题干

如图所示,铜质金属环从条形磁铁的正上方由静止开始下落,下列说法中正确的是

A:当金属环经过磁铁的上端位置A以后,环中便不再有电流。

B:当金属环在磁铁的中间位置O时,磁铁对桌面的压力等于其自身的重力。

C:当金属环经过磁铁的上端位置A时,环中的电流将改变方向。

D:金属环在下落过程磁铁对桌面的压力始终大于其自身的重力。来源:学|科|网

上一题 下一题 0.0难度 选择题 更新时间:2018-04-21 09:01:40

答案(点此获取答案解析)

B

同类题4

阅读理解

    When I spent the summer with my grandmother, she always set me down to the general store with a list. Behind the counter was a lady like no one I'd ever seen.

    “Excuse me,” I said. She looked up and said, “I'm Miss Bee.”

    “I need to get these.” I said, holding up my list. “So? Go get them. ” Miss Bee pointed to a sign. “There's no one here except you and me and I'm not your servant, so get yourself a basket from that pile.”

    I visited Miss Bee twice a week that summer. Sometimes she shortcharged me. Other times she overcharged. Going to the store was like going into battle. All summer long she found ways to trick me. No sooner had I learned how to pronounce “bicarbonate of soda” and memorized its location on the shelves than she made me hunt for it all over again. But by summer's end the shopping trip that had once taken me an hour was done in 15 minutes.

    “All right, little girl,” she said. “What did you learn this summer?” “That you're a meanie!” I replied. Miss Bee just laughed and said, “I know what you think of me. Well, I don't care! My job is to teach every child I meet life lessons. When you get older you'll be glad!” Glad I met Miss Bee? Ha! The idea was absurd…

    Until one day my daughter came to me with homework troubles. “It's too hard,” she said. “Could you finish my math problems for me?”

    “If I do it for you, how will you ever learn to do it yourself?” I said. Suddenly, I was back at that general store where I had learned the hard way to add up my bill by myself. Had I ever been overcharged since?