题干

参照图简述扬声器的工作原理.

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答:扬声器是把电信号转换成声信号的一种装置,它主要由固定的永久磁体、线圈和锥形纸盆构成.当线圈中通过随声音变化的电流时,据通电导线在磁场中受力的作用的原理知,通电线圈会在永久磁铁的磁场中受到变化的力的作用,即线圈所受力时而排斥,时而吸引,从而使得线圈振动,将电能转化为机械能,同时线圈的振动也会带动纸盆也来回振动,又将电信号转化成声信号,于是扬声器就发出了声音.

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 We all know that exercise is good for your health. But some kinds of exercise may be better than others. Running, for example, may help to protect against heart disease and other health problems. Running may also help you live longer.

 Recently, researchers studied more than 55,000 adults. About a quarter of the adults have a good habit of running. The study found these runners were much healthier than non-runners. In fact, the runners lived, on average(平均), three years longer than the non-runners.

 Running may be good exercise, but it can be difficult on the body. Here are commonly shared ideas among fitness experts(健身专家) to lower the risk of injury(受伤) for people who are new to running.

⊙Take it easy.

 Do not run too much, too soon or too fast. Most people get running injuries when they push themselves too hard. The body needs time to get used to increases in distance(里程) or speed.

⊙ Get good running shoes.

 There is no single best shoe for every runner. You should find the shoe that offers the best fit and support for your feet. More importantly, you should have a new pair of shoes every 500 to 800 kilometers.

⊙Take good notes.

 Take time after each run to write down notes about what you did and how you felt. Look for things that happen again and again. For example, you may find that your knees hurt when you run sometimes. But perhaps you feel great when you have a rest between running days. These notes will help you make the best plan for running.