A- level物理中不管哪个考局，都有Electric field及Coulomb’s law的介绍。在应用中不少同学对该部分的知识有各种各样的问题。那么怎么减少问题呢?很简单我们需要把基础夯实。
电场强度
Electric field strength:
the force experienced by a unit positive charge at a point in the field.
这个基础知识是我们必须背过的，同时该公式也是定义电场的基础公式，任何条件下都可以使用。
在使用过程中我们需要注意方向性。
关于这一点我们有如下总结：
◆ This equation is a general definition, applicable for any kind of electric field.
◆ The direction of field, same to the direction of force, if the charge is positive.
◆ The direction of field, opposite to the direction of force, if the charge is negative.
点电荷的电场强度
For a point charge, the field produced:图片
这个公式只适用于点电荷产生的电场，并不适用于其他形式的电场，如平行板电场。
同时我们需要注意该公式暗含的物理意义：
◆ The field strength at infinity is zero.
◆ The field strength is inversely proportional to图片.
◆ Only applicable for point charge. Here point charge means the distance between the point and charge is much larger than the size of charge carrier itself.
库伦定律Coulomb’s law
Any two charges exert an electric force on each other that is proportional to the product of their charges and inversely proportional to the square of the distance between them.
A-level物理的学习需要我们特别的注意定理，理解概念，也就是说该定义也是我们需要完全记住的。
注意这里的radius是点电荷中心点之间的距离。
电势Electric potential
Electric potential 是一个比较新的概念，其实我们在AS部分就接触过。在一个普通的circuit中我们将电阻两端的电压称为potential difference，那里的potential就是我们在这里提到的potential.
下面我们总结下电势在使用中需要注意的点：
◆ Definition: the work done per unit positive charge on a positive test charge.
◆ The electric potential at a point is equal to the work done in bringing unit positive charge from infinity to that point.
◆ Equipotential: surfaces of constant potential.
◆ The potential gradient at any position in an electric field is the change of potential per unit change of distance in a given direction.
电势的定义来源于能量/功，所以我们可以通过计算potential difference，用charge乘以potential difference就可以得到work done.
电势和电场的关系
The electric field strength is equal to the negative of the potential gradient.
Electric field strength
所表达出来的概念就是电场强度是和电势随着距离的变化率有关系的。 |
