A- level物理中不管哪个考局，都有Electric field及Coulomb’s law的介绍。

1

Electric field strength:
the force experienced by a unit positive charge at a point in the field.

◆ 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.
2

For a point charge, the field produced:

◆ The field strength at infinity is zero.
◆ The field strength is inversely proportional to
.
◆ 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.
3

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。

4

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.

◆ 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.

5

The electric field strength is equal to the negative of the potential gradient.
Electric field strength

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