Electric and Magnetic Fields | 电场与磁场

📚 Electric and Magnetic Fields | 电场与磁场

Electric and magnetic fields are central to the CIE A-Level Physics syllabus. An electric field is a region around a charged object where another charge experiences an electrostatic force. A magnetic field is a region around a magnet or a current-carrying conductor where a moving charge or a current experiences a magnetic force. Both fields are vector fields and can be represented by field lines.

电场与磁场是 CIE A-Level 物理课程的核心内容。电场是带电体周围使其他电荷受到静电力的区域。磁场是磁体或载流导体周围使运动电荷或电流受到磁力的区域。两者都是矢量场，可以用场线来表示。

1. Electric Field Strength | 电场强度

Electric field strength E at a point is defined as the electrostatic force per unit positive charge placed at that point.

电场强度 E 定义为放置在一点的单位正电荷所受的静电力。

E = F / q

It is a vector quantity, so it has both magnitude and direction. The SI unit of electric field strength is newton per coulomb (N C⁻¹), which is equivalent to volt per metre (V m⁻¹).

它是一个矢量，既有大小又有方向。电场强度的国际单位是牛顿每库仑 (N C⁻¹)，等价于伏特每米 (V m⁻¹)。

The direction of E at a point is defined as the direction of the force on a small positive test charge. Field lines therefore point away from positive charges and towards negative charges.

某点 E 的方向规定为小正检验电荷在该点受力的方向。因此电场线从正电荷出发并指向负电荷。

Where field lines are closer together, the electric field is stronger. Where they are further apart, the field is weaker.

电场线越密集，电场越强；电场线越稀疏，电场越弱。

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2. Electric Potential and Potential Difference | 电势与电势差

Electric potential V at a point is the work done per unit positive charge in bringing a small positive test charge from infinity to that point.

电势 V 定义为将单位正电荷从无穷远移到该点所做的功。

V = W / q

The unit of potential is the volt (V), where 1 V = 1 J C⁻¹. Since potential is a scalar, potentials from multiple charges can be added algebraically.

电势的单位是伏特 (V)，1 V = 1 J C⁻¹。电势是标量，因此多个电荷产生的电势可以代数相加。

The potential difference ΔV between two points is the work done per unit charge moving between them. A charge q moving through a potential difference ΔV gains or loses potential energy ΔU = qΔV.

两点之间的电势差 ΔV 是单位电荷在两点之间移动时所做的功。电荷 q 经过电势差 ΔV 时获得或失去电势能 ΔU = qΔV。

Equipotential surfaces are surfaces of constant potential. The electric field is always perpendicular to equipotential surfaces.

等势面是电势恒定的表面。电场总是垂直于等势面。

In a uniform field, the relationship between field strength and potential gradient is E = -ΔV/Δr, which simplifies to E = V/d between parallel plates.

在匀强电场中，场强与电势梯度的关系为 E = -ΔV/Δr，对于平行板间可简化为 E = V/d。

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3. Uniform Electric Fields between Parallel Plates | 平行板间的匀强电场

A uniform electric field is produced between two parallel metal plates connected to a potential difference. The field is constant in magnitude and direction in the central region, with edge effects only near the plate boundaries.

两块接有电势差的平行金属板之间可以产生匀强电场。在中心区域场的大小和方向恒定，仅在靠近板边缘处有边缘效应。

E = V / d

Here V is the potential difference between the plates and d is the separation between them. This equation shows that field strength can also be expressed in volts per metre.

其中 V 是两板之间的电势差，d 是两板间距。该公式表明电场强度也可以用伏特每米表示。

The field lines are straight, parallel and evenly spaced, directed from the positive plate to the negative plate. This uniform field is used to accelerate or deflect charged particles in devices such as oscilloscopes.

电场线为直线、平行且均匀分布，方向从正极板指向负极板。这种匀强电场用于在示波器等设备中加速或偏转带电粒子。

For a charge q in a uniform field, the force is constant: F = qE = qV/d. The direction of the force is from the positive plate to the negative plate for a positive charge.

匀强电场中的电荷 q 受到恒定的力：F = qE = qV/d。对于正电荷，力的方向从正极板指向负极板。

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4. Coulomb’s Law and Radial Fields | 库仑定律与辐射状电场

The electric field around a point charge Q is radial. Its magnitude at a

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