题干

如图所示,虚线为磁感应强度大小均为B的两匀强磁场的分界线,实线MN为它们的理想下边界。边长为L的正方形线圈电阻为R,ab边与MN重合,且可以绕过a点并垂直线圈平面的轴以角速度匀速转动,则下列说法正确的是(    )

A:从图示的位置开始逆时针转动180°的过程中,线框中感应电流方向始终为逆时针

B:从图示的位置开始顺时针转动90°到180°这段时间内,线圈完全在磁场中无感应电流

C:从图示的位置顺时针转动180°的过程中,线框中感应电流的最大值为

D:从图示的位置开始顺时针方向转动270°的过程中,通过线圈的电量为

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C

同类题4

   When you're not at home, many worries may start to crowd your mind. Did I turn the coffee maker off? Did I lock the door?Are the kids doing their homework or watching television? With a smart home, you can quiet all of these worries.

    A smart home is a home with a communication network. This network connects devices(装置), such as lights and TV sets, and allows them to be controlled from far away through electrical wiring, mobile phone communication or WiFi over the internet.

    More and more people may start to consider owning a smart home, because it makes life much more convenient. It can help keep your room at a certain temperature. It can record what happens inside the home and send the video to your phone. When you are on vacation abroad, you can use a smart home controller to switch on or off the electricity when necessary. Some smart homes can receive a visitor, allowing him to come in and offering him a drink. They can even feed the cat and water the plants.

    Besides, smart homes are easy to fix. Most smart home technology and devices are wireless and can be set up with a minimum of tools,using only the guiding information. When a problem appears,you can deal with it yourself without paying a professional.

    However, for home-users,the smart home technology is far from perfect.It can be rather expensive to own the technology and the devices. Also, because the smart home system(系统) allow its owner to get home information from anywhere, it leaves the home easy to be attacked by hackers(黑客), who may secretly use or change the information in the system.

    Now,many scientists are excited at the future of smart home technology. Imagine being able to get fashion advice from your mirror, or receive food shopping suggestions from your refrigerator. While there are others who worry that those smart devices will make people lazier and lonelier. Whatever it is, one thing is for sure---smart home technology will change the way we live and work.