2018年A-Level物理备考要点：电场与电场力

　　A-Level课程证书被几乎所有英语授课的大学作为招收新生的入学标准。在中国开设A-Level课程旨在为中国学生提供进入国外大学的有效途径，具体目标为：培养在国内初高中成绩优秀的学生进入世界顶尖大学;培养在国内初高中成绩中等的学生进入世界一流大学;培养在国内初高中成绩一般的学生考取适合自己的大学。

      大部分英国学生都是用两年的时间修完这种课程，但能力很强的学生有时也可在更短的时间内修完。学生的考试成绩及其所选修的A-Level课程在很大程度上决定着能否进入理想的大学和学习所选择的学位课程。为了方便同学们的A-Level备考，课窝考试网小编为大家收集整理了A-Level考试的相关信息，供大家学习参考，希望能为同学们的考试提供帮助。

　　Part 1 Electric Field Shapes 电场概念及表示

　　Electric Field

　　“An electric field is a region in which particles with charge experience a force”

　　Field Shape Diagrams

　　We represent electric fields with diagrams that have conventions (or rules) that need to be followed.

　　For example:

　　The field lines (and arrows) show the direction of the force that would be felt by a positive charge at that point in the field. Note that unlike gravitational fields, electric fields can attract or repel objects.

　　The spacing between the lines shows how strong the field is at that point. In this case it's strongest near to the charge because the lines are closest there.

　　This field is uniform because the spacing between the lines remains the same throughout the field shown. The field lines are parallel and equispaced.

　　Part 2 Electric field strength 电场强度

　　This is defined as the force per unit charge acting at a point in the field. So an equation for it is:

　　Where:.

　　E = electric field strength

　　F = force acting in newtons

　　q = the charge in coulombs

　　Units of E: NC-1 or Vm-1 (these are the same thing!!)

　　(1)匀强电场Uniform Fields

　　There is one special example you also need to know :

　　This is a uniform field. The field strength at any point in this field is:

　　where

　　V = the pd between the plates

　　d = the distance separating the plates.

　　Field strength is a vector - it has direction as well as magnitude.

　　This is important to remember because in electric fields you can have field strengths acting in different directions due to different signs of charge.

　　Radial Fields

　　A particularly useful equation to find field strength around a point charge (note - the first pictures is the field diagram section were point charges) is:

　　where

　　Q = the charge causing the field

　　r = the separation between the charge and the point you are considering

　　ε = permittivity (see below for notes).

　　You can also write this as

　　This shows that:

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