember Shirley Temple all the time . |
[ ] |
| A. as a little girl B. as a pretty actress C. as an attractive woman D. as a young politician |
| 4. Why did Richard M. Nixon appoint Shirley Temple as representative to the United Nations? |
[ ] |
| A. Because she was a successful actress. B. Because she was an intelligent woman who knows much about movies. C. Because she was intelligent and had rich knowledge. D. Because she had been a little movie star. |
| 5. George Murphy retired from making movies ______. |
[ ] |
| A. in 1969 B. at the age of 47 C. when he was 26 D. in 1975 |