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引例如图1所示,接地无限大平面导体板附近有一点电荷q, 怎样求空间中的电场分布? 空间电场是由q及板上的感应电荷共同激发的,但感应电荷激发的电场难以确定,可设想感应电荷对空间电场的贡献能用一个假想的电荷来代替．如果在导体板下方与q对称的地方放置一个电荷量为-q的点电荷,就能满足导体是等势体且电势为零的要求．于是问题转化为求两个点电荷在空间的电场分布．因为-q与q数值相等且位置对称,所以将-q称为q的电像(也称镜像),这也就是电像法名称的由来．
For example, as shown in Fig. 1, there is a little charge q near the grounded infinite planar conductor plate. How to find the electric field distribution in space? The space electric field is excited by q and the induced charge on the board, but the electric field induced by the induced charge is difficult to determine. It can be assumed that the contribution of the induced charge to the space electric field can be replaced by an imaginary charge. If a point charge with a charge quantity of -q is placed below the conductor plate and symmetrical with q, the requirement that the conductor is equipotential and the potential is zero can be satisfied. So the question turns into the electric field distribution of two point charges in space. Because the -q and q values are equal and symmetrical, the -q is called the q image (also known as the image), which is the origin of the name of the electrographic method.