C

    There are many kinds of clubs in the US middle schools, such as the music club, the sports club, the chess club, the swimming club… But do you know the homework club? It is a popular club in many US middle schools. More and more students spend their afternoons in the homework clubs. The US middle school students need a quiet place to study in. But games, computers and TVs are all around them at home. So the homework club is just for them to do their homework. The club is quiet and it's good for students to study in it. Some teachers are there, too. They can answer students' questions and help with their homework. If students finish their homework on time, they will get a snack. It's always children's favorite food. Then they can go home and play games or watch TV happily.

①此式只适用于点电荷产生的电场

②式中q是放入电场中的点电荷的电荷量，F是该点电荷在电场中某点受到的电场力，E是该点的电场强度

③式中q是产生电场的点电荷的电荷量，F是放在电场中的点电荷受到的电场力，E是电场强度

④在库仑定律的表达式F= $\frac{k{q}_1{q}_2}{r^2}$ 中，可以把 $\frac{k{q}_2}{r^2}$ 看作是点电荷q₂产生的电场在点电荷q₁处的场强大小，也可以把 $\frac{k{q}_1}{r^2}$ 看作是点电荷q₁产生的电场在点电荷q₂处的场强大小．

古诗文默写．

在下列个小题的横线上，写出相应的诗文名句或作家、作品。

①默而识之，____，诲人不倦，何有于我哉！（《论语》）

②穷则独善其身，____。（《孟子》）

③海内存知己，____。（王勃《送杜少府之任蜀州》）

④山光悦鸟性，____。（常建《题破山寺后禅院》）

⑤____，在乎山水之间也。（____《醉翁亭记》）

⑥出淤泥而不染，____。（周敦颐《____》）

⑦人生自古谁无死，____。（文天祥《过零丁洋》）

⑧____，化作春泥更护花。（龚自珍《己亥杂诗》）