在电磁学里，电介质因响应外电场的施加而极化的程度，可以用电极化率（electric susceptibility， ）来衡量。电极化率又可以用来计算物质的电容率。因此，电极化率会影响这物质内各种其它可能发生的现象，像电容器的电容、光波传播于物质内部的光速等等。

对于均向性、线性、均匀的电介质，电极化率定义为

其中， 是电场， 是电极化强度， 是电常数。

由于电位移 定义为

所以，电位移与电场成正比：

其中， 是电容率。

定义相对电容率 为电容率与电常数的比例：

那么，一个电介质的电极化率与相对电容率的关系式为

In electricity electromagnetism, the electric susceptibility (latin: susceptibilis “receptive”) is a dimensionless proportionality constant that indicates the degree of polarization of a dielectric material in response to an applied electric field. The greater the electric susceptibility, the greater the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material (and store energy). It is in this way that the electric susceptibility influences the electric permittivity of the material and thus influences many other phenomena in that medium, from the capacitance of capacitors to the speed of light.