题干

如图甲为风力发电的简易模型,在风力的作用下,风叶带动与其固定在一起的永磁铁转动,转速与风速成正比.某一风速时,线圈中产生的正弦式电流 如图乙所示,则(   )

A:磁铁的转速为10r/s

B:电流的表达式为i=0.6sin10πt(A)

C:风速加倍时电流的表达式为i=1.2sin10πt(A)

D:风速加倍时线圈中电流的有效值为0.6 2 A

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B,D

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    Helen Thayer, one of the greatest explorers of the 20th century, loves challenges. She says, “I like to see what's on the other side of the hill.” She has gone almost everywhere to do that.

    In 1988, at the age of 50, she became the first woman to travel alone to the North Pole. She pulled her own sled (雪橇) piled with 160 pounds of supplies, and during her trip no one brought her fresh supplies. Accompanied (陪伴) only by her dog Charlie, she survived cold weather and meetings with polar bears. In fact, Charlie saved her life when one of them attacked her. Near the end of her trip, a forceful wind blew away the majority of her supplies. The last week of the trip, she survived on a handful of nuts and a little water each day.

    Helen goes to challenging places not only for adventure, but also for education. Before her Arctic journey, she started a website called Adventure Classroom. On the site, she shares her adventures in order to motivate (激发) students. She explains, “Although kids often see the world in a negative way, without hope for their future, we work to inspire them to set goals, plan for success and never give up…” Helen grew up in New Zealand. Her parents were athletes and mountain climbers. Following how parents' example, she climbed her first mountain at 9. Later, she climbed the highest mountains in North and South America, the former USSR and New Zealand.

    In 1996, she took on another challenge-the Sahara Desert. She and her husband, Bill, walked 2,400 miles across it! In 2001, she and Bill traveled on foot from west to east through the Gobi Desert in Mongolia. They hope to travel in mainland China into Sichuan and Tibet to study pandas this year.

    Helen plans to continue taking trips. She'll use her colorations, writing, photography and environmental work to create programs for her Adventure Classroom website. She wants to inspire her students never to stop facing challenges!