题目内容
(6分)阅读下面短文
会发光的电磁翘翘板
小明制作了如图1 6(a)所示的电磁翘翘板参加学校的科技节活动．该电磁翘翘板是在两端封闭的透明塑料管中放有一块磁性很强的磁铁，塑料管外绕有金属丝作为线圈，在线圈两端并联两只发光二极管(符号
)，其电路如图l 6(b)所示．二极管具有单向导电性，电流只能从二极管的正极流入，负极流出．演示时，将塑料管上下翘动，磁铁便在线圈中左右移动，相当于线圈导体在磁场中做切割磁感线运动，这时会看见两只发光二极管轮流发光．
请回答下列问题：
(1)电磁翘翘板中发光二极管轮流发光是根据 原理工作的， 也是根据这一原理工作的(填“电动机”或“发电机")．
(2)电磁翘翘板上下翘动时，为什么发光二极管会轮流发光?
(1)电磁感应；发电机。
(2)因为当磁铁向低处滑动时，磁铁串过闭合线圈，线圈相对在做切割磁感线运动，产生感应电流，此时，电流通过两个二极管中的一个，故该二极管发亮；当另一端低时，磁铁又向相反方向串过线圈，产生相反方向的电流，故另一个二极管也会发亮。所以，二极管会轮流发光。
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【小题6】.
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Certainly dustmen prefer to be known as "Reuse Collection and Disposal Officers". You may think that this is rather 36 , and it is better to call a spade a spade. But dustmen can be as 37 as people of any other occupation, though we must 38 that their job is not a 39 one in the world. We often take dustmen for granted. Perhaps because they usually come very early in the morning, before most people are 40 .We are likely to forget their 41 .Our dustbins are 42 regularly, but we 43 stop to think about the men who do this. However, it is one of the most important jobs in the world, and when there are no dustmen to 44 the rubbish, the general 45 soon becomes aware that something is wrong. Recently, the dustmen of England went on strike for higher wages.
During the first few days it was regarded as a 46 .But when the first two weeks had passed, and the dustbins were overflowing in nearly every backyard in the country, the joke did not seem so 47 any more. 48 the strike continued, people could not 49 the hills of rubbish around their dustbins, and they looked for other places in which to 50 it. Even Leicester Square, in the heart of West End of London, was 51 high with plastic bags full of smelly rubbish. This was a（n） 52 attraction that the people of London were not at all 53 to see. Even when the strike was over, it took several weeks for the country to get cleaned up completely. Perhaps now the English people appreciate the work of 54 dustmen rather more 55 and won't take them for granted any more.
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