题干

如图所示,在边长为L的正方形四个顶点A、B、C、D上依次放置电荷量为+q、+q、+q和-q的点电荷,求正方形中心O点的电场强度?

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答案(点此获取答案解析)

由中心对称可知:若正方形四个顶点处均放置相同电荷量的电荷,则中心 O点的场强为零,
因此可把D点的电荷-q等效为两部分:+q和-2q.+q和另外三个点电荷在中心O点的合场强为零,-2q在中心O点的场强为 E=kQr2=

同类题2

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    I do think that dieting does not work. What it does do is to make you uneasy about food. Imagine you are prepared to abandon your belief and imagine yourself like a slim person...how do you get there?

Transform your focus

    ____Focus on what you really want, not what you don't want. Imagine how you would like to be. See a big picture of yourself, looking fantastic in your bathing suit, how good would you be feeling?____

Understand your hunger

    Slim people eat based on how hungry they are and when lunch time or tea time or dinner time around; they don't just fill up their plate because the food is there. The next time hunger pangs (剧痛) strike, imagine you have a scale in your mind from one to ten. ____Check where you are on that scale before you put something into your mouth. You can also use the scale to judge how full you're getting.

    ____

    Hunger is the only good reason for eating. Most overweight people have got into the habit of eating    when they are not hungry. ____Many people have forgotten how to eat when they're hungry and most    diets tell you to ignore pleasure and ignore hunger and they also tell you when/how much and exactly what to eat.

A. Recognize real hunger

B. Don't watch TV or read books while eating.

C. One means you're starving; ten means you're not hungry.

D. Slim people eat based on how hungry they feel before a meal.

E. Exercise is an efficient way of losing weight.

F. Start by believing in yourself as a slim person.

G. Get that great confident feeling by creating how you would like to be.