题干

如图所示,电阻为r的矩形线圈面积为S,在匀强磁场中绕垂直于磁场的轴以角速度ω匀速转动.匀强磁场的磁感应强度为B,t=0时刻线圈平面与磁场垂直,各电表均为理想交流电表,则(  )

A:滑片P下滑时,电压表的读数不变

B:图示位置线圈中的感应电动势最大

C:图示位置磁通量的变化率最大

D:1s内流过R的电流方向改变ωπ

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D

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    On a hot summer day, my friend Kathy and I drove slowly towards Indiana to our friend's house. After two hours' driving, we stopped before a parking lot to order coffee there. “It's my treat,” I said, trying to find my purse in the back seat, but it wasn't there. I knew it could easily slip under a seat, so I searched around. But nothing was found. At last I remembered I had left my purse on the roof of the car, in the Dunkin' Donuts parking lot. I panicked and was sure that my credit cards had already gone. But something told me to call the Dunkin' Donuts just in case.

     “Oh thank goodness,” the clerk said when I called. “We were going through your purse, trying to find a way to get in touch with you. Someone found your purse lying in the parking lot and turned it in.” Refusing any kind of reward, the shop owner express-mailed it to our friend's home.

    A few years later, on a snowy Minnesota winter night, my car suddenly broke down and I had to walk home. Before leaving my car, I remembered my purse was still in my pocket. But when I got home, it wasn't there. I searched again around the seats but my purse didn't appear. Had I lost my purse again? Knowing my good fortune might not repeat itself, I called the credit card company and my bank to freeze my accounts.

    But the next morning I got a call. “Is this Heidi Grosch? We found your purse lying on the ground.” Again, everything was there and the finder refused to accept any payment.

    I will always be grateful to those who are kind to restore my purse.