题目: 真空中有两个静止的点电荷.若保持它们的电荷量不变,而把它们之间的距离变为原来的2倍,则两电荷间的库仑力将变为原来的( )A.12B.14C.18D.116 答案: 开始时根据库仑定律得:F=kq1q2r2 ①当电荷间距离变为2倍后有:F1= kq1q2(2r)2 ②联立①②得:F1=F4,故ACD错误,B正确.故选B.
题型:选择题 —It's very important for you to arrive on time.—I ____ be there at seven o'clock, I promise.[ ]A. willB. canC. mustD. should 查看答案
题型:选择题 There are desks in Li Lei's classroom.[ ]A. forty-fifth B. fourty C. forty-five D. forty five 查看答案