The Concept of an Electric Field | 电场的概念

📚 The Concept of an Electric Field | 电场的概念

An electric field is a region of space around a charged object where another charge experiences an electric force. The field idea is fundamental in CIE A-Level Physics because it replaces ‘action at a distance’ with a local, measurable quantity defined at every point.

电场是带电物体周围空间中使另一个电荷受到电场力的区域。场的概念在 CIE A-Level 物理中十分基础，因为它用逐点定义、可测量的物理量取代了“超距作用”。

1. Electric Field as a Field Concept | 电场作为场的概念

In physics, a field is a quantity assigned to every point in space. An electric field arises from electric charges and can exist even in a vacuum. Unlike a contact force, the electric force is transmitted by the field: a source charge alters the space around it, and a test charge responds to the field at its location.

在物理学中，场是赋予空间每一点的物理量。电场由电荷产生，即使在真空中也能存在。与接触力不同，电场力通过场来传递：场源电荷改变了周围空间，试探电荷在其所在位置对场作出响应。

The field concept is useful because we can describe the effect of a charge distribution without continually referring to all source charges. The electric field vector at a point tells us the force per unit charge that any small positive charge would experience if placed there.

场概念很有用，因为我们可以描述电荷分布的作用，而不必不断提及所有场源电荷。某点的电场矢量告诉我们，若在该点放置一个微小正电荷，它每单位电荷所受的力。

2. Defining Electric Field Strength | 电场强度的定义

Electric field strength, symbol E, is defined as the electric force per unit positive charge. If a small positive test charge q experiences a force F at a point, the electric field strength at that point is:

电场强度，符号 E，定义为单位正电荷所受的电场力。若微小正试探电荷 q 在某点受到力 F，则该点的电场强度为：

E = F / q

The SI unit of electric field strength is newton per coulomb, N C⁻¹. Because potential difference is measured in joules per coulomb and electric field is also potential gradient, the same unit can be written as volt per metre, V m⁻¹.

电场强度的国际单位是牛顿每库仑，即 N C⁻¹。由于电势差以焦耳每库仑计量，而电场也是电势梯度，因此同一单位也可写作伏特每米，即 V m⁻¹。

Strictly, the test charge must be small enough that its own field does not significantly disturb the source charge distribution. In CIE exam definitions, you should state ‘force per unit positive charge’ and mention the test charge is small.

严格地说，试探电荷必须足够小，以免其自身电场明显干扰场源电荷分布。在 CIE 考试的定义中，你应说明“单位正电荷所受的力”，并提到试探电荷很小。

3. Vector Nature and Direction | 矢量性质与方向

Electric field strength is a vector quantity. Its direction is defined as the direction of the electric force acting on a small positive test charge. Therefore, fields point radially outward from positive charges and radially inward toward negative charges.

电场强度是矢量。其方向定义为作用在微小正试探电荷上的电场力方向。因此，电场线从正电荷径向外指，指向负电荷径向内指。

This convention does not depend on the sign of the charges in the problem. If an electron is placed in a field directed to the right, the electron experiences a force to the left because F = qE and q is negative.

这一规定与问题中电荷的正负无关。若将一个电子放入方向向右的电场中，电子将受到向左的力，因为 F = qE 且 q 为负。

When you draw field vectors, always use a positive test charge convention. This avoids confusion in vector addition and in determining the sign of work done on moving charges.

画场矢量时，始终采用正试探电荷的规定。这样可以避免矢量加法以及判断移动电荷做功正负时产生混淆。

4. Electric Field Lines | 电场线

Electric field lines are a visual representation of the electric field. A field line is drawn so that its tangent at any point gives the direction of the electric field vector at that point. The density of lines indicates the magnitude of the field: closely spaced lines mean a strong field, widely spaced lines mean a weak field.

电场线是电场的直观表示。电场线上任意点的切线方向给出该点电场矢量的方向。电场线的疏密表示电场的大小：线越密场越强，线越疏场越弱。

• Field lines start on positive charges and end on negative charges.
• Field lines never cross each other because the field has a unique direction at every point.
• In an electrostatic field, field lines do not form closed loops.
• The number of lines leaving or entering a charge is proportional to the magnitude of that charge.

• 电场线始于正电荷，止于负电荷。
• 电场线彼此不相交，因为每一点的场方向是唯一的。
• 在静电场中，电场线不形成闭合回线。
• 离开或进入电荷的电场线数目与该电荷量的大小成正比。

Use these rules when sketching fields for point charges, parallel plates, and combinations of charges. CIE questions often ask you to draw or interpret field

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