真空中一长为t的均匀带电细直杆,总电荷-|||-为q,试求在直杆延长线上距杆的-|||-一端距离为d的p点的电场强度大小为 ()-|||-P-|||-A dfrac (q)(4pi {varepsilon )_(0)((dfrac {L)(n)+d)}^2} 一 L →+→ d-|||-dfrac (q)(4pi {varepsilon )_(0)(L+d)d}-|||-C dfrac (qL)(4pi {varepsilon )_(0)(L+d)(d)^2}-|||-D dfrac (q)(4pi {varepsilon )_(0)Ld}