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．

    I have a friend. His name is Jack. ③He is healthy because(因为) he plays sports. He runs every day. He runs to his ___①____ in the morning and he runs ____②____ after school in the afternoon. He can run fast. He likes playing soccer, volleyball, tennis, basketball , ping-pong and more. He has three soccer balls, two volleyballs, six tennis rackets, four basketballs and many ping-pong balls. They are in his room. He plays sports with his friends every day. He likes many sports stars and he often watches them on TV. All my classmates say Jack is sporty (爱好体育)。